Beating the Stock Market – With DIY Insulation

A Honda Odyssey with all seats including front removed, holds exactly 41 bags of insulation.

We learned that a Honda Odyssey with all seats including front removed, holds exactly 41 bags of insulation.

Well, I’m almost done rebuilding that house I have been working on since last fall. There was a big push through the electrical and insulation stages, and then I happily handed the place off to Agustin and his crew for drywall*, which is now in its final days as well.

Insulation was a big step for me, because I’ve been talking a big game throughout this project on energy efficiency. Sure, we have plenty of high-solar-gain glass on the South side, radiant heat, loads of thermal mass from interior concrete and brick features, and a metal roof to keep heat out in the summer. But all of this goes to waste if you don’t control the flow of heat between inside and outside – which means a tight air seal** and a lot of insulation where it counts.

Where it Counts

In most houses, the attic is the first place to seal and insulate. In the winter, your warm air rises and tries to sneak through the ceilings. In summer, the sun blasts down upon the roof and your ceiling becomes a radiant heater when it is least needed. And the area is especially large: in my long, flat house, ceiling is the dominant feature.

How Expensive is a Leaky Attic?

As explained in the old classic about Destroying your Heating bill, this depends on how fast it leaks, and what you pay for energy.  But using figures from where I live in Colorado is a good generalization, since it has a climate about midway between the cool and warm parts of the United States.

In the dead of winter, we might have an average temperature of 32F (0C) here. The days are warmer than that, but the nights are cooler, and in energy consumption it is the average that matters. If we maintain the interior of the house at 68F, it means we are keeping our house constantly 36F warmer than the outside.

Now comes the fun part: for every square foot of house that is exposed to the cold (or heat in summer), we have heat leakage. The speed of this heat leakage depends on the r-value of the walls and ceiling. Higher R-value is better. Skipping directly to the answer to the puzzle above, a 1000 square foot ceiling with just drywall (roughly R-1 insulation)** loses 36,000 BTU of heat per hour, 24 hours a day. That is about $260 per month of energy loss.

Adding another R-1 of blown cellulose, one of the cheapest and best kinds of insulation, cuts this in half. This amount of cellulose over 1000 square feet costs about 14 bucks. This is a silly theoretical example because that would only be about 1/3 of an inch of cellulose, an amount too small to apply practically. But it sets us up for the handy chart below. We’ll approximate the total annual heating/cooling cost by using my “Four Januarys” method, which usually works out well.

Insulation Return on Investment (ROI)

(values shown are per 1000 square feet of surface area)

R ValueExtra Cost to upgrade to this levelAnnual Heating CostAnnual Return on Investment for this upgrade
0$0 Infinite0%
50$252$21About 8%

Pretty interesting stuff: every time you double your insulation, your costs double and your additional lump of savings is only half as big. But even so, the math works out that you should still insulate your attic to at least R-50, because cellulose is so cheap. And this table is for a relatively moderate area of the US, a country with the cheapest natural gas in the world. In other countries (or  if you use oil or electric heat), this equation tilts even more in favor of better insulation. In other words, you can generally accomplish that almost-impossible feat of beating the stock market, just by insulating your own house. 

All this is why my friend Mike, who reads this blog, needs to get with the program and insulate the shit out of the leaky attic in his 100-year-old house before the next winter season hits – no excuses!

With all that theory out of the way, here’s what I actually did in my own house. Since I built my ceilings in vaulted style, there is no traditional attic. Just 14″ roof rafters with roof deck and metal roof screwed to the top, and drywall screwed to the bottom. With about 20 recessed light fixtures in this ceiling, there is lots of potential for air leakage. So I did this:

A cross section of my own roof. 2" of spray foam plus 12" of cellulose gives me almost R-60 total insulation!

A cross section of my own roof. 2″ of spray foam plus 12″ of cellulose gives me almost R-60 total insulation! With this method, you don’t need the plastic vapor barrier below the rafters.

Spray Foam Insulation

Here is my ceiling with 2" of the amazing foam applied.

Here is my ceiling with 2″ of the amazing foam applied.

When you want to go really hardcore on insulation, closed-cell spray foam is the way to do it. It is about 10 times more expensive than cellulose for a given amount of R-value, but it has advantages that nothing else can match: a perfect air seal, double the R-value per inch, and structural rigidity.

I started my roof insulation job with 2 inches of spray foam to seal the numerous gaps in the roof framing and create an airtight roof structure that needs no venting. I also had the foam contractor fill assorted cracks around the rest of the house, and spray all exterior joist bays in the crawl space. This was expensive ($2700 for the whole project), but it accomplished the most important part of insulating: stopping air leaks. You can buy do-it-yourself spray foam kits from Amazon and Ebay, but I found that hiring a local contractor ended up being about the same price (90 cents per square foot for each 1″ of thickness), so the decision to outsource that task was easy. I hired Denver-based RG Insulation and was very happy with their work. Special shout out to my efficient estimator Phil Trimm – email ptrimm (at) usiinc (dot) com.

Blowing your own Cellulose

Now for the most useful part of this article: contractors were bidding upwards of $3000 to blow the 12″ of cellulose insulation into that area beneath my spray foam. Cellulose is just shredded newspapers, so it is incredibly cheap. It is also incredibly easy to install. I calculated that about $700 of the stuff was needed in my place, which left $2300 to cover a day or two of labor.

Mrs. MM loads a 20-pound chunk into a homemade cardboard hopper (long story)

Mrs. MM loads a 20-pound chunk into a homemade cardboard hopper atop the blowing machine (long story)

Anybody with functional arms and legs can blow cellulose, so it is an ideal DIY task. You just follow these four steps:

  1. Buy the bags of stuff at the store. It looks like this, but the Lowe’s in my area has it at the ripoff price of $14.25 per 18-pound bag. The same stuff was $9.47 at Longmont’s Budget Home Center. Each bag gives you 20 square feet at R-38. I bought 71 bags for my house. Consider delivery, because the stuff is bulky.
  2. Rent the blowing machine. This is just a giant reversed vacuum cleaner with a hopper on top. Most building material stores will lend you one of these for free if you buy at least 10 bags of insulation.
  3. Round up your spouse and have each of you put on a good breathing mask. This stuff is dusty – too dusty for the crappy fabric masks, in my opinion.
  4. You climb up into the attic with one end of the firehose, while your spouse hangs around outside opening the bags and heaving them into the machine. Blow the light fluffy material evenly across your attic to at least a foot deep. Pro-tip: a good headlamp makes this easier. Use mobile phones to communicate (or a remote controlled power switch) so the attic person can turn the machine on and off.


Cellulose in a Vaulted Ceiling

Here I'm blowing cellulose into the vaulted ceiling, supported by fabric.

Here I’m blowing cellulose into the vaulted ceiling, supported by fabric.

My own job was a bit more complicated, since there is no attic. I picked up a huge roll of landscape fabric and stapled it tightly to the bottoms of the rafters with the help of a friend. Then I cut ‘X’-holes near the top of each bay and fed the blowing hose down to the bottom of the slope. Turned on the machine, and slowly filled each cavity from bottom to top in order to pack the stuff in as densely as possible. It was a bit of a pain, but in the end a rewarding day of high exertion and a worthwhile way to save a couple thousand dollars while learning a new skill.

To round out this (hopefully) well-insulated house, I caulked the interior faces of all multi-stud columns to reduce air leakage, used R-15 and R-19 batts as appropriate in the exterior walls, stapled up sheets of plastic vapor barrier across all walls before drywall, and glued up sheets of foil-faced rigid foam insulation in strategic areas where batts would not fit. I’m also adding a 1″ layer of rigid foam on the outside of the new wood framing before adding siding, which adds even more R-value and even more importantly will cut the thermal bridging effect of all the studs and headers.

If I have done my job right, we should end up with a house where the winter sun (plus the heat given off by the people and appliances) provides for most of our warmth needs, and simply opening the windows at night provides all of our summer cooling. But if I’m wrong and further tinkering is required, that won’t be such a bad outcome either.



* This is always a semi-religious experience for a Colorado housebuilder, as you typically get a word-of-mouth drywaller referral from another builder. An earnest Mexican guy comes out to appraise your project and gives you a very fair (yet very informal sounding) estimate. You’re not sure if you conveyed everything properly due to your respective lack of Spanish and English comprehension. But the next day, when you show up at 9am to make sure they are doing well, you enter a scene of blaring mariachi music, flying drywall panels and cutting tools, and a house that is already almost done. I’ve worked with every housebuilding trade and learned to do each of them myself over the years. But the speed and skill of Colorado’s Mexican drywall crews remains beyond my comprehension. Maybe it is an informal brotherly competition that arises just like in real sports, since hanging the sheets it is probably the most physical and athletic trade. Either way, these guys are the superheroes of construction.

**Airtight houses still need ventilation. In the olden days, houses were so leaky that you’d get more than enough fresh air just through the cracks. Modern houses fixed the leaks, but that caused indoor air to become stale: the various smelly and toxic gases from products, cooking, and humans build up and you end up with a building that can actually make you sick. In my climate, you leave your windows open for 6 months of the year, which leaves the 6 cooler ones to deal with. I plan to vent out the house during daylight hours (warmer), leaving it more sealed during winter nights.  Part of the strategy involves these silent Panasonic Whispergreen bath fans, which can be set to run at a very low rate without you hearing them.

*** Sounds pretty ridiculous, but it is not far off the truth for my new house when I bought it. It had drywall ceilings, and a 1″ thick layer of sawdusty wood fiber batts from the 1950s (labeled “extra thick!”). Then a vented attic right above. And even that was poorly installed with no vapor barrier and lots of gaps and holes in it. Net effect might have been R-2 or 3. I cringe when imagining this home’s previous 55 years of energy bills.

  • Bob Werner May 2, 2014, 9:56 am

    PS. Don’t forget to blow door test before the dry wall is up. (or after if need be) This will identify any leaks. Then seal and caulk them. A big leaker is the gap between the floor and stud plate. (I figure you already know this)

    I haven’t kept up, but if your making the home as tight as I think you are, you’ll need a heat exchanger or at least an air intake of some sort. Otherwise you’re likely to be short on oxygen in such a small enclosure.

    • Andres May 2, 2014, 2:24 pm

      I’ve been wondering how MMM is going to deal with this as well. Since he’s doing radiant heat, I’m assuming that he doesn’t have or want ducts running through the place. That means either having a well-placed Heat/Energy Recovery Ventilator combined with an open floor plan, or a bunch of them in each room. A few companies make them, though I haven’t researched price (google for “ductless ventilators hrv”).

      I’ve been looking into this stuff since I just bought a house from 1927 with oil forced air, and I’d *really* like to switch to a minisplit heat pumps/radiant flooring combination after doing some serious insulating. I’d like remove the duct work in the process, freeing up lots of space in the house (and no longer blowing old stale dusty air all over the house). However, how to properly ventilate those well-insulated spaces without ducts then becomes a concern.

  • micha May 2, 2014, 10:05 am

    Did anybody else have trouble interpreting the investment returns on the chart? I think it is showing that for spending $14 you get to save $520 which is a 3700% return. (this is over 4 years, not annually as indicated in the chart.
    Do the investment return numbers seem like they are multiplied by 4? and the last one is more like 1.1% annual return which is not a good return. It seems like the cross-over for beating the stock market happens somewhere between R-16 and R-32 if you look at annual returns.

    • Mr. Money Mustache May 2, 2014, 10:46 am

      Sorry for the confusion. I estimated total annual heating+cooling costs as being 4 x the heating cost in January (four Januarys method). This is just because my own energy bills tend to align this way. So it would indeed be $520/year and a 3700% return. Better returns in more extreme climates.

      • Micha May 2, 2014, 12:02 pm

        Thanks for the clarification, I didn’t understand the 4 January rule as a yearly estimate.

  • D Foltz May 2, 2014, 10:27 am

    Just a nitpick on your table of heat leakage through the roof:

    The R-value of an uninsulated roof isn’t zero.
    Even neglecting the (admittedly, pitiful) R-value of the building materials themselves, there’s a couple air gaps.
    Pulling out my heat transfer book and calculating an R-value for a roof-with-separated ceiling gives me R6 (+-2 or so) – about R0.5 from the top surface of the roof, R1 from the bottom surface, R3.5 from the top surface of the ceiling, and R1 from the bottom surface.

    Plus probably R1 from the OSB and drywall, so call that R7 +-2.

    Doesn’t change the conclusion much, but it might be worth fixing. (infinitesimal payback periods just look silly in a table)

    WRT heat loss through the vertical members connecting the ceiling and roof, they are about R20 and pretty small. I wouldn’t worry too much about hot (cold?) spots due to them.
    (The structural elements of the ceiling and roof, themselves, might matter a bit, but I think the ceiling is already built, so we won’t worry about the cost/benefit of using 2x3s rather than 2x4s to support the ceiling)

    • Mr. Money Mustache May 2, 2014, 10:42 am

      Haha.. I knew I would get busted for that :-) … the uninsulated roof (line zero in my table) is actually supposed to be the case of having NO roof – just blasting your furnace out into the open sky.

      I do find it amazing to think that the 14″ engineered TJIs would only be R-20, though. In theory they will conduct through tiny warmer spots on the metal roof.

      • D Foltz May 2, 2014, 11:08 am

        I think 14$ is a pretty cheap roofing job. I suppose sheet of plastic is about 14$ and gives you about R1… :-p


        Well, I didn’t even use an envelope for the R20 value. Let’s see here…
        14″*(1ft/12″)*(1m/3.28ft)/(0.12W/m^2 [softwood])= SI R value = 2.9
        2.9*5.67(SI R -value > US R value) = US R value = 17
        R19 including air gaps, OSB, drywall.

        Actually lower than that, because they are encased in another material (the insulation), but that’s 3D and I’m not doing writing up an FEA for this.

        But… the TJIs are what, 1.5″3.5″, spaced every 2’x2′? That’s about 1% of the area of the roof, so you’ll loose maybe 2-3% of your heat through them.

        NPV of 0.5$/year is what, less than 12 bucks? I can’t think of a solution which would have cost less than that.

  • OhioDoug May 3, 2014, 6:33 am

    Any recommendations on an all brick 2nd story (all my living space is upstairs) home without any current attic vents? The entire exterior is 3-deep brick and the slightly pitched roof is straight above without any overhang for soffits. I think I can figure out the insulation part myself, just not sure about the venting. One bonus for me is that there are businesses below me that pay for their own heating, which will help a ton. I’m finishing up framing now and hoping to start insulation in early fall. I haven’t started research yet so this article is a great kickoff for me!

  • Hobbes1 May 3, 2014, 7:15 am

    Thanks for this article. My wife and I have been considering this very topic lately.
    We have a two story, all brick (double thickness) home built in 1925. It has a full attic, with exposed rafters that are about 10″ wide, under a slate roof. I have seen numerous opinions online about the ability to insulate the roof on the inside but some ideas are conflicting.

    Any tips on the best and most cost effective manner to insulate our roof in the attic to a degree that we could proceed with turning it into living space? We don’t want to ruin (or replace) the slate roof but from what I’ve read, back in the day, the roofs were installed in a manner that allowed them to “breathe” but without soffit vents. We have a standard sized window at each end of the attic for more ventilation.


  • EconNerd May 3, 2014, 11:01 pm

    The insulation ROI table is a nice example of diminishing marginal returns. Schools should teach econ w/ examples like that!

  • James May 4, 2014, 11:21 am

    An important thing keep in mind when blowing insulation (be it cellulose or fiberglass) into a closed cavity, like the roof system mentioned in this article, is to fill that cavity at a high enough density (a dense packed install). If installed at too low of a density the insulation will settle and lose contact with the upper surface (spray foam on the roof deck in this case) and create an air space. This air gap makes the insulation less effective and in this case can create a misaligned pressure/thermal boundary where the air tight surface (spray foam) is not in contact with the majority of the insulation (cellulose). The spray foam serves as backup protection here and you should still see good results but it would be a good idea to air seal the ceiling plane at the drywall in addition. Caulk around the housings of can lights, plumbing vent stacks, fan housings, etc. For fiberglass , dense packed, is around 1.5 pounds per cubic foot. For cellulose it is around 3.5 pounds per cubic foot but should push closer to 4 for large cavities greater than 6-8″.

    To accomplish these densities a little math to determine a bag count is very important. Figure out the weight of the bags, the volume of the cavity to be filled, subtract framing, do a little dimensional analysis and you should know how many bags need to be installed. To blow at theses densities, a good machine with powerful blowers is necessary to accomplish this. I have no personal experience with store rental machines but the ones I’ve seen at box stores seem unlikely to be able to accomplish these densities.

    I’m not sure of the dimensions of MMM’s house but let’s consider a 1000 sq. ft ceiling with a 10″ space to fill. That’s 833 cubic ft. Subtract 10%for framing and such: 750 cubic feet. He got 18 pound bags… so (3.5lbs/cu. ft x 750 cubic ft.) / 18 lbs/bag = 145 or so bags. At a bare minimum this is how many bags should be used if this space was 100o square feet.

    When installing insulation in the method MMM used the netting fabric should bulge down between each rafter bay after installation and require a roller to make it flat prior to installing drywall. If the drywall can be installed without rolling the bays, it is likely not installed at a high enough density.

    I’m a huge fan of an un-vented roof system as described, I think the whole building industry should change to a similar style, just be sure to make it air tight and install insulation under your foam that will remain in contact with the spray foam.

    • Mr. Money Mustache May 4, 2014, 4:02 pm

      Excellent stuff James. Looks like I didn’t dense pack it nearly that much – just blasted as much in there as I could fit. I’ll let you know how well it performs next winter. Still very glad I got the spray foam as a backup!

      Update: and based on your recommendation, I caulked all recessed light openings before putting on the trim kits. With the housings themselves being highly rated airtight ones, we should have very little air leakage into this ceiling!

  • ElectricEagle May 4, 2014, 7:38 pm

    I’m surprised that nobody has mentioned fire risk.

    Cellulose insulation is an organic material which is treated with borates to reduce flammability. Even with this treatment, it can support a smouldering fire (who knows what you get if your cellulose is made-in-china and doesn’t have the fire rating that is claimed on the bag). When combined with flames that spread quickly through non fire-rated landscaping fabric, this could be a big deal.


    The financial benefits of a well-insulated house are real and significant. That said, I think its worth springing for either silica (glass) or mineral (rock) insulation.

    • PatrickGSR94 May 4, 2014, 10:25 pm

      The insulation is protected by the Sheetrock. Spray foam insulation is required to be protected by either an intumescent coating or noncombustible covering like Sheetrock. So it will also protect the cellulose as well.

      • Electriceagle May 5, 2014, 8:50 am

        Many fires start inside the wall, near lighting fixtures or other electrical items. Besides, unless you use typeX, sheetrock is not fireproof

        • paddedhat May 7, 2014, 5:09 pm

          Commodity grade 1/2″ sheetrock has a 1/2 hour rating. The only problem with your thinking on this issue is that not only do you have a rated sheetrock surface in the assembly, you also have IC rated, air-tight cans and a sealed ceiling/roof assembly. Therefore, your lack one vital component to generate and sustain your “smouldering fire” scenario……oxygen. Now, I’m not claiming that there is a vacuum and total absence of air in the space, but there is not enough to sustain an ongoing event. Had this same assembly been constructed with a fully vented roof, I would agree that the possibility of a sustained burn is much more likely. This btw, isn’t just supposition, but demolition. As I have observed “self-extinguishing” electrical fires on numerous occasions, while rewiring and replacing aluminum Romex. Finally, the flame spread rating of the landscape fabric, in this case is a non-event.

    • PeterK2003 May 8, 2014, 9:22 am

      I would be much more worried about other thing catching on fire than treated insulation.

      The carpet padding in my house(early 80s most likely) was extremely flammable. After removing it I burnt it and it easily started with just a match and was an extremely hot fire in just a few mins. my house would have been gone before the fire company even got out the door.

      If you ever burned a piece of furniture(at least older ones). They are alarmingly combustible.

  • Matt the architect May 5, 2014, 10:47 am

    My criticism is from the standpoint that your vapor retarder is on the wrong side. In northern cold climates you want the vapor retarder on the warm inside side seeing water vapor travels form warm to cold. In southern climates you want the vapor retarder like where you have it with the premise that you would be cooling more of the interior when the exterior is hot and humid. Our dew point calculator is having licensing issues in our office so I am unable to use it for you. My guess is the dew point is right about the location of the foam insulation.

    I would suggest doing this calculation or have it done because my 30 year gut experience is telling me you could be in trouble and all the money you saved will be use remediating mold.

    • Mr. Money Mustache May 5, 2014, 10:57 pm

      Don’t worry – did the calculations, checked with the building department, built everything right for my climate, and passed the inspection. Remember I did start a custom house building company and I’ve been building things since I was a little kid! There will be no mold.

      • Hack-gineer February 6, 2015, 5:04 pm

        He isnt the first guy to ask about the vapor barrier here and you did own up to not understanding how a dense packed wall works. Can you take some time to draw a sketch and explain how your system will function please? His is a valid question since the building dept. hasn’t inspected your yet to be installed foam under the siding (something most wouldn’t permit). Thanks in advance for explaining your approach. I am studying on my own about PassiveHaus technology and this is a major consideration for building longevity.

        • skstrobel January 23, 2024, 10:55 am

          The contractor that insulated our house in Montana (cold climate) used spray foam selectively in places where it was hard to get enough R-value in other ways (rim joists, edges of the roof). I suggested spraying just enough on the outside walls (on the sheeting) to seal them up before installing the fiberglass batts in the walls, rather than using a vapor barrier behind the drywall. He said that when you seal the outside surface (at least in a cold climate), that you have to use thick enough spray insulation to keep the dew point outside of the vapor barrier. Otherwise, when warm moist air inside the house reaches the surface of the foam, it will condense and you will get water in your walls. If I remember right, he said that we could use spray foam on the outside surface (in addition to the batts inside), but the foam would need to be at least 2″ thick to deal with the dewpoint issue. That makes a lot of sense to me.

          If you really wanted to use spray foam to seal everything up while keeping the vapor barrier on the living space side of the bulk of the insulation, I suppose you could mix things up like this… Frame the house but leave the sheathing off while you rough in the electrical and plumbing, then put up the drywall and spray a thin layer of foam onto it from outside the house, then add insulation batts (from the outside) and finally put up the sheathing. It’s pretty clear why no one does that :)

          • Mr. Money Mustache January 23, 2024, 1:55 pm

            Thanks for the story SK, your contractor’s advice is right on!

            I’m a big fan of adding a 1″ layer of rigid foam board insulation on the outside of the sheathing before the siding. It’s cheap and fast, breaks the “thermal bridging” through all of the studs, and yet it doesn’t have the dewpoint issue since you still have the vapor barrier on the inside of the house. The only hitch is that the windows and siding need to be installed appropriately to deal with the extra material, but I’ve made it work before with great results.

  • Stephen May 5, 2014, 11:45 am

    I’ll add another check mark onto the people you influenced to do the insulation. I did the attic and it took about a day but we actually had no insulation when we bought our house. I still feel bad for the 50+ years of discomfort or high energy bills due to no insulation for the previous owners. Also, we were able to get a GA rebate that paid for over 1/2 the cost of the insulation so our out of pocket was about $160.

  • Rob May 5, 2014, 1:04 pm

    In planning out the insulation & energy efficiency of our new build, I decided to hire a company to actually build an energy model of our home. I was planning R30 walls and an R60 ceiling. When we ran the modelling, it turned out than anything above an R40 ceiling was just extra expense that didn’t do much at all to lower my heating costs. Instead or spending that money insulating the ceiling we’ll be paying closer attention to the air tightness of the building and getting a good HRV. The extra cost for the up-front modelling paid for itself rather quickly.

    And on the spray-foam front, not all areas allow spray foam in the same way. The MMM example wouldn’t be allowed here in Calgary, for example. Here you still need to (stupidly) have a air gap for venting when you use spray foam, which adds even more to the expense! So we’re opting for rock wool in the ceiling instead and will save the spray foam for the really tricky spots like along the floor joists.

  • Peter May 6, 2014, 9:08 am

    So i have an attic and there loose insulation of some sort filling the voids between to the joists. I would like to add more insulation but I would also like to be able to store stuff up there. So making the layer any deeper than the joists would hamper storage. So i guess the solution would be to add “Rafter Vent baffle”(think that is what you call it) and then tack batts between the rafters. Yes? No? Better idea?

    Related question. If I would get rid of my asphalt shingle roof and do a metal roof is the ventilation still needed?

    Thanks All.

    • Joseph May 7, 2014, 6:33 am

      Well, technically there’s nothing wrong with doing it that way as long as there isn’t more than one vapor barrier (which shouldn’t be a problem if what’s there already is blown in). That being said, two separate thin layers of insulation probably won’t be as effective as one thicker layer. One way of doing it would be to heighten the joists by adding 2x4s. If you attach them along the joists you would add almost four inches. Then you could add more insulation and still store things up there.

      • PeterK2003 May 7, 2014, 7:05 am

        ahhhh true that would be easier too…though I it would be a B#&*% to put all those 2x4s up there.

        I’m curious why 2 distinct layers would be less efficient. Like what would be the physics of that?

        • Joseph May 8, 2014, 7:54 am

          To be completely honest, I’m mostly going off of what the “pros” in the industry around me have said without doing much research on my own, so I couldn’t be certain of the “why” in this situation. I was told it had to do with it being one solid thermal mass versus two layers with a transitional air pocket in between. At least that’s what one of the insulation salesman in my office said. When I talked to a guy who actually sprays foam he said that if they spray foam in the rafters they would completely rip out any insulation in the floor joists so as not to create a second envelope apart from the rest of the house. I think that had more to do with kraft faced batts though, since multiple vapor barriers could create moisture and mold issues in the space.

      • TomTX June 5, 2014, 8:11 am

        I would put the 2×4 across the joists, not along. Actually, I did, though it was 2×6, not 2×4. Easier to install, and breaks most of the thermal bridging.

        • Joseph June 5, 2014, 8:31 am

          True. I was thinking about it like batt insulation, which would need the boards to be installed along the existing ones to hold a thicker batt, but with blown insulation I suppose it doesn’t matter.

  • maury young May 6, 2014, 10:39 am

    I’ve hesitated telling you about this because you’ve already started the house.


  • paddedhat May 7, 2014, 5:20 pm

    Loved your take on the Mexican rockers. I had a kick Ass crew, Manuel and his posse, for about five years. They always had crazy tunes on the boom box and they would cook lunch on the salamander heater. I typically build modest places, with 100-110 boards, per. Manny would show up early, with four or five guys. The rock would be up, and the place would be swept clean by noon, with workmanship that was about as close to perfect as humanly possible. Anymore, when I hear a local clown whining about Mexicans being lazy, or doing shit work, I just shake my head. Those guys were the fastest and best tradesmen I have ever seen. It was about two years ago, when my good friend, and sheetrock company owner, hit Manny’s number on his speed dial, to schedule a hang job, only to find out that the whole crew went home. Our loss, and I hope they are all safe and well.

  • Paul Ford May 8, 2014, 12:19 pm

    I love the number of comments the article is getting… and I’m hoping for some help. Basically, should I spray foam the attic floor or the underside of the roof deck? I have a 150 year old home with a ridge vent and a couple of box vents. No sofits or gable venting. The heating bills are out of control (12′ ceilings and 3k sqf) and I see that the previous blown in insulation has settled to the level of the roof joists. I’ve recently installed a 2nd HVAC system for split-floor control. The system is in a closet on the 2nd floor with all ducting run through the attic and registers penetrating the ceiling. I’ve later learned this may not have been smart, but with plaster on brick walls I didn’t have many choices to get conditioned air to the 2nd floor.

    So, if I spray the underside of the roof deck, I won’t get a great air seal because I doubt I want to cover the ridge/box vents… but it would be easiest application. I could then blow in cellulose to hit R60. I suppose I could consider rigid foam and great stuff for the under deck application. Alternatively, I can vacuum out the existing blown-in and spray foam the attic floor/joist area. This would have the benefit of sealing all the ceiling penetrations, but would be much more costly accounting for removal of existing blow in and operating around all the ducting. What say you?

    • Cam May 11, 2014, 7:37 am

      Paul, you are right, routing the ductwork through the attic is not a terrific idea. Since it is there, you have to either insulate the ductwork or insulate the bottom of the roof deck. Otherwise the nice conditioned air in those ducts gets wrecked by the extreme temps in the attic. So you should consider doing the bottom of the roof deck and kill two birds with one stone.

      • Mary G. May 28, 2014, 10:03 am

        I agree with Cam. Just to clarify – if the attic space remains ventilated, then insulation at the level of the roof deck has NO insulating value. In order to get your $$ worth out of insulation on the roof deck, the attic space must be brought within the thermal boundary and sealed off.

        A ventilated attic space is thermally similar to the great outdoors. Ducts belong indoors. If I were you, I would look at the layout and decide whether it would be doable to create a sealed, insulated space out of the currently ventilated attic or, alternately, whether it would be possible to add enough spray foam and/or cellulose so that the ducts are effectively covered with R-60. You are losing a lot more heat through those ducts than you are through the ceiling.

  • Jocelyn May 9, 2014, 8:20 pm

    To echo your Colorado Mexican crew, I had a similar experience in California. Had to redo the fence in my backyard after a container truck went through it, nicely stopping a few feet from the house. The (old) Asian guy came, gave an estimate about half what others had quoted, but I couldn’t understand what he said, was not sure he understood we were speaking about the fence other then “yeah yeah, good good, no worry” (which had me worry…). Out of insanity we went with them. I have never seen a team work like that (and I did extreme remodeling a few times). Your mariachi induced transe description is very accurate. Now, 15 years later, I am looking to have a fence done (in Montreal) and I have no idea how to find such a fun, good natured, hard working and efficient crew. I am thus expecting that the experience will suck, as usual.

  • TinaP May 14, 2014, 2:35 pm

    My insulation sucks in my 1988 home – I’ve wanted to improve it for years, but was never quite sure how without paying an arm and a leg to hire someone. And it’s not a huge house, so the savings wasn’t really out-weighing the cost. Can you use this spray foam and cellulose to “redo” insulation on an already established home?

  • Andrew W May 20, 2014, 10:15 pm

    Dear MMM,
    I recently read an article in Fine Homebuilding (Issue 242; pp 54-58; March 5, 2014) about creating a balanced ventilation system within an airtight home. MMM, you mentioned your choice of the Panasonic bath fan as part of your ventilation strategy, but what, besides opening up windows, is your plan to provide your home with fresh air, especially during those cold winter nights? Have you considered either a HRV (heat-recovery ventilator), or an ERV (enthalpy-recovery ventilator)? You could add a supply-only ventilator, but that comes at the expense of losing the heated air inside your home.

    Your Guardian Mustachian

  • Andreas June 26, 2014, 9:00 am

    Hello MMM, great article – and yes, insulating your house is a phantastic investment. Here in Germany, energy is comparatively expensive, hence we had our house insulated some years ago. What I have learnt since is that insulating your house is not only a great investment, it also a kind of luxury that I wouldn’t want to miss anymore: The house is warm and cozy in winter, and it stays cool in summer – we hardly need to turn on our heating in winter, and in summer we have no desire at all for air-condition, even after long periods of heat (well, of what we call heat here in Germany).

    What we did was to not only insulate the attic. We also added facade insulation and covered the house from the outside with 15cm polystyrol plates, on which a thin layer of brick was added. Additionally, and very important to obtain the desired insulation effects, we exchanged all windows of our house. Previously, the house had 40 year old windows with double glazing, only. With such old, hardly insulated windows, all insulation attempts may be futile. We hence switched to state-of the art windows, i.e. with triple glazing and krypton gas filling. The effects are tremendous, and in spite of high cots it is a very good investment in the long run (at least considering the high energy cost here in Germany).

    Here are some informative links (sorry, the automatic translation in the 2nd link isn’t perfect, but the photos speak for themselves):
    – Windows: http://en.wikipedia.org/wiki/Insulated_glazing
    – Facade insulation: https://translate.google.de/translate?sl=de&tl=en&js=y&prev=_t&hl=de&ie=UTF-8&u=http%3A%2F%2Fwww.mein-eigenheim.de%2Fdaemmung%2Fitems%2Ffassade-daemmen-3-systeme-zur-auswahl.html&edit-text=

    Regards, Andreas

    • Mr. Money Mustache June 26, 2014, 3:18 pm

      Wow, triple glazing and 15cm foam! Here in Colorado, most people don’t even bother replacing 100 year old SINGLE pane windows, because energy is so cheap (plus they are bad at math :-))

      • Andreas July 5, 2014, 12:14 pm

        Hello MMM, well, double glazing has been standard here since I can remember. I don’t know any building here with single pane windows. Well, there are some buildings from before the world war that have single pane windows, but usually, these have double windows, each with a single pane.

        Our kind of insulation has become very common here: Many Germans are insulating their homes and upgrading their windows as interest rates are so low and as energy is getting more and more expensive all the time.

        For example, we would never measure our fuel in gallons and rather use liters, because our fuel prices for gallons would be too intimidating :-) I personally think it is a good thing that fuel and energy is getting so expensive, cause it gives an incentive not to waste it.


  • Adrian July 7, 2014, 8:44 pm

    Do you have any recommendations on how to insulate an attic that has knob and tube wiring? The K&T is in good shape and replacing would be very difficult (access impossible at eaves, requires opening up walls and ceilings)

    We haven’t insulated the attic yet because 1) i like to hang out up there… well not. but working up there on lights and outlets is easier without insulation. 2) because of the K&T wiring. We have about 1 month of cold climate a year (about 35F outside temp). 3) we like the warm floors from the hydronic radiant heat and like to circulate a lot of fresh air in the house, so don’t mind the heat losses so much. We figure (perhaps wrongly) that if we insulated the attic, the air would get too hot if we had the radiant on as often as we like warm feet… 4) we plan to heat the hydronic water with a few solar thermal vacum tube panels. For now the heat uses btw $10-$30 of gas per month. Thanks for the great article!

    • Evie July 10, 2014, 3:57 pm

      Blowing insulation around knob&tube wiring is against building code in many places…

      • Adrian July 15, 2014, 5:48 pm

        Yes, exactly. It’s against code here too. If someone has done it, please share how you did it.

  • Evie July 10, 2014, 3:55 pm

    The DIY approach may NOT always be cheaper than working with a contractor. In fact, many states incentive air sealing/insulation work, but you can only access the subsidies/rebates if you work with a certified contractor from a specific list and only if you get an energy audit first. For example, in Maryland (where i work), the utilities provide 50% rebates up to the cost of $2000. The contractors also get a much better deal on the cost of materials since they buy in bulk. Once you factor in the rebates and the cheaper cost of materials–the shear math works out in favor of hiring a contractor over doing it yourself.

    • Oh Yonghao July 14, 2014, 11:02 am

      My experience so far looking up how much contractors cost they have this funny way of eating up the entire rebate. I’ve looked into it for solar and I could do it where it costs me nothing if I could get the rebates, but since I have to use a contractor the price skyrockets. It seems like most contractor’s find buying in bulk means more padding for their own pocket, being on the list means no competition, and the rebate means they just have to beat the price of a non-listed contractor after rebates.

      Luckily the insulation rebates did not require a contractor, and the home energy audit is a self audit. The only thing which would require a contractor is a blow test which is 100% refunded. YMMV.

  • Danny C. November 19, 2014, 11:43 am

    MMM. Great article, We, just yesterday, broke ground on our home up here in Fort Collins that we are building ourselves. However, I have yet to decide on our insulation due to our vaulted ceilings. Quick question: I don’t see how you elected to vent your vaulted ceilings. I’m trying to figure out how to best created needed air circulation under the decking, while still achieving at least R-50 in the rafters. How did you vent your rafters?

    • Mr. Money Mustache November 19, 2014, 12:37 pm

      Unvented is the way I like to build cathedral ceilings these days. The spray foam on the underside protects you from condensation (and is in line with what the building code requires for an unvented roof), plus you get a much better seal.

    • Rob November 19, 2014, 5:26 pm

      Annoyingly the building code here (Calgary) doesn’t allow for unvented cathedral ceilings, so even with spray foam we still need to use vent baffles and roof vents.

      On the house we are building right now I paid to have an energy model created. It paid for itself right off the bat when we calculated that R50 insulation had almost no difference over R40 for our roof. The air barrier was far more important.

  • Eric January 23, 2015, 9:04 pm

    Our 1976 house in Washington State is very cold in the winter, and very hot in the summer. I think the original insulation is in the attic and walls, and there is NO insulation beneath the floor. Obviously we need to do some work. Any thoughts on the advantages and/or disadvantages of removing the old insulation before adding new insulation?

    • Mary G January 24, 2015, 7:40 am

      It depends on the type and condition of the old insulation. Another important factor is that air sealing is the prerequisite for adding insulation, so if the old insulation interferes with that, then moving it out of the way, at least temporarily, may be wise.

      Lack of insulation is easy to see. Failure to air seal is easier to overlook, but much more costly. It’s the best place to start because it is inexpensive to do and has a great payback. It’s also difficult to do after you add insulation.

      Here’s a good place to start for information: http://energy.gov/energysaver/articles/air-sealing-your-home

  • BodesWell February 22, 2015, 4:49 pm

    I don’t see it in you drawing, so I’m curious. Was your metal roof directly screwed into the OSB, or did you add a small venting gap? (as recommended here: http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling).

  • NE Magyar January 5, 2017, 8:25 am

    We just had all our insulation removed, as it possibly contained asbestos, and now we are air sealing and re-insulating ourselves. For the blown-in insulation, you can get the 19-pound bags from Lowe’s or Home Depot for $10.00/bag. I called the manufacturer, Green Fiber, and asked if they had a distributor in our area that wasn’t a big box store. They put me in touch with Allied Building Products out of New Haven CT, which sold us 30-pound bags for $8.00/each, and delivered to our home for $35. Saved us a bundle.

    Also, this: https://www.greentechmedia.com/articles/read/Next-Step-Living-Out-of-Cash-is-Shutting-Its-Doors-This-Week

  • JeffD September 3, 2020, 8:15 pm

    The article covers the “before” but what about the “after”? Did the energy costs work out as calculated? Was it “too” airtight? Did any of the potential downsides pas out, and how much time a day did you have to devote to managing fresh airflow?

  • Eric Miller January 4, 2021, 2:39 pm

    Are people seeing any bowing in the drywall after doing this?

    I’ve heard most drywallers are suggesting 5/8″ instead of 1/2″ sheetrock on ceilings to avoid bowing even when there isn’t weight on the drywall. Adding the weight of cellulose seems like it would increase the risk of it bowing out between the trusses after a year or two.

    As a result, I was thinking of doing this method but putting the landscaping fabric *above* the trusses, rather than stapling it to the underside. This is a more annoying installation method though, and steals a couple of inches of insulate-able space.

    Any comments or experience one way or another on ceiling drywall bowing from the weight of cellulose?


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