Thoughts on insulation

When it comes to insulation, more is better. Or so they say. Of course, I’m always skeptical when people say “more is better.” More may be better in some ways, but there’s always a cost to having more, and it turns out you usually can get away with less. But how much is enough? That is what I want to know.

I’ve been doing some research on insulation, and as it turns out, it’s a rather complicated subject. On the one hand, there’s this deceptively simple formula:

H = ( 1 / R) x A x T
H : heat loss in BTU/hour
R : R-value
A : surface area in square ft
T : temperature difference in Fahrenheit

Using this formula, I can calculate the theoretical heat loss of my cabin. For instance, Hut 2.0 will have a surface area of around 750 square ft, and if I manage to wrap it all up with R-10, and there’s a 50F temperature difference between the interior and exterior, I can expect to lose (1/10) x 750 x 50 = 3750BTU/hour. That doesn’t sound like much. For instance, even a tiny stove designed for boats is rated at 3000 – 8000BTU. In fact, I can even go down to R-5, and will be under 8000BTU/hour.

The reality, of course, isn’t so simple. I just assumed a single R-value for the entire structure, but the reality is that windows will have a much lower R-value, the door another value, and perhaps the walls, floor, roof will all have different values too. On top of that, R-values give you an idea of how slowly heat will transfer through surfaces, but that only accounts for a fraction of actual heat exchange. In a structure, one huge source of heat loss is through air exchange. For ventilation, outside air needs to be brought in, and that necessarily displaces internal air. At the very least, in order to use a stove, I’d need to suck in enough cold air to supply oxygen for the fire (and myself). So the kind of calculation I did above is useful for setting a baseline, that is, I know my heat loss won’t be any less than the calculated figure, but doesn’t produce anywhere near an accurate or realistic number.

On the other hand, I can’t afford to go and buy tons of insulation. Also, the structure is tiny as it is, so to maximize space, I’d like to keep the wall cavities open instead of filling them in with insulation. There’s also the environmental cost too, since most common forms of insulation are made of toxic materials, or at least materials that are non-biodegradable and difficult to recycle. There are “green” insulation options, but as batt or blown insulation materials, and not rigid boards that I can use. I might get away with less insulation if I decrease air exchange by using housewrap, but housewrap is made of plastic, so that’s less than ideal in my opinion. But then, if I really care about green materials, I should probably be building a straw-bale structure, so perhaps there are limits to how green (or warm) of a structure I can build out of timber framing.

My situation is also different to those of typical homes, because I live in the woods and have a practically infinite and renewable source of firewood. For me, firewood is free, so the cost of heating is also free (if I ignore labor, which I do). From an ecological perspective, I have no qualms burning dry dead wood on my property, since if I weren’t burning the fuel, a natural forest fire very well may instead. So while typical houses may be able to justify the financial and ecological cost of additional insulation by factoring in the cost of heating, for me, the cost of insulation is just that: a cost. The only consideration I have, is to make sure that my heat loss doesn’t outstrip my heating option. Though, if that’s all I’m worried about, I think an old fashioned cast iron stove that the local antique store sells for a little over $100 will probably keep my hut warm either way.

So that was a rather long way to say, I’m going to go light on insulation, and instead depend on good heating to stay warm. Stay tuned to find out how that works out come winter (assuming I stick around for winter, which isn’t yet certain).

19 thoughts on “Thoughts on insulation

  1. Just keep in mind, it’s not just for cold nights, with a high desert summer climate [same as where I’m building] the insulation on the roof helps keep things cooler inside, especially nice when it’s uncomfortably hot outside. Floor insulation is not really much of a factor for cooling with no sunlight heating it up, but when trying to stay warm it’s obviously nice to have a well insulated floor to keep the windchill factor down. I went for max insulation and the entire floor with R30 was only $60, so roof will be a little more than that and walls will probably round it all off to under $250 total. With no real green options locally, I went for fiberglass and at least it’s quick to put in and doesn’t burn.

    • Yup, I’m planning on using a little more insulation on the roof, precisely to beat back the hot summer sun. My current plan is to use rigid foam boards on top of the sheathing (but under the roofing), as well as some mylar or reflectix to reflect back radiant heat from the sun. I also may use batt insulation in the space between the rafters, since (unlike wall cavities) that space isn’t useful for much anyway.

  2. You might want to check out some of the new ceramic insulative paints based on technology developed by NASA for the Space Shuttle. The indoor paint prevents heat from escaping and the outdoor paint prevents heat from entering during summer.

    This site sells it and also has some very interesting technical articles on thermal transfer and R values:

  3. With your chosen construction technique and available resources, your reasoning is sound. The benefit vs the time and expense required to super insulate would not likely pay big dividends.

    We used conventional platform framing so insulating was easy and inexpensive. I’m quite pleased with the performance of the building. We’ve used it in Winter when the night time temperatures reached -3F and the daytime temps stayed in the teens. For the better part of four days and three nights I used less than $5 of propane to keep the place 65F or above.

    RT’s point is good too. During the hottest days of summer we open up the cabin during the morning and then close it up during the heat of the day. Depending on how often we go in and out the temperature stays 15-25F cooler than outside.

  4. We went rather light on insulation on our tiny house (fiberglass in the walls and under the floor) and ended up putting rigid insulation in the roof this summer because the temp inside the house fluctuated too much. Now that the roof is insulated, too, the temp is more even. Last winter we’d wake up in the morning and if the fire from the night before had burned out (which is often did), it sometimes was below or barely above freezing. INSIDE!

  5. Yes, the irony of building small is that it’s cheaper to insulate well but less important to do so – or to put it the other way, big buildings take a double hit in needing the insulation more and it being more expensive.

    The same sort of double hit applies to the use of green materials over conventional ones; the excess cost for green materials on a small building are less but the damage of using conventional ones is less, too. On the other hand, the proportion of wall thickness taken up by green materials is larger and may out-weigh (e.g., in the cost of transporting extra lumber) the harm of using PUR or whatever.

    You and the other commenters make some good points but there are some other considerations.

    Adding more insulation will reduce the amount of heating required in the coldest part of the year but, as you point out, that’s not terribly significant. However, it will also greatly reduce the length of the heating season, because solar gain and incidental heat (cooking, hot water use, body heat) are sufficient, thereby reducing the hassle of using the wood burner at all.

    With a small building having enough ventilation is more critical than in a bigger one as there is less volume to buffer any spikes in “contamination” – cooking, showering, etc. Therefore, you’re more likely to over-ventilate making the losses that way greater. It seems to me that this makes a case for the use of heat-recovery ventilation.

    What I’m thinking of for your sort of hut are the small heat-recovery ventilators sold for use in bathrooms. When in background trickle mode they don’t use a lot of power.

  6. Hi Ryo.
    Just recently tripped over this site and as it touches on one of my areas of fascination (tiny buildings) I keep coming back. I designed and had built a smallish house ten years ago and did all the conventional things with insulation. My big surprise was how really poor even double glazed windows are at keeping the heat. I now routinely use that awful plastic wrap on my windows come winter. I’m glad I put most of my glazing on the south side as I get terrific solar gain (even on the reasonably rainy BC coast) and in the hot summer my house acts like a chimney ( all the heat rises and goes out the attic windows)
    I am inspired by your site although it make me wish I was 28 instead of 58…I would surely be doing something similar.
    Check out this site:
    Terribly expensive but if you built a house “around” one you could almost justify the cost. Fire, stoves, cooking…..more areas of fascination.

  7. Congrats on your progress, man!

    I looked at heating solutions (and being from Vermont) realized that a small wood stove burning in a small building can get unbearably hot and be very difficult to regulate. Believe it or not, several candles burning, combined with the large windows facing the sun, keep my (very well insulated) 10×12 liveably warm much of the time.

    Insulation will more than pay back it’s cost in comfort if it’s installed properly and the structure is built well. It’s much easier to insulate now rather than in mid-January with a foot of snow on the ground! Hopefully you can insulate the floor too, it will make a HUGE difference when the temps drop.

    Also Ry, don’t neglect any spaces where your studs meet or leave any gaps in the sheathing/siding. A badly sealed building is a magnet for vermin and insects to enter and nest. If you have not cut all your lumber to fit tightly, you’ll soon be rousting creppy crawlies out of your walls (and food, bedding, and clothing). Unfortunately I learned this the hard way with my first shed! Steel wool, expandable foam and silicone caulk can go a long way to keeping critters out and heat in!

  8. I think that’s good reasoning on the insulation issue vs the amount of firewood you have available. I lived with a wood stove for heating for over 20 years and found that it was both exceptionally warm and very cosy too.

    Plus, there’s something about collecting firewood that’s satisfying and problem at the same time.

    If you stay for the winter, I imagine you’ll be warm and content in your house.

  9. It’s great to see all these people sharing their experiences with their small homes!

    Since I live in a very mild climate I’ve been wondering how important insulation will be for my tiny house. The feedback here has proved very useful. 🙂

    Thanks all!

  10. Before you put your roof on, have you considered putting a Velux type skylight in? They are double insulated and open up for ventilation. Home Depot stocks them. This would eliminate the need to burn candles inside during the summer for light, like you did in the video.

    • thanks Ed, but I double-built the floor with an insulated deck over the 2×6 joists plus made sure there were no air gaps anywhere in my frame. The only un-insulated part of my cabin is the glass in the wall of windows (my glass door is low-w double-glazed). I even sprayed expandable foam into the hollow aluminum window frames to keep them from leaking heat.

      If it’s done right, insulation is WELL worth the cost, time and effort!

  11. I love your project! Keep up the adventure.

    Permit me to share a few thoughts on your insulation post.

    How much insulation is enough? There are calculations to determine the “economic thickness” of insulation. It is an optimization routine. As an engineer, you should have no trouble following that logic. I’m not thinking the calculation is worth that much effort. Insulation comes in standard sizes and you only have so much room available. Just put a 3.5 inch in the floor and walls and be done with it.

    However, insulation does more than reduce heat flow. It also controls condensation of water vapor on the wall surface. I propose in your situation, this becomes the over-riding consideration. You want enough insulation to keep the inside wall temperature above the dew point during the coldest season the hut is occupied. Again, this is an easy calculation. This is important because you don’t want condensate running down the inside walls. It can lead to wood rot and mold growth and other unhealthy or unpleasant things.

    Which means we also need to talk about vapor barriers. If you don’t have a good vapor barrier on the inside of the wall, then all that moist air will penetrate the insulation. When it reaches a point in the insulation with a temperature equal to the dew point – presto! – the water vapor turns into liquid water, right in the middle of your insulation. The R value will rapidly approach zero and we’re back to talking about mold and rot again. Unless you have used some kind of closed cell foam insulation which you really don’t want to leave exposed in a hut that has open flames.

    So the recommendation is batt insulation with a vapor barrier on the inside. That means something that resembles a wall. I know that takes up your vital storage space, but it would make the hut much more livable.

    On the subject of a wood burning stove – I’m thinking that would turn your hut into an oven. Maybe you can find a “micro-stove” that would be appropriately sized for your hut, but anything approaching a real wood burner would be far too large and hot and take up space you don’t want to give up.

    As you say “For me, firewood is free, so the cost of heating is also free (if I ignore labor, which I do).” There is value in collecting firewood. It puts you in touch with your land, it clear out the tinder. It also costs labor and time is all a man really has. It is the one thing that can never be replaced. Aren’t there other things you would rather do with your time? Stick some insulation in the walls,minimize the amount of time you spend collecting fuel, and go do something else.

    As to priority, insulation is most important in the ceiling, second in the floor (because warm feet are nice), and finally the walls.

    Another poster mentioned the space age insulation paint. I’ve tested those products on steam pipes and found them to be worthless, utterly worthless. Granted, your hut won’t be operating at 400 degrees F. But I don’t think the insulating paint will work any better at lower temps. And it’s expensive.

  12. One can install a pipe from the exterior to the intake of the stove so the fire is drawing cool air from outside rather than inside air, less risk of suffocation.

  13. This would have been more helpful to you earlier, but you should look into cordwood building. Rob Roy has some books out there (Cordwood Building: The State of the Art). Mother Earth News and BackHome magazine have done a few articles. As seen in Rob Roy’s book, you could add a cordwood shell around your Hut for additional insulation. Check it out. Labor intensive but cheap.

    Here’s a website with articles you can check out for free.

  14. When I insulated my 600sf Sprinter I used this:

    I used the can spray of the same (isocynurate & polyol) to fill the gaps.

    I considered the time to install the cheapest (basically styrofoam), the blue sheets, pink sheets and yellow (mine) is all the same. Since your labor is 2/3 of the investment on anything it only makes since to use the best. It takes the same space as well.

    Mr. Plywood in Portland, OR sells the 3″ thick yellow sheets for $65. No brainer.

  15. Pingback: More thoughts on insulation « Laptop and a Rifle

  16. Pingback: Insulating Hut 2.1 « Laptop and a Rifle

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