More thoughts on insulation

I got a lot of great comments on my recent post on insulation, so I thought I’d write another post to summarize some of the common issues that have been pointed out, and to also elaborate on my plan.


A few readers pointed out the higher labor cost of gathering more firewood. I said in my post that I was ignoring that, but I think it deserves a few more words…

Economists call it opportunity cost. When I was in college, students would queue up at one of the campus coffee shops, which served milkshakes for a dollar on Wednesdays. Obviously, this tradition, knowns as “Shake Day”, was a popular diversion among students who would wait in these long lines with their friends, socializing (or simply pondering silently in solitude) as they waited for their tiny cup of sugary molten goop. An Economics professor once criticized this tradition, by invoking the concept of opportunity cost. The professor argued that the cost of waiting in line outweighed the potential upside of buying a shake for a dollar. Instead, presumably, students should be doing homework to prepare for high salaried careers, or perhaps be peddling their time to low-wage campus jobs for $10/hour.

Of course, this “criticism” wasn’t entirely serious (I hope), but in my eyes, it represented a common perspective in our society that I find troublesome, as it is the very reason we have lots of fat wealthy people who are unhappy and unhealthy. Yes, I can be sitting at a desk, selling my time for $125/hour (or more). But if that’s what I wanted, I wouldn’t be living in the woods. For me, an excuse to get outside, be in the woods, and do a little physical work, is worth far more than what money can buy. More generally, gathering my own fuel makes me more aware of my resource consumption, and having to go out to the woods to gather fuel will also give me better insight into how quickly (or slowly) I am depleting the resources I have, and in turn, get a better assessment of how sustainable (or unsustainable) my lifestyle is.

And yes, it is also entirely possible that I’ll decide at some point that I’d rather spend less time gathering wood. If that’s the case, I’ll change something, but until I try it, I won’t know.

Insulation is for summer too

I focused mostly on how insulation will impact my life in the woods should I stay for the winter, but, of course, insulation matters in the summer too. However, as far as I understand, insulation in the winter and in the summer are actually two different problems.

In the winter, the goal is to keep the cold air outside, from cooling down the interior. Heat is transfered mostly through conduction and convection. That is, the warm air inside heats up the structure’s sufaces, which in turn conduct (and radiate) heat to the outside cold. Or, cold air gets into the structure, displacing warm air. So the common solutions are to use insulation materials that prevent conduction, like foam and batt insulation, and prevent air exchange.

In the summer, the goal is to keep the interior cool, but the main problem isn’t the warm air outside, but rather direct radiant heat from the sun. Up in my area, the air is very dry in the summer, and at 4200ft elevation, the air stays fairly cool most of the time. But the sun beats down relentlessly, heating anything it touches. So the goal is to reflect that heat away from the structure, and to prevent it from heating up the surfaces. To reflect radiant heat, you don’t need thick batt insulation; a coat of white paint, or shiny material like mylar will do the job quite well.

Granted, from what I understand, most homes don’t make a distinction between the different heat transfer characteristics. And indeed, you don’t have to. In the summer, you could let the sun heat up your roof, and then prevent that heat from getting conducted inside by using a ton of batt insulation in the roof and attic. That way, you’re dealing with conduction in the summer and winter, and can use the same insulation for both scenarios. The kind of insulation that works well in the winter can also be beneficial in the summer if you want to make efficient use of air conditioning (which I don’t have), or want to keep the structure from heating up during the day, once it has been cooled at night.

In my particular case, since I am trying to minimize insulation, I plan on trying to reflect sun as much as possible during the summer, instead of relying on insulation. I’m planning on buying light-colored roofing panels, and also lay down a layer of mylar (which I have l left over from Hut 1.0) under the roofing panels to keep the roof from getting too warm in the first place. I won’t be able to expect the structure to be any cooler than the ambient shade temperature, but that’s good enough for me (for now). If I need additional cooling, I might make a swamp cooler, but if this summer was fairly typical, I probably won’t need it for more than a few weeks each summer.


Another issue that I didn’t really address is moisture/condensation. I considered using housewrap, but decided instead to seal up the cabin through other means (namely, by taping up seams between the exterior insulation boards, and by using spray foam insulation and caulk). However, that still leaves the issue of moisture, since sealing up the cabin will simply keep moisture from getting out, which in turn could cause condensation and all sorts of other problems.

Wood stoves too hot?

A couple of commenters also pointed out that a wood burning stove might get too hot. I guess this sort of depends on how big/hot of a stove I get, but right now, I’m leaning towards getting an old fashioned cast iron stove from the local antique shop. I have no idea how much heat those things give off, but I could see how it could get kind of warm.

An Idea

So, it seems like I have two open problems: controlling moisture, and keeping the cabin from getting too hot.

Fortunately, there’s a common answer to both problems: ventilation. Pumping fresh dry air in and moist air out solves the condensation problem, and will probably help with the heat problem too. The plan is to have an air intake (possibly with a small 12V fan) near the stove, so that the air that gets sucked in gets heated immediately. The idea is to pump more air into the cabin than the stove needs, and thereby create an over-pressure (this will also prevent cold air from getting in from undesirable gaps). I’ll have a vent at the top of the hut, where hot moist air gets pushed out. Most of this air movement will happen by convection, since the cold fresh air will rise once it gets warmed by the stove.

I should only need to actively vent air when I’m actually producing lots of moisture, for instance, when I’m cooking or drying wet clothes. At night, I’ll probably stop the air exchange to conserve heat, and while I’ll generate some moisture, I could probably dry out the interior again the next morning by getting the stove going and turning on the fans (or by opening the windows if it’s warm enough). If I decide that I need more insulation, I can always fill in the wall cavities, which I plan on leaving open for now. Adding a moisture barrier later won’t be an option, but hey, there’s always Hut 3.0.

15 thoughts on “More thoughts on insulation

  1. I think you’re right on with your article. The wood stove no matter how small will put out more than you need to heat your hut with a tiny amount of wood. We had an uninsulated yurt with a smallish stove and it would get cooking hot. Now I think it’s too big for yours even. Maybe a boat stove would be the ticket and take up less room.

    How many square feet? 120?

  2. Also consider with a wood stove you will need to use steal plating or some sort of metal behind it along the walls or you may cause another unintentional fire …. also consider putting water storage in attic area above wood stove as a just in-case .. also it will warm you water….

    If you do decide to buy a thick plastic barrel for rain water consider a small awning built off the side and rain gutter to fill the barrel…adding a small ply wood wall and put the barrel next to the spot you place your wood stove (outside of course)…that may help with radiant heat to keep the barrel water from freezing..

    • I’m thinking of surrounding the stove with a brick wall, to keep the heat away from the walls, and also to act as thermal mass that’ll slowly release the heat later once the fire has burned down.

      • This actually goes along with what I was going to write. I have grown up using nothing but wood stoves for heat (and, if you put a kettle or metal bowl of water on top, it will keep your humidity levels up) and this was in Northern Michigan where it can get really cold! You definately need something, ie: bricks, that will not only reflect heat in to your living area but will also absorb it and radiate it back out when your fire inevitably dies at 0230. Cast iron not only lasts nearly forever, it is famous for holding heat. That’s why it is (was) so commonly used in cookware. Basically, your stove material, the cast iron, and your backing, the bricks, will both hold and radiate heat back into your domicile. As an added benefit, stone/brick is virtually unaffected by heat and will not pose a fire hazard to your hut. Good luck.

  3. Old fashioned wood stoves may not be air tight, which can add smoke and other elements to the inside of the hut. I switched from a wood stove to a propane stove years ago and have been happy with my decision. I am no longer hauling five cords a year off the ridge, and coming home to frozen pipes. My propane stove has a thermostat that is not dependent on electricity, so I am able to leave for several days in the winter, and return to a warm house. A propane boat heater would be ideal for your hut. You will have to haul tanks, but you may be able to work out delivery.

    • Yeah, I’m slightly concerned about smoke getting out. Hopefully it won’t be too bad, though good ventilation will hopefully also solve that problem too. The modern propane boat stoves I’ve seen seem somewhat more pricey (and I’d also have to buy propane), so I think I’ll try the cheap wood stove first, and if that doesn’t work, consider propane.

  4. Ryo,

    We have a 12×12 room which I added onto our house. The smallest boxwood stove made by Vogelzang heats it up quite well, and can easily be way too much. There’s a fine art that you will learn in no time to balancing the right mixture. If you choose the wood stove route, I’m sure that you will enjoy learning about the types of wood, their BTU ratings, and which varieties should be avoided. The sappy woods will leave a think layer of creosote inside of your stovepipe, especially when tampered down to a slow, smoky burn during the nighttime.

    I’m sure that you will perform your due diligence, and if we’re voting on it, my vote is that you stay there this winter. Those crisp, sunny mornings would be quite a scene to view.

  5. Heat always moves to cold.

    There is no doubt that closed cell insulation like polystyrene which is one of the best available insulations at a reasonable price, is what you need.

    Polystyrene is to all intents wind and waterproof, being a good insulator water vapour does not invade.

    Lining the inside with sheets of polystyrene and almost water vapor proof plastic sheet, covered by wood or whatever,will solve the heat loss problem, stop the drafts and solve the condensation problem in one go.

    Condensation only forms on cold surfaces, if you keep the hut warm all the time, the insulated warm walls will not attract water vapor and condensation will not form. (except on the windows)

    Although you will need an air source for the stove and ventilation for yourself.

    The best way to go, is to fit an underfloor pipe from one side of the hut to the other, with a “T” riser under the fire to provide immediate fresh cold air to the fire, this will help prevent drafts and cut down on the amount of heat you will lose by burning air you have already heated.

    At the same time you need to locate the chimney inside the hut, to ensure that as much of it as possible is kept warm, the centre of the floor is the best place for the fire and the chimney. Make sure that the length of the chimney is correct, it should extend no less than 16 feet from the highest opening in the stove and at least 3 feet above the highest point of the roof. To help in lighting the stove the chimney must be insulated as the kindling has to lift 16 feet or more of cold air up and out of the chimney to get it working, then the chimney needs to be as warm as possible all the time to ensure the smoke rises up the chimney when the fire has burnt low and there is a local area of high pressure.

    In the summer, the goal is to keep the interior cool.
    Lining the walls, ceiling and floors with polystyrene, will stop the conducted heat coming in. The passing wind will pull the hot outside air up the floor vent and down the chimney, if the hut is not airtight. Therefore, the floor vent and the chimney need to be sealed. Shutters and or some other means are needed to keep the sun out.
    At this time of year, north facing findows help.

    You may have heard of SIPS? This is a system based on a polystyrene sandwich. At least 5 inches of polystyrene sheet sealed between two peices of waterproof plywood or oriented strand board are placed over the roof and or walls, completely covering and isolating the frame of the hut from the effects of the sun, there is no metal or wood bridging the hot outside to the frame of the home.

    The prefered roof is fitted over the SIPS panels.

    The winter water vapor problem caused by cooking, washing, breathing, sweating disappears once the walls, ceilings and floors are warm, opening a window will change the air and allow the colder drier air to come in, as you wish.

    A central wood burning stove will throw out a lot of heat, but that heat does not travel very far, as long as the air supply, gaps and cracks in the walls, floors and ceiling and the chimney are as prescribed, you will avoid smoke problems.

    Having any holes in the roof is a bad idea, and will pull air down the chimney causing smoke. The passing wind creates an area of low pressure to the lee of the hut, pulling the air from inside, air then comes down the chimney to relace the removed warm air.

  6. One old but effective way to deal with the sun heating up a roof is to build a double roof, like they did on safari vehicles in Africa before AC. Build a roof that will keep moisture and dirt out, then put the metal roofing or whatever you end up using on top with a couple inches between the 2. The top will heat up and radiate some of the heat down but the cool air gap will minimize the effect. Simple & not much more $ than a standard basic roof but you’ll be glad you have it in the summer.

  7. so Ry, you want to stay all winter in a hut, covered in snow, surrounded by 3/4″ of plywood and heated with a wood stove? The minute the stove revvs up you’ll be smoked out (or heated out from the intense heat), soon as it goes out the temp will drop in a matter of minutes. There is also a huge danger of burning the whole place down unless you ventilate properly and seriously protect the area around the stove. Judging by your past building techniques, I’d be REEEEally careful adding fire to the mix.

    I vote for insulating…there’s a reason everyone does it, LOL

    good luck either way!

  8. Unless you’re prepared to spend some money to have the antique stove vetted, nix it and buy a nice, tiny, modern one. One that comes certified to modern safety standards and whose condition has been inspected. I have relatives who use woodstoves and not one ever considered using one of the antique stoves in the family. Also, not to sound like nag, I sure hope you have a CO2 detector for your tiny space.

    How are you storing your wood?

  9. A wood stove in a small cabin is like a boats rudder. It needs constant attention. Using a thermometer as compass it will guide you to cozy warmth. Avoiding turning the place into a sauna or burning it to the ground. Packing the firebox full is not always wise with a small place. Wood type is also important. Dry old non resinous hardwood is best. Good luck. Enjoy the winter.

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

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