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.
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.