Insulating Hut 2.1

I posted some thoughts on insulation a while back, but now that I’m actually in the process of insulating Hut 2.1, I figured I’d do another post talking about what I’ve ended up doing.

As you can see in the photo above, I’m using R-5 3/4″ thick polyisocyanurate rigid foam insulation boards on the exterior. If you recall, I used that stuff on the roof as well, so basically the whole structure, save for the windows, door, and floor, will be encased in those boards. Normally, insulation goes inside the wall cavities, but I first thought of putting insulation on the outside while working on Hut 1.0, and implemented the idea in Hut 2.0. The reasons for doing so are manyfold:

  1. Putting insulation on the outside leaves the wall cavities available for use. I’ve filled in some of the walls with “shelves”, but I also have the option of filling them in with batt insulation if the rigid boards on the outside prove to be insufficient. The same applies for the space between my rafters, which I may fill with batt insulation later.
  2. Instead of using a traditional air barrier, I’m taping the seams between the insulation panels together so that the insulation panels themselves form an air barrier. The boards on the roof also act as a waterproof layer, in the event that moisture leaks through the roofing panels. This cuts down on building materials, which lowers the financial cost as well as the total environmental footprint.
  3. By wrapping the entire structure in insulation, I am increasing the thermal mass within the thermal envelope. That is, all the posts and beams and OSB sheathing inside the insulation act as a thermal mass, which can absorb heat and release it slowly. Obviously, wood isn’t as effective of a thermal mass as, say, a concrete slab, but I think it counts for something. The downside is that, if I’m heating the structure from a dead cold, it takes longer to warm up, because, for a while, the structure itself is going to be absorbing some of that heat.
  4. Covering every square inch of the exterior in insulation (again, except for windows and doors) prevents thermal bridging. Thermal bridging, in the context of structures, is when heat conducts through structural members, bypassing insulation. For instance, in a traditional 2×4 stud wall construction, only the space between studs typically have insulation, so the studs themselves can conduct heat in or out. While that may not seem like much, if studs are spaced 16″ apart, that’s 1.5″ out of every 16″, or close to 10% of the surface area that’s left uninsulated.

When I first thought of the idea, I thought it was all new and radical, but I’ve since learned that this exterior insulation thing is… well, a thing. For instance, a related and somewhat similar concept is SIP –or Structured Insulated Panel— construction, which is a fancy way to say “insulation boards sandwiched between structural sheathing.” The one interesting thing about SIP is that the insulation boards are glued to the sheathing, thus increasing rigidity and eliminating any gaps. Gaps in insulation are bad, because it could allow for air circulation, which can render insulation moot by carrying heat in or out. For Hut 2.1, I originally tried using a spray-on glue to attach the polyiso boards to the underlying OSB sheathing, but the glue I got didn’t stick too well. So, instead, I’ve been nailing the boards onto the sheathing, and, where possible, through to the 4×4 posts (the gray patches of duct tape in the photo above are where I’ve put in nails). That doesn’t eliminate the gaps, but, in my case, I’m not terribly concerned because those gaps will be inside the thermal envelope, and as I mentioned above, the foam boards should, in theory, form a mostly air-tight enclosure and relatively little of that heat should escape outside.

For the floor, I’m thinking of doing something different as well. Normally, batt insulation would be stuffed in the spaces between the floor joists. But, I don’t like fiberglass, and ever since I saw those cubes of recycled cellulose blow-in insulation at the hardware store (pictured below), I’ve been wanting to use them. The stuff is made of recycled materials, supposedly uses far less energy to manufacture than traditional alternatives, and is pretty cheap ($8 per bag), so it sounds pretty awesome all around. The current plan is to lay down more 2×4 “joists” on top of the existing floor (though running perpendicularly to the existing joists for strength), then put another layer of OSB on top of that, and fill in the 3.5″ tall gaps with the blow-in insulation, which should give me about R-13. Since the insulation will be sandwiched between two layers of OSB, I also won’t have to worry (as much) about critters getting in there or stealing my insulation, which I hear are concerns for the more typical exposed under-the-floor batt insulation.

So, that’s basically the plan so far. I’m considering getting some batt insulation that’s made of recycled materials, similar to the blow-in insulation I got, to stuff in between my rafters. But, I think I’ll hold off on that until I get my stove going, and see how well the existing insulation works (or doesn’t work).

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.

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).