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 : 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).