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:
- 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.
- 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.
- 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.
- 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).