Tiny House 7: Insulation and a word about the design

Something I should have mentioned at the beginning of this series: the design of this tiny house is due to David Howard of FirstDay Cottage. My ex-wife and I built one of his house “kits” in Vermont in 2012-2013.

I use the word “kit” loosely. We received several shipments of uncut building materials and a hand-written instruction book. So we worked this way: I interpreted the instructions (often having to call David to ask questions), and cut most of the wood myself. The two of us shared the actual nailing of the wood in place, but my wife did more of that than I did, and she also did most of the really hard stuff, like installing the roofing materials. You can see some pictures of the house-building project here.

So this tiny house that I’m building is my miniature version of a David Howard design. Because it’s tiny, I was able to use 2x6 lumber instead of 2x10s in the floor beams, and 2x4 lumber instead of 2x6s in the rafters. I also used a 45 degree roof instead of the 38 degree (4 in 5) roof in the Vermont house, which made doing the math for the joinery much simpler:

While I’m waiting for more wood and house wrap to show up, I’m making progress by installing insulation. So far I’ve done the west wall, the only surface that is fully sheathed and wrapped. As with the Vermont house, I used rigid foil-backed foam, but 1.5 inches thick instead of two inches. That’s probably overkill for winter here, but it might keep the house cool in the hot Sierra summer.

I had to custom-cut each piece of foam to fit the various spaces on the wall. It’s like making a puzzle from scratch. The pieces are awkward for someone working alone to hold in place against the wall, so to help things a little, I screwed little pieces of scrap wood to the underside of the floor beams. Then the foam pieces can rest on the scrap while I’m attaching them to the wall:

In the Vermont house, we installed the insulation by placing strapping (1x4 and 1x6 rough lumber) over the foam and nailing the strapping to the underlying posts with 5-inch gutter spikes. I did things slightly differently with the tiny house, by using 3- and 4-inch #10 construction screws instead of nails. Here’s what the west wall looks like now:

The important things about this insulation design are:

• You first have to completely frame the window and door openings with strapping.

• The rest of the strapping consists of horizontal strips placed no more than two feet apart.

I used the four inch screws to fasten the strapping to the posts. Four inches is long enough to go through the strapping, foam, sheathing, and one inch into the post. The horizontal strips are about 18 inches apart, dividing the wall into four horizontal sections (dividing the wall into three sections would have placed the strapping more than two feet apart).

One thing I didn’t really like much in the Vermont house was how David told us to use metal strips to attach horizontal strapping to the vertical strapping around the doors and windows. Here’s what that looked like:

I never really liked that system; it didn’t seem sturdy enough, especially considering that the siding will eventually be attached to the strapping. So in my tiny house, where I was unable to screw strapping to a post with four inch screws, I attached the strapping to the underlying sheathing using three inch screws.

Another thing we should have done in the Vermont house was to use sealing tape over the joints between the foam pieces. I believe that omission was one reason why one of the windows in the kitchen would often leak in heavy rain. So I’m taping the joints in the tiny house.