The key to modern green building often comes down to the details of what they call “the thermal envelope” or the building envelope. If you imagine a building like a great bubble, the envelope is the outer skin of the bubble. Its the part of your house that keeps the outside, well, outside. If you want to have a highly efficient home then you need a highly efficient envelope.

Many of the design challenges of our house (and a few of its pricier components) are related to building a high-quality thermal envelope. We originally planned to put 4″ of insulation on the outside of the sheathing and none on the inside. That would have made for both a thermally efficient design and the convenience of open stud bays everywhere. (Essentially every wall is an interior wall). We were planning on using polyisocyanurate, or just polyiso for short, which is a very high r-value insulation. They do buildings like this in the interior of Alaska.

This original design fell to the realities of modern building codes in combination with our brick exterior. Apparently nobody builds brick houses in Alaska. Anyway, the code officials can’t deal with the brick sitting 5″ away from the structure of the house. They make fancy brick ties just for this purpose, but the International Residential Code has a max distance that just wasn’t going to cut it. Rather than have the whole thing engineered, we backed off to just 2″ of polyiso on the outside and to fill the interior cavities with spray-foam.

As a sidebar for technical nerds, it turns out you shouldn’t put foil-faced polyiso on the outside and closed-cell foam on the inside, even though closed-cell is higher r-value than open cell. Closed cell can’t breathe, and neither does the foil-faced insulation. If any moisture finds its way back there the wood can’t ever dry out and it will rot right inside your super-insulated wall. Bad news. So we opted for open-cell spray foam on the inside, which lets some water vapor through and would therefore let the sheathing dry to the inside should it ever get wet.

Calculating total wall r-value is an eye-opening experience. Because much of your wall is made up of pine boards (r value of about R1.25 per inch), the total insulating value for a wall with insulation between the studs is not so great. As in this example:

That’s one reason why exterior board insulation is such a boon. The other reason is air sealing. Hot or cold air escaping through gaps is a much bigger potential loss than heat radiating straight through your insulated walls. Putting up polyiso boards and taping them creates a solid air bubble around the house. Spray foam on the inside does the same. Like a sealed tupperware container inside a ziplock bag!

Our 2″ of polyiso (R6.5 per inch) nets us R13. Given that our previous houses had simple fiberglass between 2X4 studs there is no way they ever made R13 for the total wall assembly, even if that fiberglass were the most meticulously installed insulation ever. (I wouldn’t put it past old Earl who built our previous house to have detailed that insulation by hand himself.) To that R13 we’ve added a wall made up of 2X6 studs set a full 24″ apart, meaning more insulation between them. And that insulation adds about R 3.7 per inch over 5.5 inches, or another R20. Given the wide spacing, our total wall R value is still over 19, plus the 13 on the outside foam for a total of R32 walls.

For attics its hard to beat blown cellulose. Because you don’t really care how much space it takes up, its R3.2 per inch is very cheap to stack up. Heat rises and ceilings can therefore lose a lot of heat. On top of that, attics get absurdly hot in the summer sun as I can personally attest to. So a well-insulated attic is a must. We are using canned spray foam to seal around any penetrations (which we went to considerable lengths to minimize in the first place). Then on top of that we’re piling enough fluffy insulation to net R49 ceilings. To do this properly you need raised heal trusses so you get the full height of insulation all the way out to the edge. And in our case we brought the board insulation up the full 12″ on the outside to connect with the attic insulation piled up on the inside

We have some vaulted ceilings on the east wing, so we needed a different solution there. We took leftover polyiso panels from the house exterior (we always end up with near zero waste) and created overlapping 3-layer sandwiches of insulation in each of the bays. That’s an R value of 39 (albeit somewhat less due to thermal transfer through the rafters.) 39 is a hell of a solid barrier for a ceiling with no attic, only possible because we built it with extra-deep rafters.

All told there are seven different kinds of insulation used in this house. There’s mineral wool insulation in the garage walls because its fireproof. There’s fiberglass batt insulation in the ceilings because its cheap. The attic gets blown cellulose, as I mentioned. There’s open-cell spray foam in the exterior walls along with two different kinds of canned foam – one used around windows and doors and the other for firestopping. There are polyiso boards on the outside of the walls and XPS (Extruded Polystyrene) under the basement slab. And we have yet another formulation of XPS that will eventually go on the inside of the basement walls.

Despite all that insulation, the Achilles heal of any modern thermal design is the windows. Even high quality windows have a U value of .25 or .28. U value is just 1/R (why do they do it that way? Just to be annoying?) So even really good windows have an R value of 3 or 4. Buh. Our house has quite a bit of glass, so that’s one of the major concessions to pure efficiency. In fact, the two windows on the stair landing contribute more head load in the summer than the entire (non window or door) wall assemblies on the whole house combined. Yeah, <30 sq feet of glass costs more in A/C load than >3000 sq ft of walls, at least if they are insulated like ours are.

The other two blatant concessions I can think of are the wood-burning fireplace and the mail slot. Even with a modern factory-built unit its well nigh impossible to seal up a fireplace. It has a giant 12″ hole straight to the outside, with at best a tight metal damper on one end. The chimney and the fireplace are definite weak points. And the one that really left me shaking my head was the mail slot. Amy wanted to get one like we had on the old house. I must admit, it is very convenient and very quaint – befitting a house of the style we are building. But we’ve gone to all this trouble to insulate the walls and then cut a 3″ X 10″ hole straight through it! I figure if we have a blower door test its the mail slot that will pop our otherwise hermetically sealed bubble. Cest la vie. The mail slot got installed end of last month and I must say it does look charming. 🙂

So with our few deliberately chosen concessions aside, this should prove to be a remarkably tight house. The furnace is so small it looks like a dorm fridge, the A/C draws only 5 amps (about the same as a household vacuum cleaner) and the building inspector said he was going to come knocking on our door next year just to find out what our power bills turned out to be. He thinks ours is probably the most well insulated house in the whole city, and it seems like he would know.