We’re finally ready to start on what I consider to be the defining feature of a Zero Energy Home. Our walls are only 4″ across, leaving precious little room for insulation. While most folks doing energy improvements on their homes would do what they can within the wall thickness they have, to do zero energy right you generally need more than 4- or even 6″ of insulation. Keep in mind I’m talking about the Vermont climate: about 7000 heating degree days. In milder climates 6″ of well-installed insulation might be enough.
How do you put more insulation in a wall than will fit in a wall? You have to make the wall thicker. And that is why most energy improvement projects just color within the lines when it comes to the walls. A thicker wall means you either rebuild the inside of the wall: very messy, disruptive and expensive; or you rebuild the outside of the wall: requires new siding, moving the windows, and also very expensive. Between the two, thickening the wall to the outside is generally more effective and slightly less expensive. Plus, it’s a chance to renew the look of your house.
The reason I say a thickened wall is a defining feature of a Zero Energy Home is that the whole zero energy thing is a little over the top at almost every level: oversized solar, super energy efficient appliances, whole home energy monitoring… But the whole is greater than the sum of the parts, so you have to go the extra mile on all this stuff or you don’t get the zero energy special sauce. Having superinsulated walls makes something magic happen: the house keeps itself warm. Some day, once we get these walls done, the house will be warm everywhere, with just a little bit of heat introduced in the dining room.
Designing a thickened wall is no easy feat. There are too many competing considerations: cost, aesthetics, effectiveness, greenness, longevity… And there’s an overriding threat that if you do it wrong the wall will load up with water and become a giant moldy diaper draped around your house. As an aside, we have exactly this diaper situation with the wall behind our bed. Whenever it rains and then gets warm we can smell the house composting as we snuggle down to sleep.
Here’s my first stab at our thickened wall design, layer-by-layer, from the inside
- Original plaster wall – very stout and heavy: not gypsum board!
- No plastic behind the wall board. I don’t think plastic had been invented when this house was built.
- Original 2X4 stud wall with a messy combination of fiberglass (original) and densepack cellulose (added two years ago) between the studs.
- Original 3/4″ wood board sheathing. This is the good stuff. No plywood in this house. Except the diaper wall on the addition.
- Ice and Water Shield, peel-n-stick membrane. This will be my air tight layer and my water/vapor-proof layer. Where you put this layer is very important for diaper avoidance. I think this is a good place to put it.
- At this point in the wall there is currently tar paper and siding, but that will be removed.
- A new 6″ or 8″ framed wall stuck on the outside of the building, filled with cellulose.
- OSB – not my first choice, but my builder has talked me into it.
- Some kind of drainage plane house wrap: allows water behind the siding to drain away while keeping the OSB dry.
- New siding: hopefully cedar, but maybe vinyl.
We’re getting the whole thing priced out right now. I’m sure the design will evolve at least a little. We’ll let you know what the price is per square foot.