Challenges in Designing an Energy Efficient Home - Exterior Insulation

I am trying to finalize plans for John’s house. I have to order the kit by January 31^st for delivery in April. I backed up the start date to April just because I’m not ready AND I might be taking boards in March. I do not need to be trying to build a house and studying for boards in the same month. That would absolutely drive me crazy.

Today's topic will be exterior insulation.

Exterior insulation is used to prevent thermal bridging. I learned in my Net Zero energy class that for a standard 2 x 6 wall insulated with dense pack cellulose you lose 25% of the insulation value from thermal bridging through the wood studs because the R-value of the wood is only about 1 per inch. There are a number of strategies to prevent thermal bridging, but exterior insulation outside of the sheathing makes the most sense to me.

Polyiso covering house from the Building Science Case studies

When I installed new siding on the my house, we covered the sheathing with Tyvek housewrap and then  2” polyisocyanurate (polyiso) foam to give me an uninterrupted R-value of 12 around the house. My house looked similar to the house above before the siding was installed. The advantages to using polyiso foam are that it is relatively easy to find, lightweight, relatively easy to install using cap nails, good R-value of about 6 per inch, uses more environmentally benign blowing agents than EPS or XPS foam. There are some serious downsides to polyiso though– ants and termites can tunnel through foam. They don’t eat it but they love to nest in it if it is wet. Although polyiso is less flammable than EPS and XPS, it is still flammable. But the most important disadvantage is that it is derived from petroleum. For those reasons, we’re going to use a different product on John’s house.

I’m voting for Roxul ComfortBoard which is made from volcanic rock. I've talked about Mineral wool insulation before when I talked about insulating John's shop. Below is a photo of exterior insulation with Roxul ComfortBoard.

Roxul ComfortBoard

I’m choosing this because it is non-combustible, does not rot, insects cannot tunnel through it, cannot get moldy, vapor permeable, water resistant. It is probably harder to install the polyiso and it is itchy. I’ve ordered Roxul batt insulation from a local store when I was working on the Lenore house so I think I can get the ComfortBoard which is a compressed version of the Roxul batt. R-value is about 4/inch. Cost is supposedly similar to the foam.

Schematic of Roxul with rainscreen to allow rain to drain from behind the siding. This keeps the wall sheathing drier as well as allows the siding to dry on both sides prolonging the paint and the siding. This photo is from Builder Online  in a slideshow on 10 Hot Products from Greenbuild 2012.

Using exterior insulation solves the thermal bridging problem, but it brings a whole new set of challenges - detailing around the windows and doors, and what to do about the roof overhangs. I hope to cover some of those issues in the next post.