Challenges in Designing an Energy Efficient Home - Roof Details

I am trying to visualize the details for applying exterior insulation on the roof in my head all the time. Sometimes I have an Aha! moment and think I have it solved. And then realize what a pain it would be to execute while you are on a roof or scaffolding 20 feet in the air.

First a little bit about roof anatomy. This photo is borrowed from www.hometips.com.

This diagram shows several different kinds of roofs - a hip roof,  a regular gable roof with a doghouse dormer on it. We will have a simpler roof than the example above.

Simpler gable roof from a different website - Container Home Consultants

Ok, so the terms we need to know to talk about a gable roof for the purposes of this discussion are eaves, rakes, rafters, and rafter tails.

The eaves are the part of the gable roof that are horizontal. They are lowest part of the roof and serve to allow water to run off the roof without draining directly on the siding.

The gable end or rake end is the sloped end of the roof over the wall. It is usually extended to prevent rain from running down the siding as well. In heavy snow load areas, eaves and rakes do not usually extend as much as they do in the south because the snow can be so heavy and can mass on these areas causing ice dams, etc. Not a problem here. We need eave and rake overhangs to block the rain and the summer sun.

The eaves are usually formed from rafter tails, extensions of the rafters themselves.

The problem with the eaves and gable overhangs in the design of John's house is that there is no attic in John's house so the rafters themselves form the base of ceiling inside the house. The area between the rafters will be filled with dense pack cellulose just like the walls. Thermal bridging raises its ugly head once again. If the rafter tails protrude outside the thermal envelope they will conduct heat out of the house (or in summer heat INTO the house). So the issue is how to make overhangs that will not violate the thermal envelope. It's not easy.

The other issue about the rafter tails is that it is difficult to air seal around them. If your house is a simple gable shape as shown below, you can much more easily air seal around the exterior of the house than if you have rafter tails protruding.

Drawing borrowed from 2009 Solar Decathlon Gable House designed by students from the University of Illinois

One of the guys at Shelter Kit where we are buying the house kit from, suggested that this would not be a big issue in our part of the country since we are in a cooling dominated environment. I know that is true, but I still think it is important to consider these details and try to minimize the thermal bridging as much as possible.

Here is the latest drawing. (Please note: It is hard for me to draw overlapping wood 2 x 4s in 2-D). The top drawing shows the roof with plywood sheathing with parallel 2 x 4s that form the overhangs on the eave side of the roof. The bottom drawing shows a cross-section of the layers of the roof.

The roof itself is a simple rectangle covered with 1 inch of foam that has 2 x 4s standing up forming the rafter tails. 2 x 4s are nailed to the rafters below, but are separated by an inch of foam. The only penetration of the thermal envelope is the nails or screws holding the 2 x 4s in place. The area between the 2 x4s is covered with either foam insulation or Roxul mineral wool insulation. If you want to get really fancy (and you have a lot of money) you can add another layer of insulation over the 2 x 4s, before you add the last layer of sheathing, then the metal roof.

The rafter tails along the eaves can be covered with siding underneath when the siding is applied.

To make gable end overhangs, a ladder-like extension could be attached to the 2 x 4 near the gable edge of the roof and could extend past the roof-wall intersection by 8-12 inches. We don't need a wide overhang because of the porches around 3 sides of the house.

This design is complicated, but less so than some of the ones have seen on my favorite websites - Green Building Advisor, Fine Homebuilding and Journal of Light Construction. I do have to run it by some real builders though. I'll let you know.

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.

After Christmas Creativity - Pom Pom Snowman

A lot of times right after Christmas when things have slowed down a bit, I just get all these ideas about things I want to make for the next year. It often prompts me to go ahead and start making things.

I got a Clover Pom Pom maker because I want to make a bunch of garlands with pom poms. Man, is that tool seriously fun. I left it at work one night so in case everyone had a little time they could make some pom poms. One of my co-workers was especially talented at making multi-colored pom poms so they started calling her a "pom pom overachiever".

I made a pom pom snowman that I think looks pretty cool. (Aside - I had my walls painted about a year ago and they used the wrong color paint. It was supposed to be a much milder version of this yellow. When I take photos inside it gives everything a strong YELLOW cast. I will fix that one of these days and learn to take better photos as well.)

But you can still see that the pom pom snowman is cute. I made the hat out of black felt. I rolled a strip of felt to form the body of the hat and sewed 2 circles, one large and one the diameter of the rolled felt to the top and bottom. I got a large needle, threaded it with the white yarn, sewed it through the head, then through the hat to form the hanger. I glued on the felt eyes, orange nose, mouth and buttons. The stick arms are just wedged in there. I cut the scarf from felt, but I think it would look better knitted.

Other post-Christmas projects - cleaning, planning for next year's party, planning for John's house, trying to exercise some so when we ski in early February, I will be able to button my ski pants and actually ski.