Roof Assembly Details - Challenges in Designing an Energy Efficient Home
I have been looking on the internet for days trying to find good photos that explain in more detail how I want to do the foam insulation on the roof. I have found a few photos that are similar to what I want to do, but not exactly. I've included a list of related references and links at the end of this blog post which might be useful if I don't explain things adequately. I have also noted the reference in the photo caption if I have posted a photo from the internet and listed it in the reference section as well.
So now for the actual roof assembly discussion.
I'd like to use a type of foam insulation called polyisocyanurate for a number of reasons. Polyiso (as it is usually called) has the highest R value/inch of the commonly used foams (EPS, XPS). It is more fire resistant than the other 2 and more environmentally benign. That is, the blowing agents used to make polyiso contribute are thought to contribute less to global warming than the blowing agents for EPS or XPS. Polyiso has some disadvantages though. It is less dense than the other 2 foam boards and cannot take much weight on top. It shrinks somewhat. See 2 articles below to Dr. Joe's barn. Dr. Lsturibek wrote about it in his article in Fine Homebuilding. And finally polyiso is still a fossil fuel product which we are trying to avoid for the most part in John's building assembly.
The plan is to place about 3.5 inches of polyisocyanurate insulation on top of the roof sheathing to provide about R-21 insulation in addition to the dense pack cellulose insulation between the rafters which provides R-33 giving a total insulation of R-54. On top of the insulation will go another layer of plywood sheathing to provide a stable base for the metal roof.
One of the challenges of installing foam insulation on the roof is that you are working at height. Another is trying to install the insulation without causing "thermal bridges" where heat is transferred easily. And finally you have to figure out how to attach the metal roof securely through 3.5 inches of foam and 2 layers of plywood sheathing which usually entails using some long ass screws.
One of the best articles I found that shows one way to install polyiso on the roof is on a house in California called the "CLAM" Blue House, the first passive house in California. First they airsealed the roof sheathing. Second they installed 2 x 4 sleepers (installed on the narrow edge of the 2 x 4) to the roof sheathing nailed from the UNDERSIDE of the roof sheathing between the rafters to prevent thermal bridging. The function of the sleepers is to provide an attachment for the plywood layer onto which the metal roof is attached. There is still some minimal thermal bridging by using the 2 x 4 sleepers but not much. Remember the R-value of solid wood is about 1/inch. It has little resistance to heat flow.
See the air sealed roof sheathing. The gentleman is spraying the sheathing with a boric acid solution to inhibit mold and prevent insect infestations. This photo is from the CLAM Blue House blog.
Photo of the installation of the 2 x 4 sleepers on the CLAM Blue House roof. Remember these sleepers were installed from the underside between the rafters to decrease the thermal bridging through the rafters.
Photo of the installation of 3.5" of polyiso between the sleepers on the CLAM Blue House
This photo shows the sheathing over the polyiso and sleepers on the CLAM Blue House
You can see in the photos of the CLAM Blue House that they made the roof overhangs by extending the sleepers over the edge of the roof. This prevents the thermal bridging problem of using the rafters themselves for overhangs.
Overhangs established by using the sleepers on the roof sheathing on the CLAM Blue House
Advantage of using this method of insulating the roof - relatively easy design with polyiso. Disadvantage is primarily thermal bridging and loss of insulation with the 2 x 4 sleepers.
Sometimes you have to make a "ladder extension" to make gable end overhangs. Martin Holladay wrote about it in a Green Building Advisor. These ladder extensions are built on the ground and installed on the roof sheathing. I hope to avoid ladder extensions because I think they would be heavy and awkward to get up on the roof.
After the 2 x 4 sleepers are installed then you add the polyiso between the sleepers. In the CLAM Blue House they used one layer of 3.5" polyiso, but I would prefer to use 2 layers because polyiso can shrink. Seams between the polyiso would be foamed and taped to provide a continuous air-sealed layer. Then sheathing on top, then roofing felt followed by the metal roof.
Another good article is "Retrofitting an Insulated Cold Roof" from the November 2008 Journal of Light Construction written by Dan Perkins, a builder in Michigan. He used a similar design except that he used a different foam, XPS, as a solid layer on the roof abutted against one or two layers of 2 x 4 nailer(s) on the edge of the roof.
Layer of XPS installed on roof sheathing covered by a synthetic roof underlayment in Nov. 2008 JLC article "Retrofitting an Insulated Cold Roof" by Dan Perkins. Please note the double 2 x 4 nailer on the edge of the roof. I cannot tell if the author taped the sheets of XPS together or not.
XPS has higher compressive strength than does polyiso, so if you are going to install 2 x 4s directly over it, it will compress less.
The 2 x 4 sleepers which the author calls purlins were then attached on top of the first layer of XPS perpendicular to the slope of the roof. The areas between the purlins are filled in the another layer of 1.5" XPS. The author uses the 2 x 4 purlins to so he can attach a 2 x4 perpendicular to them to provide the venting for the roof.
This form of roof provides a more continuous layer of insulation and avoids the thermal bridging with the 2 x 4 sleepers shown on the CLAM Blue House. Also it looks like it is a little easier to install. I don't know if you can use polyiso under the 2 x4s or if you have to use more dense foam (which we are trying to avoid because of increased global warming potential in the blowing agents of these foams.)
I also like the way he retrofits this roof as a vented roof with a perforated metal edging. He uses this method because one of the major roofing concerns in Michigan is ice dams which is not an issue in South Carolina. Download the article, it is a great article with a elegant design.
Here is a drawing from Green Building Advisor which shows the nailers on the edge before the foam is applied.
Another way to insulate a roof with foam is to use a product called nailbase which is similar to SIPs (Structural Insulated Panels) only with the OSB on only one side. I think you would have to use a crane to get the nailbase on the roof though. I'm not really interested in renting a crane.
How will I decide the exact insulation strategy on the roof? I've got to over the details with Randy and Carlton and John and the people at Shelter Kit. These guys know the practical aspects, not just from reading about it which is all I know. I'll keep you informed.