Hot Roof Design

Stephen Zehr
We are planning to use a hot roof design in which spray foam insulation is used between the roof rafters such that the attic is unvented. I expect that since solar heat gain absorbed by the roofing material cannot transfer through the back side of the roof (as in a vented attic design), the surface temperatures on the top of the roof will be slightly higher. Is this a concern in terms of increased expansion or material durability for metal roofing?
Todd Miller
Isaiah Industries, Inc.
Metal handles thermal differences very well as it does not break down the material. Well designed metal panels include allowance for thermal change. In my opinion, metal is a great choice for hot roofs as well as for structural insulated panels because the elevated rooftop temperatures could break down some other roofing materials over time.
Eric Novotny
An informed customer is the Best Customer!
+1 to Todd's comments. The overwhelming majority of cooling happens to the outside surface. Surface temperatures are largely a function of pitch, color, and orientation. Ex. A lighter color roof with a decent pitch (+5:12) will have a far lesser surface temperature than a darker roof with a lower pitch. In studies of sealed attic designs, the average dark color range asphalt shingle temperature difference in Florida (i.e. more exposure and more heat) was less than 2 degrees. This helps illustrate the fact that shingle temperature issues/concerns in sealed attic designs are greatly overblown. That all being said and the fact that you will likely be more than fine, I would suggest a bit of an alternative approach. If you are going the extra mile by spray foaming the roof deck, I would highly encourage you to put rigid foam to the exterior of the roof deck and completely thermally uncouple the roof from the rafters/structure. You will get far better performance out of that assembly and people often see spray foam as the cure all. If you just spray foam the rafter bays, the rafters will still be cold zones and depending on what climate you are in, you can quite easily get condensation issues. Being in Chicago, you certainly get some low temperatures and thinking that spray foam is going to be the final solution might not yield you the performance and trouble free design you are looking for. Rigid foam is typically cheaper per square foot than spray foam. You also need consider the vapor retarder question. Your home is in climate zone 5 so you need to concern yourself with vapor/moisture control. Going with rigid foam to the exterior at a sufficient thickness will allow you to use more traditional insulation to the interior. Also, if you run spray foam, you will need to have a proper thermal barrier if it is in a cathedral ceiling. Google "A Crash Course in Roof Venting"

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