Installing a metal roof on a storage attic? Keeping it light weight and reasonably cool?
Bryce Nesbitt| Posted inEnergy Efficiency and Durabilityon
What methods would you suggest for re-roofing in metal, for a storage attic?
Here zone 3c, heavy sun exposure on one side, solar panels to be added, Class A fire zone, earthquake zone, reasonably easy to air seal attic with no present insulation, 2″ dimensional x 5.5″ rafters.
What are the tradeoffs with insulation below the deck?
Insulation above the deck?
A cold roof arrangement with purlins and an air space?
There’s no desire to condition the space, just moderate summer heat gain without adding too much weight or cost. The building has no A/C and no need for it.
Replies
Hi Bryce,
I'm not too knowledgeable of details for building in a class A fire zone. I believe that there are some precautions to take with a vented roof assembly. The vent openings need to be covered with a fire-resistant mesh and it may be best practice to just build unvented assemblies. I'm sure that you are aware of those details.
Perhaps a combination of R-5 rigid foam insulation above the roof deck with the remainder of the insulation in the rafter bays for an unvented assembly is the way to go. I'm sure you have read this:How to Build an Insulated Cathedral Ceiling
Peter Pfeiffer, a experienced architect in Austin believes that metal roofing with a low-e coating installed above an air space is the best performing way to use a metal roof. That could be done over the rigid foam. And his homes do perform really well. You can read some of his thoughts on that here:Designing for a Hot, Humid City
I'm not sure what you would need to do to make sure that the above-deck venting isn't a fire hazard, probable similar to blocking soffit and ridge vents with screening. And it does keep coming up here on GBA that people can't get roofers to install above-deck rigid foam insulation, or at least not for a reasonable cost. This may be a job for a general contractor with experience in high-performance building.