Insulation for a flat roof
user-6610113
| Posted in General Questions on
I’m in climate zone 3 in southern California. My home has a flat roof with 1/4 in 12 slope. The roof sheathing is made up of 1×6 with gaps in between. The geometry of the roof is fairly complex.
My initial idea was to add a layer of plywood then foam then an additional layer of plywood.
Q1: The structural engineer is asking if I can add the foam directly on the 1x s, keeping the outside plywood but forgoing the layer of plywood directly above the current sheathing.
Q2: What is the best way to provide air sealing from on top of the roof sheathing?
Q3: Foam should be faced or unfaced?
Thank you in advance for any thoughts.
Matt Wade
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Matt,
Q. "The structural engineer is asking if I can add the [rigid] foam directly on the 1x s, keeping the outside plywood but forgoing the layer of plywood directly above the current sheathing."
A. Yes, you can do that, but only if you have a good air-sealing plan. Some builders use the rigid foam as their air barrier -- that's possible, but it's not a technique that is universally accepted, because of worries about possible foam shrinkage. (Using rigid foam as an air barrier works best if you have two layers of rigid foam with staggered seams. Foil-faced rigid foam is the easiest type of rigid foam to tape.)
Better air barrier choices are a peel-and-stick product like Grace Ice & Water Shield or a European air barrier membrane from an importer like 475 High-Performance Building Products.
For more information on this issue, see "How to Install Rigid Foam On Top of Roof Sheathing."
Q. "What is the best way to provide air sealing from on top of the roof sheathing?"
A. See the answer to the first question.
Q. "Foam should be faced or unfaced?"
A. Either type of rigid foam will work, as long as you have an air sealing plan. Remember that some types of rigid foam are easier to tape than others.
Hi Matt -
You are in a mild enough climate that your building enclosure is probably way more about managing solar heat gain than anything else. So focusing on your roof is appropriate. One thing to bear in mind is how you will connect your roof air control layer to the same layer in your walls. Overlap is the way to go, but not sure if your design/stage of construction/renoavtion makes this easy or hard.
Peter