Unvented roof with attic space
Hello all,
We are building in CZ 6A and plan an unvented roof with 2/12 pitch (truss drawing attached). The roof trusses will consist of a section that is part of a cathedral ceiling in the main living space and a section that is attic space separated from the main living space with drywall (flat ceiling). The attic space will be accessible but not used for storage. The cathedral ceiling portion will have 8-9″ of closed cell spray foam directly to the roof deck to reach approx. R60. The attic portion will also have this 8-9″ of ccSpF directly to the roof deck. There are 3 main questions/concerns I have regarding this situation:
– Venting: given the attic space is separated from the main conditioned space with only drywall, is it necessary for ventilation via ERV to avoid any condensation or code (2021 versions in effect for our town) concerns? I was not sure if the drywall only barrier (vapor permeable and poor insulator) to the heated/cooled space below is adequate to maintain proper humidity/temperature control or if it needs to be actively “conditioned”.
– Fire/Thermal barrier: Since the attic will be separated from the living space by drywall, will the ccSPF require an additional fire barrier in this situation? I have seen some SPF manufacturers claim they would not, but I am looking for an unbiased review/experiences.
– Spray foam in attic portion: Should the trusses in the attic have additional depth or can the 8-9″ completely “bury” the trusses without concern?
Appreciate your thoughts!
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Replies
BiBH,
1.
- Once you insulate at the roof-line, the attic becomes just another room in the house and needs conditioning just as they are.
- Moisture rises, so the attic will be more humid that the other spaces.
2.
- This article has a good diagram showing where protection is required: https://www.greenbuildingadvisor.com/article/thermal-barriers-and-ignition-barriers-for-spray-foam
In your case under the IRC you appear to need an ignition-barrier.
3.
- 8" to 9" is fine. You are going to end up with much more thermal bridging in the cathedral roof section that the attic.