vapor diffusion port?
ac__arch
| Posted in Energy Efficiency and Durability on
Is a vapor diffusion port recommended at the top of a hybrid roof insulation assembly? [meaning rigid insulation over the roof deck & fiber below]
assumptions:
climate zone 4c
vapor open wrb under asphalt roofing
sheathing for attaching roofing & protecting rigid insul
r-10 rigid abv roof deck
vapor barrier
structural roof sheathing
cellulose in 2×12 joist bay
smart air barrier
drywall
thanks for your assistance,
A
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Replies
I thought zone 3 was the code approved limit for the diffusion port?
Walta
Adam,
No for a couple of reasons.
- If the required ratio of exterior foam to permeable insulation is met, there should never be moisture accumulating the needs removing.
- These types of assemblies rely on a continuous Layer of exterior foam to stay safe. Interrupting it at the ridge will create an area which will fall below the dew point. So any vapour-diffusion port will in effect be dealing with problem it created.
Thank you for setting me straight. I was looking at the article indicating damage at SIP ridges and considering the sandwich of sheathing-rigid-sheathing that makes up the upper portion of the hybrid roof section to be similar and might have similar issues of accumulated vapor over time.
I'm happy to not have to add this additional detail/mass on top of the ridge which is a small single gabled roof that is otherwise quite simple.
I appreciate the replies, thanks!
Adam,
Yeah, SIPS are their own unique viper's nest of possible failures. I'm not sure there is much to be learned from them that has general applicability to other assemblies.