Adding vented fascia to a roof with XPS exterior rigid and cavity insulation
I’ve looked everywhere for an answer to this question.
I’m building a large house in central Oregon’s high desert (climate zone 5B) and I want to avoid the problems with ice dams and water damage I have had in the last two houses I have built here. After reading as much as possible and analyzing every diagram I can get my hands on, it seems like a hybrid roof with exterior rigid insulation combined with cavity insulation at the correct ratio is what I want to do. However, I would like to go one step further and add vented space over the XPS (and under the roofing) to minimize my chances of ice dam formation. I can’t seem to find an illustration that demonstrates the hybrid roof with a vented fascia anywhere. I have made a diagram (kindergarten style) showing what I understand the correct technique would be. I would appreciate any advice someone would have about this especially if I am wasting money somewhere or creating a disaster.
Thanks,
Wayne
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Replies
Wayne,
Your kindergarten-style sketch is clear, and that approach will work.
If you want, you could instead install a conventional vented soffit. Just make sure that the air space in the soffit area is connected to the vent space above your rigid foam (via vertical channels between the fascia and the bottom edges of the rigid foam).
See page 51:
https://www.buildingscience.com/sites/default/files/migrate/pdf/GM_DER_Guide_2013-01-18.pdf
Instead of 4" of XPS, which is blown with a mix of high global warming potential HFCs (predominantly HFC134a ), use polyisocyanurate (blown with pentane, at 0.01 x the damage of HFC134a). Even derated for time and temperature the polyiso will be fine. As the HFCs diffuse out of the 5" XPS over decades it's performance eventually falls to R21 (same as EPS of equal density), despite the fact that it's warranteed for R22.5 (90% of labeled R.)
Also note (particularly in forest fire zones of 5B Oregon)) polystrene (XPS, EPS) melts while burning and in a top vented roof can form burning puddles of polymer, whereas polyurethane and polyisocyanurate char in place and never melt even when fully engulfed.
Just a quick added note:
ice dams cannot be overcome with venting. They are overcome by continuous air sealing and insulation. Venting is a great phenomenon but needs its partners (air sealing and insulation) to pull their share first.
Peter