Open cell foam in vented roof cavities
Hi, I’m trying to come up with a definitive answer on whether or not open cell spray foam in a vented, cathedral ceiling would require a vapor retarder. The building codes speaks directly to unvented roof assemblies but I dont see any code language specific to my question. The roof is 2×10 rafters with thin foam baffles installed. This contractor doing the install plans to fill the rafter bays with open cell foam. I realize there may be better methods to insulate a cathedral ceiling but this is not my project nor my design. I’m simply interested in how this application might relate to current building codes. Any insight is greatly appreciated.
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It depends on where you live.
With a vented open cell foam cathedralized ceiling you don't need an interior side vapor retarder tighter than interior latex paint on gypsum board except in US climate zone 7 or higher. This is not covered by code prescriptives.
If the free-air ventilation area is less than 1/150 of the attic area, R806.2 goes into effect, which requires interior side vapor retarders tighter than that for US climate zones 6 & up:
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R806.2 Minimum Vent Area
The minimum net free ventilating area shall be 1/150 of the area of the vented space.
Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:
1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
2. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by eave or cornice vents. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.
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https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction
Thanks for the response Dana. This is Zone 5. In this specific install the room is a second story bedroom and will include unfinished eaves with no access. In this area the foam will not be covered with drywall/paint. How concerned should I be about these areas? Would they need a vapor retarder paint in the eaves only? The foam will mostly likely be sprayed deeper then the rafters in the eaves so installing poly or a smart vapor retarder wont be possible.
Hi Jon -
Dana's response tells you what the code requires for vapor management with open-cell spray foam in CZ5. And then there is also what happens in YOUR home. I have seen open-cell spray foam in CZ5 work just fine with only a Class III vapor retarder; I have seen it accommodate a real moisture problem at the roof sheathing in the same climate. I THINK the variable is what the average wintertime interior relative humidity is.
If you are running your interior relative humidity at or below 30%, you will probably be ok; if you are running significantly higher than that, best to apply that vapor retarding paint to all of your bare-naked open-cell spray foam.
Peter
>" I have seen open-cell spray foam in CZ5 work just fine with only a Class III vapor retarder; I have seen it accommodate a real moisture problem at the roof sheathing in the same climate."
You have seen a real moisture problem in zone 5 with open cell in a VENTED open cell insulated cathedralized roof?
Unless that house was kept at tropical moisture levels indoors I'd be looking for other sources of moisture!