venting closed cell spray foam insulation in dormer roof
Should we or shouldn’t we use proper vents when we have the closed cell spray foam installed between the rafters of our dormer roof? I’ve researched and seen 2 different sides to this. I spoke with a contractor the other day and he mentioned that there are special proper vents to use with the spray foam that have a flap that drops down into the soffit. This contractor said the spray foam is sprayed up against the proper vent. I also spoke with a friend the other day who mentioned someone who had major rot problems in their entire roof because they didn’t vent and the spray foam was installed right against the roof sheathing. Condensation built up between the sheathing and the spray foam. Another question would be: does the spray foam actually cause excessive heat on the under side of the roof sheathing causing the shingles to deteriorate?
Replies
Use the vents or you will have what is known as an unvented roof. This can effect the life of asphalt shingles.
The unvented roof would be what they call a "hot roof". Tests have shown that such a roof, despite the term, does not get significantly hotter than a "cold roof", and, while shingle warranties may exclude hot roofs (though I don't know that they do), there's no evidence that shingle life will actually be impacted.
In the case of rot mentioned by your friend, if the case actually occurred (you never know when you hear it from someone who heard it from someone) then the rot was not due to "condensation", but was due to water from roof leaks being trapped by the foam.
or it could have been open cell foam and condensation but not closed cell. More than likely a roof leak though.
There is a lot of info on unvented roofs. Many things come into play from your climate to HVAC ducts and equipment in the attic. It can get a bit confusing. One of the main issues is trapping moisture in the roof sheathing and framing which can lead to rot of those materials. Providing ventilation should eliminate these issues. Without it, you may not have enough air flow to dry things out. This JLC article from a GAF representative states the issue fairly simply. In the event you have moisture problems and need to replace the roof sheathing, spray foam attached directly to the underside will make for a complicated and invasive job. We don't do it.
http://www.jlconline.com/cgi-bin/jlconline.storefront/EN/UserTemplate/69?c=f9d5e3c75f370b421e4ce9430191e327
I can see three ways that moisture could get between the underside of the roof sheathing and spray foam applied directly to that surface:
1) A leak in the roofing material.
2) Condensation of water vapor that is able to get through the foam, or around it where it joins the sides of the rafters.
3) Residual moisture in new lumber trapped and unable to dry out.
No matter which way the water gets into this area, I cannot imagine any type of vent device or technique that would allow this trapped water to escape from the area.