Ventilation – Ridge Vent / Soffit / Gable
Zone 4 in NC (Raleigh)
Attic/3rd floor recently finished and the area has open cell spray foam from soffit vent to ridge vent. Prior to spraying the livable space, the foamed space had a gable vent (now sealed), ridge vents at all top points, and soffits throughout. There’s another gable vent that is not sealed on the opposite end of the livable space/house with a much lower roof that I keep the access door closed (reachable thru mechanical room, that is foamed as well). I don’t consider it part of the 3rd floor since it has a lower roofline / not livable (much duck down to walk).
Question – I’m getting a roof replacement soon and don’t want to rely on roofers since they don’t specialize in building science. Should the gable vent be sealed off so there’s a ridge vent (not spray foamed) and soffits (not spray foamed)? I’ve seen many articles that say a ridge vent + soffit is all you need and to seal the gable vent (after adding a ridge vent by roofers) – but this house built in 2007 had all 3. Best to seal the gable vent? Prior to finishing the 3rd floor, there was free flowing air opposite the remaining gable vent but now that wall is sealed with open spray foam. Does this change anything?
Links to support one way or another?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
BH,
(I will ask a couple of questions for clarification and also to give your post a bump.)
On the attic/3rd floor, it sounds like you have converted to an unvented attic by spraying open cell foam on the inside of the roof and gable sheathing. Is that correct? How much foam was installed? Ideally, you would have R-49.
It sounds like there is no connection to the second attic space. Is that correct?
Yes, you are correct. Insulation, perhaps a little over R49, is present. The only connection to the second attic space is the access door installed in the mechanical room the prevent those temps from leaking into the mechanical room/liveable space.
I've seen many different opinions and little data. If you don't see any problems in your vented attic space, I wouldn't change it.
I'm a little unclear on what you've got, despite the clarification. The first space you describe with "open cell spray foam from soffit vent to ridge vent". Does that mean both vents are foamed over and closed off? Or is there a vent channel from one to the other, and the foam separates that vent channel from the conditioned space?
Charlie, The unvented attic/liveable space/envelope has open cell spray foam from soffit to ridge vent (including one sprayed gable vent). That's at one side of the house. Attached to that space is a six foot wide mechanical room (foamed as well). On the far side of the mechanical room is an opening (now with access door) to access the smaller attic/roofline (no spray foam). In this smaller attic exists a gable vent and ridge vent.
Bhsizemo11,
Leaving aside the still-unanswered question about the spray foam installed between the rafters ("Is it a vented assembly or an unvented assembly?"), I'll address your basic question about the low attic (a vented unconditioned attic).
It's perfectly OK to leave the existing gable vent. It's also perfectly OK to vent the attic with soffit vents and a ridge vent (with or without the gable vent). Really, the venting details are irrelevant. What really matters is doing everything you can to make sure that the ceiling under the insulation has no air leaks.
For more information, see All About Attic Venting.