Venting a Low-Slope Roof
My 1100 sqft flat roof has both roof joists and ceiling joists with blown-in cellulose filling the ceiling joist cavity, and about 15″ of space between joists. I have 2 soffits (about 6×12″ each) on one side of the house, one skylight is permanently vented and overlaps with attic space, and there is a louvre vent in an interior closet (which doubles as access hatch).
From reading GBA, I need better attic venting. From reading this site, I need less (zero) venting and stay away from foam. I have plans to re-roof this year or next, probably with SPF, so I’d like to get venting taken care of before that happens.
Also, can anyone recommend a home energy expert local to me in San Francisco Bay Area (zone 3c)?
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Replies
David,
The anti-foam bias reflected on the website of 475 High-Performance Building Materials is based on tenuous arguments, not building science. You should make your own decision concerning rigid foam insulation. Especially if you choose to use reclaimed (recycled) rigid foam, it's an environmentally responsible choice.
For information on whether or not flat roofs should be vented, and instruction in ways to insulate a flat roof with rigid foam above the roof sheathing, see this article: Insulating Low-Slope Residential Roofs.
Thanks for the quick reply. As a consumer trying to educate myself, its hard to know who is "right". I'm a fan of GBA, but I don't have a reference point. Similar, some sites say 1:300 for roof venting, others say 1:150. (Hence my desire to talk to someone local)
The article Martin linked was one I had read before, and I should have linked in my orig post. One question, is there any pro/con to buying one of these vents from home depot [LINK] vs building a 2x2' doghouse with rectangular vents?
David,
Without wading into the debate as to what approach works best, can I suggest you first evaluate the alternatives in terms of whether they meet your building code. Some flat roof assemblies, which may work perfectly well, don't meet present codes.
The 1:150 and 1:350 rates come straight from the 2015 Residential Code. The default requirement for vented attics is 1:150. For attics that have vapor retarders facing the conditioned space and have balanced low and high ventilation (ridge and soffit venting), you can use 1:300.
David,
Q. "Some sites say 1:300 for roof venting, others say 1:150."
A. The 1:300 ratio is for roofs with soffit venting and ridge venting. The 1:150 ratio is for roofs with soffit venting only. For more information on code requirements for attic venting, see "All About Attic Venting."
Q. "Is there any pro/con to buying one of these vents from home depot [LINK] vs building a 2x2' doghouse with rectangular vents?"
A. The link you provided shows a mushroom vent. You definitely want a well-built doghouse (vented cupola), not a mushroom vent. Mushroom vents are associated with poor performance and water-entry problems -- which is precisely why Lstiburek recommends the doghouse approach.
Hi Martin. Circling back to the 1:300 and 1:150 rule, do these rules apply when you don’t have either ridge, soffit vents, or any ventilation on a flat roof? Our home was designed unvented but was built to be vented (the batt insulation is sitting on top of the gypsum ceiling) but are missing vents so we are trying to correct the issue. Looking at adding louvre vents on the side and square vents on top of the TPO roof. Manufacturer says one vent per 1500 sq ft but the 1:300 would suggest 5 vents on top based on the net free area.
Thank you
Mmmtin,
The article Martin linked to in post #1 is worth a look: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs