is it really absolutely necessary to vent an insulated roof?
(asphalt shingles over felt over osb then foam baffles and faced r-30 then drywall)
how critical are these baffles and the circulation they provide?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Fine Homebuilding is excited to be the official media partner of the 2024 Building Science Symposium series! This event offers builders, tradesmen, architects, designers and suppliers to discuss topics ranging…
Featured Video
Video: Build a Fireplace, Brick by BrickHighlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
venting an insulated roof is for maximizing the roof life, lower indoor air temp is a side effect. If you are not into ridge vents then invest in the solar powered exhaust vent set to a thermostat.
shingles dry from the inside out- another reason nail overs are not a great idea.
maintaining good air ventilation will also reduce the amount of moisture that collects in those spaces, ie mold reduction etc. Also it reduces the amount of heat stress on duct tape and ducting. Ive been in a lot of attics and have seen how much a batt reduces after 10 years of heat beating it down.
Not always. What you speak of is what we call around here a 'Hot Roof". Which is when the insulating job is so superior that there is no need for venting. Some roofing companies will not guarantee there roof with this system though. Also every detail of the insulating job must be done very well for this to work.
Spray foam would be my only choice for a Hot roof .
If using fiberglass I would stick with the venting.
Why do you want to get rid of your venting? Does it leak?
To pass code (and work without moisture on the inside roof deck from interior condensation) you need to have a air barrier insulation.
Only Icynene passes that in the soft foam and all the closed cell foams. Never will FG or cellulose reach the air barrier standard with current technology. Stu
How thick would the spray foam have to be, applied to the underside of the roof decking.
Never suggested F.B. for the install.
I also suggested spray foam myself.
You either have to vent or to make absolutely sure to build so as to deny any dew points and to control moisture in other ways.
The answer also depends on your AHJ and what codes and ordinances they apply
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
you don't need to vent the roof, but if you don't you will need to engineer it differrently:
The spray foam insulation folks got this code change passed in 2004. North Carolina did not adopt it until 2006.
I'd suspect the titanium membrane outside isn't going to do anything for the condensation on the inside of the roof deck. I'm figurin' most know the moisture problem that the venting is supposed to band-aid is the warm moist interier air that hits a cold roof deck condenses and leaves dew or drops on the surface that collect up over a short period of time to cause a big problem. Nothing on the outside matters unless of course your talking 3-4" of foam board or something stacked up. That would raise the plywood temp up to very close to the interier air temp so no condensation any more. Stu
I don't see how the synthetic roof membrane, Titanium UDL or Grace's product, roof guard II, etc would matter at all for venting. From what I understand, the main point of using those are for installation advantages, its much lighter than felt, does not tear, won't wrinkle when wet, stays flat, etc.Also, in my conversations with Elk, they need to air barrier from foam, fiberglass, blown in, etc. won't work and need venting.
Well, I didn't invent the engineering on this, but I did read something on it. Here's the summary (I was assuming asphalt shingles).
Asphalt shingles draw in moisture.
If you use regular felt, that moisture can penetrate the roof deck. Since there is no venting, it won't dry out, thereby causing a rot problem.
If you use titanium or equivalent low perm membrane, there is no (or close to no) penetration of moisture into the roof deck.
Don't take this personally but your post makes no sense what so ever. You say "If you use regular felt, that moisture can penetrate the roof deck. Since there is no venting, it won't dry out, thereby causing a rot problem." So basically you are saying that the roof sheathing will carry moisture from the outside, under the shingles, and through to the inside and that is why it needs to be vented. And if not vented it will cause rot. How could there be a consistent source of moisture traveling through the sheathing, without the sheathing rotting?The synthetic underlayments are fairly new. If asphalt shingles draw in moisture, how could any house not have rot problems since almost all house have felt paper under their current roof?Your really way off base.
Easy there...
I read the same thing but gave him a pass. Stu
I'm not trying to beat him up, so sorry to him if the post comes across that way. I didn't want it to pass though since I think the info is off base and could leave people with some incorrect information.
From Building Science:
"A less common failure mechanism is the result of moisture stored in the roof finishes... or held in small gaps (between asphalt shingle laps) being driven inward when heated by solar exposure. This vapour drives to the inside and can condense..."
I used some ice &snow shield (no perm underlayment) this summer and was interested to see that they REQUIRE venting of the roof when it is used as a full roof underlay....I guess so that the interior moisture can escape through the shingles....?
so I vented it.....
kinda.....
in short,read the article in JLC March '07 pg. 63 (Journal of Light Construction). there's some excellent info there.
Generally, the school of thought was venting of the roof deck underside (the prop-a-vents you mention) would prevent damage to the shingles from heat build-up behind the roof decking.
If you have an attic vs. a cathedral ceiling you may want to take a different approach for each.
with an attic situation,you want to seal the ceiling of the floor below such that no air leaks into the attic space, this means sealing all penetrations such as wiring,plumbing, lighting, hvac, etc... (don't forget exterior wall penetrations) so no air from the living space can enter, this is the biggest source of moisture in an attic, especially during heating season.
read that article, it'll help.....
Geoff
Some of the info in this thread is just flat wrong. Stu (MAsprayfoam) and Piffen (of course) are exactly right. You must vent unless you have an air barrier to keep the relatively warmer air with more moisture from condensing on the inside of the roof sheathing. As both said, the moisture in the air will condense in the winter when the air on the outside is below freezing. Fiberglass does not stop air movement and thus needs to be vented or your roof will rot right off from the inside out. The posts about lowering shingle temps are not correct. There have been tests that "hot roofs" with foam insulation and vented roof has very similar temperatures and the shingle life is reduced by 6 months or so on a 30yr shingle. Building science has some good articles on this stuff. I believe, even with spray foam you need to vent in places like Az, FL, etc. probably because the direct sunlight is so strong that there it would affect the shingles.
In a nutshell, spray foam or you need to vent.
I think everybody's got it right about only Icynene(or comparable, maybe the new soy-based clone) with an unvented roof. We're doing that in our new build. But the other info. we've gathered says roof shingle life has more to do, quality being the same, with shingle color. Those very light and white colors no one wants (except in FL, etc) are likely your best bet for shingle life.
I have been following the vented vs. unvented roof question and now I have one. Why spray the foam on the underside of the roof deck?
It seems you are then incorporating the attic area (cu. ft.) into the conditioned space. Why not foam the ceiling joist area and reduce the volume of conditioned space?
>>>>>>>>Why not foam the ceiling joist area and reduce the volume of conditioned space?Because you're creating a volume of unconditioned space that has to be vented to get the moisture out and heats up from the radiation. If you reflect the heat at the roof deck by spraying the underside, you don't let the heat in. The little bit of conditioned air you add is a small price to pay. You have to deal with it one way or the other.http://logancustomcopper.com
http://grantlogan.net/
"We have enough youth, how about a fountain of smart?"
thanks. all responses appreciated and helpful.
what is the method of choice for venting valleys? how much circulation can you get where you have 40 jack bays between two valleys? (10 per side)
in my situation there is no attic. all of the space under the roof is living space so all of the roof will be insulated.
thanks againps- what about styrofoam baffles 48 X 14.5 costing 1.68 apiece?
Are you against using foam?
>>>>>>>>>>ps- what about styrofoam baffles 48 X 14.5 costing 1.68 apiece?They're cheap and they're worth it.http://logancustomcopper.com
http://grantlogan.net/
"We have enough youth, how about a fountain of smart?"
will-
Are you still planning on using fiberglass batt insulation?
Absolutely .... let me tell you why ....
When overseas, at one point I was housed in a pre-fab block building. The building, including the roof, was insulated by 4" of rigid foam. Nevertheless, the place got HOT in the desert sun.
We constructed a very simple gable roof over the unit - corrugated panels on a simple wood frame. No attempt was made to seal the edges where the panels met the edges of the building.
The resulting shade on the roof, and the ventilation under the roof, made an enormous difference in the comfort of the "cubes."