Hello, My name is Don, and I’m a builder in Eastern Long Island, New York. This my first post.
In current issue, of FH, June,2009, I learned a lot about spray foam. However, my question is about the illustration on P34, in which both builders show the insulation line in the sloped roof, apparently above drywalled ceiling joists. I have heard that vented attics now are considered unnecessary… this maybe called “hot roof”? If this is indeed an attic, not finished space, why is this desirable? It would seem to just add to the amount of conditioned space for no increase in living space, and add to the square footage of insulation, as opposed to insulating at the ceiling line between the two spaces? I would appreciate it if someone can direct me to an in-depth article or discussion on this topic. I’m building a house with a target of net-zero energy usage with shed roofs at a pitch of 2 in 12. My intent was foam in the ceiling and a vented attic space. Is this behind the times? Thanks you. Don
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To get you started until others have better advice and links, you can go to http://www.buildingscience.com and I think they will have info on insulating roofs.
The short answer is that with adequate R-value tempering the differential to eliminate dew points and with closed cell foam also providing a vapour barrier, there is no need for venting to deal with moisture escape vs condensation.
And most shingle manufacturers are less concerned about the increased temperature of the shingle. They know that it does shorten the lifespan but not appreciably so.
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I really like hot roofs for a few reasons.
Most times we have air handelers and duct work in the attic, better to keep them in the conditioned spaces.
Most times there is about a bazillion recessed lights,fans,ducts,and other nonsence cut into the ceiling, nice not having to futz around trying to air seal and insulate around them.
Add to it that you don't have to mess around with soffit-ridge-gable or other vents,makes the price of a hot roof easier to swallow.
Well, down here in Texas, the sun is the enemy. If an ignored one for being the 700# gorilla in the room.
The sun heats the shingles, which heats the deck, which then heats all of the framing. Does this for more than 12 hours a day, Equinox to Equinox.
Right now, 1400 28 may here in Bryan, it's 84º and very mild, Dew Point is only 54º (down almost 18º from yesterday)--but, my attic is holding at 108º.
In my case, the deltaT on my walls is only a few degrees; but the deltaT for my ceiling is almost 30 degrees. As it gets to real summer, with mid and upper 90s for ambient, that attic will go to 135, 140, and only cool to about 90-92 about 0600, in time to get the morning sun to repeat the process.
Historically, it was what it was--water tightness was a bigger issue. This is changing. Particularly from how, given the basementless construction so prevalent here, all of the ductwork and AHU are in that sweltering attic.
That's here, how does that apply to LI?
Well, ceilings are notoriously hard to seal up, both for air-tight and for vapor-tight. The roof deck plane can be much simpler to seal up. Also, it can be easier to carry the vapor-tight plane up the walls and into the roof. (That's just "can" YMMV)
Part of this in the science part of buildingscience is that we are examining buildings as complete systems; no longer just insualting in the "waste" spaces left over in framing, but actually examining where the insulation plane, where the vapor-tight plan ought to go.
Any of that help, or just muddy the water?
No matter which system you go with, it will help to use light-colored shingles. Dark shingles over a vented attic heat up more than light shingles on an unvented one.
Thanks to all above for interesting thoughts. I also plan to populate this roof with photovoltaics and solar thermal systems. In the case of the photovoltaics, I have read that the efficiency decreases as they heat up, so I wonder if the extra heat on the roof is significant in this vein.Last house I built was flat roof modern. The insulation sub said he would never recommend closed cell foam in roof, as if a leak did develop, one would not know it was there, or if it became apparent, it might be showing itself twenty feet away from the leak and impossible to locate; whereas, with open cell, if a leak develops, the water will come straight down. Sort of made sense to me. But this vapor barrier thing complicates that decision, since open cell is not a vapor barrier, right? For THIS house, given the necessary penetrations to secure all the equipment on the roof, I'm thinking maybe it is worth the trouble to go with vented attic, if only because it will give me an open underside of the roof to make finding any leak easier. Roofing will be white EPDM, very secure until one contemplates all the screws going through it. There will be no equipment or ductwork in my attics, and I may just reject ceiling cans as an option because of the concerns mentioned in some of your replies. I plan to ban incandescent bulbs, so if fluorescent or LED bulbs used, perhaps I could recess sealed boxes in the ceiling joist space without overheating problems..? I mean like a 12" x12" x 12" plywood box, and put CF bulbs behind plexiglass or some such....? Maybe even cf bulbs would heat up too much in that situation? Does anybody put exhaust fans in attics to cool them some and is that worth it; say a vent on one end and a fan on the other?Thanks for your help, and I welcome any comments that find LEAKS in my reasoning thus far above.