For as long as I have been building, 45N latitude, I have been operating under the understanding that a wood frame building envelope should have a vapor retarder under the finish, insulation in the cavity and then vent, vent, vent between insulation and sheathing. I’ve seen this concept universally accepted for roof structures (and have often wondered why the same logic isn’t equally applied to walls, but that is not my question).
I’m presently doing a remodel in a somewhat remote area where “standard practice” is a bit different than what i’m used to. Here it’s common to solid foam fill wall and sloped ceiling cavities and not to allow for the ventilation that I have always understood to be critical. “We’ve never had a proplem” is a common response. I’m still scratching my head about the dynamics of this. It’s a wonderfull way to get a tight building shell, but I feel uneasy about how this practice will find the building 20 years down the road.
Thank you for your thoughts on this.
C135
Replies
When you start using foam a lot of the old rules go out the window.
The main reason for venting an attic is to allow heat/moisture coming through a poorly-sealed, poorly insulated ceiling (and also to a degree heat from sunlight on other parts of the roof) to escape before it causes ice damming and the like. Secondarily, when the insulation is poor, the venting helps reduce the amount of heat that enters the home during the summer. A third reason is to allow moisture from minor roof leaks to evaporate before it causes rot.
If instead the roof deck is insulated (eg, with foam) and is WELL-insulated, using materials which are reasonably impervious to water vapor, the first two reasons go out the window. Whether the third is a concern or not depends on the quality of the roofing job and how well it's maintained.
What you have is what is now reffered to as a "hot roof". Around here, we called it a "sandwhich roof" and the concept has been around for a while. The first ones I saw were rigid styrofoam ontop of roof decking with a water proof membrane ontop of that. Used alot in commercial buildings. With the increased use of sprayfoam insulation, now its used in rafter bays, to the underside of sheathing, to basically create the same thing.
I think one of the downsides to this method is it will reduce the life of ashphalt shingles, because the shingles will be subjected to higher temperatures.
If you have one, leave it. The cost to vent would be prohibitive, IMO.
> I think one of the downsides to this method is it will reduce the life of ashphalt shingles, because the shingles will be subjected to higher temperatures.I believe the consensus is building that the shortened shingle life is urban legend more than anything else.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I just spent the past 2 days demo'ing a "hot roof". This was a 150+ yr. old cape with original vertical pine board roof deck (laid over purlins) with 2 layers of asphalt shingles on top of the original deck. On top of the two layers of roofing, someone laid down a layer of 3/8th plywood, covered that with #15 felt, then 4" sleepers laid horizontally on 24" centers on top of that, then 2 layers of 2" foil faced poly iso foam in between the sleepers. Another layer of 3/8th plywood over that, then a layer of three tab asphalt shingles (no felt) right on top of the plywood. I think the built up roof was probably done in the late 70s. No ridge or soffit vents of course, so this was effectively a "hot roof". House is on a hill, no shade at all, and the ridge runs roughly E-W so one side of the roof gets full southern exposure. The final layer of roofing was grayish-green. My observation was that aside from being ~30 yrs old, the asphalt shingles were in ok shape. The 3/8th plywood directly under the outermost layer of shingles (on top of the foam) was in ok shape on the lower half of the roof, but nearer to the ridge (top 2-4 feet), the plywood was toast! Not water damage, just cooked to fibers. In many areas it just crumbled in my hands. Pretty much the same story with the foam. On the lower parts of the roof it was almost new looking, but the further up the roof you went, the more deteriorated (crumbly) it was.
FWIW, based on my observations on this particular roof, the asphalt shingles showed no unusual damage that I could attribute to heat buildup, although the underlying decking and foam did.
By the way, it was a bear to demolish.
BUTThjat roof was a goobldey gobledy glob of junk. There was nothing about it that comnes close to any recommended hot roof, insulated roof, or foamed roof. They did tons of things wrong to begin with. I know you are hurting from having to deal with it and I agree that your cconclusion that hot roofs are not necessarily bad for shingles is correct, but this job you demoed is so far from any kind of accepted practice that no conbclusions based on it should be considered valid
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You are right my friend, this was the roof from hell. And I didn't bother to mention that there was drywall/plaster attached to the underside of the original deck. Each 2x2' chunk of "roof" weighed about 100 lbs. You'd be equally impressed with the walls. So far, the walls have yielded insulation in the form of blown in cellulose, fiberglass (yellow, pink, and green (have never even seen green before), vermiculite, newspaper, rags, brick, sand and straw). One gable wall that appeared plumb from the outside was actually an entire second wall (firring, sheathing and siding), scabbed onto the original wall that was leaning at least 10 degrees into the house. In an effort to tighten up the floors, someone drove 2" drywall screws through the wide pine planks about every 12". The whole mess was built on rubble, and small rubble at that. We are hand demolishing because it butts up against a brand new "addition" which is really a stand-alone house. The owner built the "addition" while living in the hovel, and now that the addition is done, the hovel is coming down. On the upside, we were able to get our 30 yd cans right up against the house so nothing has to be lugged more than 20 feet or so, and the view is spectacular. 180 degrees and at least 50 miles to the horizon on a clear day.
I was LMAO reading the first half of this and thinking "Nothing he can't fix with five gallons and a match!"The "addition" scenario makes it all clear what is going on
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It is not urban legend, but the amount that shingle life span is reduced is so negligable that it is not worth conmsidering except maybe in places like the deep south.I read the reports that ccame out on this topic re the results of the tests. I forget now, but the aging was more rapid on the hot roofs in the range of something like .08%. I ran some calculations and found that on a typical roof, this meaant premature death in the neighborhood of 5-8 months because of the extra heat cooking the volatile oils out of the shingles. So no not myth, just not a practical cconcern.
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Last I read about it, the thinking is that shingle color has more to do with its life than "hot" or "cold" roof. Where's ShelterNerd? Go as light as the homeowner can bear to look at. Most folks seem to want dark. Except in FL, AZ, etc. where they can't deal with the cooling bills anyway and will look at even white metal to get some $ relief.
<<Where's ShelterNerd?>>I'm here. The deal is pretty much as stated above. At this point only Elk and Certainteed (which just bought Elk) will warrantee shingles when the underside of the deck is spray foamed. But the color of the shingles and their reflectance (Energy star listed shingles have high reflectivity) is the primary factor with overheating. Venting in a northern climate keeps the roof cool to prevent ice damming but the superior insulation and air tightening of spray foam also keeps a roof cool so to call a foamed roof a "hot roof" is inaccurate. Closed vrs open cell is a very hot discussion when it comes to "conditioned attics" and the Hydrophobic quality of Icenene open cell definitely gives it an edge over the cheap Demelak that I use. That jury is still out in my opinion.My feeling is that the best practice is to be damned sure that your flashings are well done and no leaks can get into the attics. Small amounts of snow can be pulled up hill by drafts into an attic and can melt so "thinking like a drip" is not enough. Wind driven snow is one kind of water that does flow up hill. Sealing the attic air tight helps to minimize the pressure differentials that can drive moisture into an attic from the exterior and from the warm humid air in the interior. It also allows for the location of ductwork and AC equipment in the attic when you have a slab house with radiant heat that still needs AC. Locally some folks have tried to use spray foam in renovation jobs with disastrous results. The leaks that were no problem in a drafty un-insulated house were trapped by the foam and lead to huge rot problems. Need to be sure you get your flashings and house wrap right when using spray foam. But it's an amazing product if done right. Another thing with spray foam is how it effects your manual J calculation. It's impact on the heat loss/ gain is far in excess of its stated R-value so it is important to have the heat loss calculations done by someone who has experience with foam or you will get over sized HVAC equipment. If using spray foam you should definitely spring for a variable speed air handler. I have my HVAC guy run the Manual J calculation and send it to me as a PDF which I then E-mail to my energy star rater for review prior to installing the equipment. Finally a sealed attic has some amazingly beneficial fire retarding benefits which are further discussed in the "flammable House" thread. Basically a lot of fires spread through the attic because there is plenty of air and plenty of room and flammable material. By sealing the attic you cut off the air supply and greatly retard the spread of fire. All the bestM------------------"You cannot work hard enough to make up for a sloppy estimate."
Edited 7/6/2007 6:57 pm ET by ShelterNerd
At this point only Elk and Certainteed (which just bought Elk) will warrantee shingles when the underside of the deck is spray foamed.
Certainteed didn't buy Elk. GAF did.http://grantlogan.net/
Sometimes, when I lie in bed at night and look up at the stars, I think to myself, "Man! I really need to fix that roof."
Always good to get your expertise. This whole thing is getting personal with me because I'm getting ready to have a builder build my own house here in Atlanta, GA. Hot,humid. We're doing radiant hydronic heat; installer already told us he'd be able to downsize the HVAC (which we need for A/C; ductwork in the attic). Using Icynene.I want to use Elk's (soon GAF) Cool Color Series Energy Star rated roof shingles ). All this stuff is incredibly expensive and I'm not rich. Now I know why people don't "build things right in the first place." After cutting square footage to the bone, I still need to reduce costs. My builder was working on that and said to save money, just Icynene the walls and let him blow in some cheaper insulation and do it above the ceiling rather than at the roof. I'm convinced that's a BAD, BAD idea, but can't get my argument together very well. Can you help me out with that?Many thanks.
If anything I'd switch it around -- blown cells in the walls and foam for the roof.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Why? I have a hunch myself that Icynene in the roof is critical to get the performance, but can't explain that well to the builder. He's installed Icynene, but not all over. This cobbled together thing grates on me. I like to consider the whole structure as a piece. A nice, tight, Icynened building envelope with HVAC ductwork in the attic, inside that envelope.Seems to me you'd lose something in the way of tightly sealing everything, which is a big part of the Icynene--not just it's insulating value.Builder's trying to help us out to get this thing affordable, so that's where he's coming from.
It's fairly easy to get tightly-sealed walls, and blowing in cellulose makes them tighter still. Harder to tightly seal a ceiling.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Harder to tightly seal a ceiling.
I blew about a foot of cells on top of 6" of loose fiberglass and it made quite a difference in the amount of air exchange in the house. Not perfect, but the can lights aren't sucking all the air out anymore.http://grantlogan.net/
Sometimes, when I lie in bed at night and look up at the stars, I think to myself, "Man! I really need to fix that roof."
Yeah, you can smother it with cellulose and achieve a reasonable seal, but still nowhere near "tight", if the original construction wasn't tight. And making a ceiling tight even during original construction is a challenge.On the other hand, if you pack the walls full of cellulose, and also do the usual vapor barrier and a housewrap on the outside, all well taped, the walls will be plenty tight.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
And making a ceiling tight even during original construction is a challenge.
Ayuh. I've never owned a house newer than 50 yrs old and the one I'm in now is the newest one. I'm concidering an addition and cells or foam is the question.http://grantlogan.net/
Sometimes, when I lie in bed at night and look up at the stars, I think to myself, "Man! I really need to fix that roof."
Have you checked in to see if you can get energy star rebates?Also, are you doing in the floor radiant heat?
We're going through the commissioning process to get it certified as an American Lung Assn Health House. (Builder is trained/certified by ALA and has added on a new company solely to build these houses; he's done 2 I know of; I went through 1 of them.) Theirs are more stringent standards than Energy Star; you pretty much have to meet and exceed Energy Star to meet ALA's. Also are more stringent than Southface.org's Earthcraft program (a regional green building certifying group). So we'll surpass those as well. Don't know if that's where it's from or just in the components we're using, but he says we can get a $2000. tax credit. Credits are good; come off the bottom of Uncle Sam's due.Unfortunately, the ALA charges $2500. to go through the cert. process plus about $1000. in fees to have their required inspections and performance tests done (3 in all). We'd want most of them done anyway, so that's ok. Taxes being what we are, we should about net out.Yes, down here there is one experienced hydronic radiant infloor installer who is listed with several manufacturers. He gets most of his business up in north GA mtns and maybe adjacent colder states' mtn. areas. Haven't worked it out with builder yet but I at least hope this radiant guy who knows something will agree to do the calculations for us, in a consulting capacity or something. For him to come in and lay it is probably gonna be cost-prohibitive. Don't know. It's a one-story slab on grade, 3200 sq.ft. now but that's gonna go down by 300-400 sq.ft. I think. The systems are real interesting and big doins' in these parts.Edited 7/7/2007 9:28 am ET by bjAtlanta
Edited 7/7/2007 9:29 am ET by bjAtlanta
I'm with DanH blow the roof and batt the walls after caulking the heck out of the plates and all penetrations. Foam the windows and doors. There is a new system coming on in NC called Flash and Batt. And go ahead and send me your phone number by e-mail through my website and I can call you and talk over some ways to save money. I have to drive to DC Sunday evening and can call from the road while driving.Be wellMichael------------------
"You cannot work hard enough to make up for a sloppy estimate."
Will do. Very interesting stuff.
I agree on thatThe shingle color and the latitude of the home makes A FAR GREATER DIFFERENCE
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corbond has done studies that back up this pactice and has gone to baat to help old fashdioned inspectors understand this.. Their primary book, Walls That Work, explains why.
The reason for VBs is to keep moisture from getting to the loaction in the wall where the dew point lets it oondense to become liquid water and start to cause trouble.
A polyurethene foam insulation IS a vb to begin with.
But inaddition, the polyur foams and the icythene foams reduce or eliminate the dew point situation so the great majority of moisture will not condense
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