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I’m planning to finish the attic in a 100-year old house in Massachusetts and make living space for a bedroom and bathroom. There is about 800 sq ft of space and the rafters are completely exposed. I recently had soffit vents added during a gutter repair. There is presently no other ventilation except for leaky windows that will be replaced when the attic is finished.
What would be the best way to insulate and vent the roof? If venting the rafters to a ridge vent is appropriate, I have a second question. The roof structure includes two gables that cross, producing a very large roof valley. Consequently, there are several rafters in each gable that do not extend down to the soffits. Is it normal to ignore the venting of such rafters, and if so, should they be insulated differently from the rafters that are vented?
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Tom-
you're in driving range for a company in North Thetford, Vermont that I've had a lot of experience with (Foam-Tech, Inc.) They spray closed cell polyurethane foam primarily. Check out their website at http://www.foam-tech.com and/or call Henri Fennell, the owner.
The site also has a very informative case-study of improperly vented roofs to show you what not to do.
Most importantly, you could insulate your roof without venting it using poly foam. The R-value is about 7/inch, and it's not uncommon for poly roofs to be in the R-25 range (whereas R-48 - 60 fiberglass is common up here in VT). We have a polyurethane insulated roof (in between rafters on 24" centers) and it's very, very impressive.
Good luck and let me know if you have any other questions.
*Tom, You have a roof that cannot effectively be vented. So block off the soffit vents and make yourself an airtight, unvented roof assembly.Foam is fast, easy, effective and expensive. Dense packed cellulose is cheap, slower, a little more attention to details, and nearly as effective as foam.Fred Lugano is in Charlotte, Vermont and Bruce Torrey is in Massachustetts. Don't know if they are near you or not.I routinely travel to Clinton and Marlboro. I could stop in and offer my opinion on the whole package if you'd like.-Rob
*You didn't say if space was a premium and if you were able to drop the ceiling down in the center.Most attics can be insulated with regular inexpensive FG batts, just remember to install baffles or spacers between the batts and the roof sheating. You're in a cold climate so the VB goes on the inside or the warm side of the wall/roof.Another advantage to using FG is that you can do it on your own, at your own pace. Do half now and the rest later or whatever.Gabe
*Tom. Ask yourself what is the purpose of attic ventilation? To get rid of moisture and heat leaking into the attic space. But suppose I stop the moisture and heat leaking into the attic spaces? Then, of course, attic ventilation becomes unnecessary. Now the question is which is the most effective and least expensive wayy to insulate and seal the attic space. GeneL.
*Gene you forgot another reason is to prevent heat buildup under the roofing shingles. BTW In a perfect world, moist air can be easily contained, but in an older home, it will leak into attics from places as far away as the basement. Better to give the moist air a place to go and not be trapped to do damage.:)Gabe
*At the risk of being one of "those guys", the primary purpose of venting a roof -- in New England, at least -- is to keep the surface of the roof cold to prevent ice dams. "Venting" does not mean actually venting from the inside of the building out -- that's exactly what should be prevented -- and increasing the longevity of roofing materials is a by-product of the primary purpose which is to keep snow on the roof, not melting and damming.Having said that, make sure to have the right combination of air leakage prevention and R-value and you won't need to vent the roof. I know closed-cell, sprayed foam accomplishes both and about 4" worth is all you'll need.My two cents...
*Shawn, I'll keep my post brief because I have been typing alot today and my hands hurt!!I agree with this statement "the primary purpose of venting a roof -- in New England, at least -- is to keep the surface of the roof cold to prevent ice dams"This is the supposed reason to vent in cold climates. But the question you are not asking is "Where is the heat coming from?" The answer is that it comes from inside the house. 20 Years of weatherization programs (and lots of reasearch) has shown us this.""Venting" does not mean actually venting from the inside of the building out -- that's exactly what should be prevented" I also agree, but the reality is that venting a roof increases heat loss from the house, period. Lstiburek ahs proven it through reasearch, weatherization programs have proven it, and I have proven it to myself by UNVENTING open, underinsulated, and leaky attics on ice damming roofs to fix them!I (and others) are beginning to believe that R-value is not needed at all to stop the ice-damming. If I could find someone to sponsor the reasearch I'd love to see if I was right. I know in my own house I had, for a few years, R-20 airtight roofs constructed of 2.7" polyiso foam. No ice damming and snow atyed on roof for long, long time with small icicles (less than 1/4" diameter and 8" long) here and there.If heat removal is important for shingle life than why don't shingle warranties vary from region to region? Clearly Las Vegas roofs wouldn't last as long as mine (western NY) or yours.What about barn roofs? Many last a long long time with no venting.-Rob
*Rob, Shawn and others, I'm about to embark on an attic conversion project in Portland similar to that described by Tom (1908 builder bungalow). I'm sold on the foam in place approach with no venting, however wondering about the pros and cons of going with Icynene vs. urethane foam. Rafters are 2x4 at 32"o.c. with skip sheathing, and a roof that will need replacement in the next several years. Any thoughts would be appreciated. Are any of the foams approved for exposed application? I'd like to foam flush to bottom of rafters and leave exposed if possible.Mike
*Gabe. For many years now I have argued that the first line of defense against ice dams begins in the basement/crawl space with the sealing of all the openings in the basement/crawl space ceiling. Do the same thing to the attic floor. Otherwise, all the warm moist air originating in the basement ends up in the attic. For years and years the Asphalt Roofing Manufacturers Association (ARMA) has claimed that attic ventilation is a must to prevent premature failure of roofing shingles. But in spite of constantly asking them to document their claim all we get is " If you knew as much about shingles as we do, you would know that we are right." However, imdependent researchers have monitored (in one case the monitoring was continuous for more than ten years)the temperature difference between vented and non vented attics, and found a delta T of about 12 degrees Fahrenheit. This research shows that venting or non-venting a roof has a 5% impact on shingle durbility. But venting or non-venting is not as crucial as the color of the shingle. Finally, CertainTeed is now giving a ten year warranty on shingles installed on an unvented roof. Does this portend a significfant change in ARMA's position? GeneL
*Mike, search the archives here for several recent discussions about foam insulation. Very informative, and at least one poster was a foam installation specialist.As I recall two points: 1) Icynene is an open cell type which will allow some breathing (able to recover after a roof leak is found and corrected), and the other Urethanes are closed cell types (how can water get out if any ever did get behind it?). This open/closed cell point is pretty minor if you don't ever have any leak, I think.2) About exposed foam, the code says pretty much, "no exposed combustible material", so its up to you and your inspector to decide if its combustible or not (get the mfr's data to discuss this with your inspector).There was lots more discussed but these two points stuck with me.
*Mike-we have exposed polyurethane foam in our home (a renovated barn) and here are a few things to consider.First, polyurethane foam degrades (slowly) in sunlight. It gets dry and brittle and turns dark yellow. This happens immediately after it's installed. If you leave polyurethane foam exposed, it must be painted, which prevents the degrading. However, I'm almost certain poly foam can't be left exposed anymore and must be covered with a 15-minute fire barrier (sheetrock, 5/8" solid wood or 7/16" OSB/plywood, etc.)Second, sprayed in urethane foam is tenacious. If you're planning on replacing your roof sheathing (or did I misunderstand?), place something in the bays to spray against and flare the edges. (I probably misunderstood and you most likely meant you're replacing your exterior roof, right?)Third, I've been using an unconventional approach to foam because sprayed-in urethane foam is too expensive, and I can't offset the costs with sweat equity if a contractor sprays it in. I buy rigid, foil-faced foam (Celotex, Thermax, whatever), cut it to within 1/4" of the bays I'm insulating, secure it in place with redcaps, then spray the edges with two-part polyurethane foam that I get from EFI. EFI sells it in two sizes, I think: the 280 kit and the 600 kit. I haven't used the former, but the 600 kit costs $490 for 600 bdft of closed-cell polyurethane foam at 1.75 lb density (approximately R-7/inch). The also sell the gun ($90, which is reusable and includes 10 tips). I use the fantip which sprays a 4-6" wide swath of poly foam and I go over the edges of the rigid foam. I get the benefits of foam insulation (air barrier, high r-value, low r-value drift, etc.) at a fraction of the cost.It takes a couple of minutes to get the hang of using the gun (which includes a 15' hose), but after that you'll be spraying like a pro.This pricing might not apply to your situation, but I got an estimate to do a floor/ceiling in our barn for $4600 (R-19, sprayed in place). I decided to do it myself with two 1 1/2" of Celotex with a 3/4" airspace (theoretical R-value of nearly R-25.75) and my materials costs were $1250 (not including sweat equity).Sorry this is so long-winded. Hope it helps.s.
*Rob-I'd hate to see what happens when your hands don't hurt!I think you and I agree completely. I do have one question: in your experience (and the others you cite) does venting a roof increase heat loss from the house because of the cold air washing simultaneously against the underside of the roofing and the insulation, creating in effect a cold convection current? I try to convince anyone I know who is building or renovating not to vent their roof and every bit of information helps.s.
*Gene,If you search the archieves you will find that I argued that the colour of the shingle has always been the most significant contributor to increased heat in the attic.In most modern homes, migration of air from the basement to the attic is not a consideration, unless Blue framed it. :)Variations in regions has a dramatic effect on shingle longevity as well. When you all go to Cape Breton for the Capefest next month, check out the roofing there. You will find that it's common practise to tar the underside of all shingle tabs prior to installation. Not enough heat to seal the shingles otherwise.Those of us with the expertise to understand the physics/science involved can make any insulation type work in any given situation. Most in the trade don't and a poor job with any insulation is still a poor job.Gabe
*Shawn,Lstiburek says that venting a typicalhouse increases heating load by ten percent.I can see the cold washing effect of the outdoor air through the ends of the batt with the infra red camera. I think this is the lesser of two evils though.I believe the big issue is that if there are air leaks, the venting gives a place for the air to go to, so that the attic cannot ever be at equal pressure to the living space. The result is that stack effect takes the hot air into the attic and then right out the ridge/gable vents. When you close off the roof venting, you now create a closed space above the living space that eliminates the gushing of the stack effect, and replaces it with a convective cell. This convective cell runs between the ceiling and the roof. This is a pretty weak convection cell at best, and exposes the roof to much lower temperature air. A vented roof lets stack effect bring room temperature air into contact with the roof just before it goes out the ridge vent.I fixed ice damming on several houses last year by unventing the roofs!I believe I see this with the infrared camera as well. At late night in winter time I will almost certainly find the ridge vent and gable vent openings are showing hot relative to the siding or remainder of the roof.gabe - I think I could make a case that many modern houses are no btter off in the attic/basement connection. At least in the last few I have been in!-Rob
*In that case Rob, you should learn how to build a better mouse trap. The only connection between a basement and the attic in most new homes is the plumber.Mr. Lstiburek would be well advised to stop making up statistics to back up his theories.Gabe
*gabe,I have been in several houses recently that had designed in bypasses from basement to attic!There is also the issue of leaky ductwork, and the electricican's spade bit.One thing I got out of discussions with Joe is that he can always cite references, much like Gene does here. Most of the innovators in the building science field can cote test data.Your own CMHC does alot of stuff like this and the people who work there also bring up facts and references in discussion. You tell me, is the CMHC political and slanted? I believe that the research that comes out of our DOE, LBNL, Oak Ridge, Building Science Corporation, Advanced Energy, etc. Is non-political and non-biased. Would you agree on this?-Rob
*Hi all,Thanks for your comments. I seem to have put an unintentional “red herring” in my original question: since the attic is completely unfinished, the windows presently provide some ventilation for the roof because the rafters are open. However, I understand that when I finish the attic, if I ventilate the roof I would not be doing that with interior air, but rather with air flowing from the soffit vents to a ridge vent. Sorry for the confusion.Rob, if I understand you correctly you’re saying that the best approach would be to insulate the roof without venting it at all to minimize heating of the roof through interior air leaking out. That makes sense, but I can’t quite follow your statement that “R-value is not needed at all to stop the ice-damming.” Won’t any form of heat transfer that raises the roof temperature above the freezing point produce melting, and potentially, ice dams? I also don’t understand how warm air brought “into contact with the roof just before it goes out the ridge vent” can cause ice dams. You mentioned Lstiburek as an opponent of roof venting. Since my original post, I have purchased his new book on construction techniques for cold climates. All of his framing and insulation illustrations show vented roofs with the exception of one example of a “hot roof.” But the text only discusses traditional soffit-to-ridge venting and the use of baffles to keep fiberglass insulation in place, and he even has an illustration indicating that the right way to frame a skylight is to use shallower lumber to preserve the flow of air from the soffit to the ridge. Can you point me to the other information you refer to?To all - the replies generally incline toward the position that roof venting is unnecessary if there is no air leakage and some insulation. I guess that leaves me puzzled as to why there are so many sources that recommend venting. Any thoughts? Also, when people do vent a roof, how do they deal with the situation in my original question, i.e. crossed gables?Thanks,Tom F.
*Hi Rob,Afraid CMHC is a little slanted. I was part of the team that built their headquarters in Ottawa. Like most organizations like this, whoever pays the piper picks the tunes.Gabe
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I'm planning to finish the attic in a 100-year old house in Massachusetts and make living space for a bedroom and bathroom. There is about 800 sq ft of space and the rafters are completely exposed. I recently had soffit vents added during a gutter repair. There is presently no other ventilation except for leaky windows that will be replaced when the attic is finished.
What would be the best way to insulate and vent the roof? If venting the rafters to a ridge vent is appropriate, I have a second question. The roof structure includes two gables that cross, producing a very large roof valley. Consequently, there are several rafters in each gable that do not extend down to the soffits. Is it normal to ignore the venting of such rafters, and if so, should they be insulated differently from the rafters that are vented?