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I have an unvented roof with 2×6 rafters and a finished cathedral wood ceiling. Over the years the roof has leaked, damaging the celing and insulation. I would like to replace both of these, and vent the roof in the process. I’ve considered 2 layersof 2″ high R ridgid foam (leaving an 1 1/2″ of vent space) or blocking down the rafters atthe expense of losing head room, so that the rafter bays could accept R-38 fibeglass. My question is should I go with either of these souloutions, or any other such variations, or go with R 19 in the roof?
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Replies
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Sheath under the rafters with 1.5" polyiso, spray foam all the seams, and dense pack cellulose into the remaining cavity. Drywall or siding over the polyiso. inspect any penetrations and seal tight. Skip the venting.
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Joe,
I live in Ashfield, Massachusettes. It is a colder region of Western Mass. Thanks for any info.
Britt
*Bill,My reasoning behind venting the roof was to possibly rid the annoying problem of ice dams in the winter. Will the tight seal prevent this? What about vapor barrier? Does the foil face act as one? If so, would the double (each side) foil faced polyiso cause a problem? Thanks for any info.Britt
*Britt, I am glad my partner Joe had asked, where you live. Now, I would like to ask you if there is going to be any recessed lighting in this cathedral ceiling? The reason for this question is that insulation needs to be handled differently depending on the type of recessed lighting you are going to install.So, it's important to know what type of "Hi-Hats" you are going to use if any. Sorry for the many questions but, it's difficult to give an accurate answer without knowing all the details. Regards, Victor VergaFusco & Verga Construction Co., Inc.
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Bill
You write "spray foam all the seams". Is there any known chemical reaction between spray PUR and Polyiso. or is there a spray Polyiso foam now available?
Seems that foaming joints in RFBI which is then going to take d/wall or siding is creating a lot of cleanup of overfill. There is a metal tape avail. (with a fire rating)that will maintain the air barrier that is generally used to seal joints in foil faced Polyiso. A careful installation should leave few gaps to foam anyway!!!
What R value will 5 1/2" of dp cells add to the R 11 of 1.5"foil faced RFBI?
*I am using a paper faced polyiso - not foil - get it from the roofing supply. I'd use 1" except the 1.5 is about 60 cents more a sheet so why not. Per Fred's suggestion - mount the poly-iso with a gap - approx 3/8" - and fill with foam. Much easier than cutting tight (especially in my case - hip roof with FOUR hipped roof dormers - inside the wall plate I couldn't put two whole shingles on without a cut!)The bard says it's R10 - gauranteed 10 years. I just don't trust tape for 100 years - since that seems like a reasonable life in my books. R value - well 3+ per inch so I'd guess with 2X6's he'd get 25 or so. Point is - absoulutely no convection because you can inspect it.But don't take me so seriously - I'm just a hacker that has found this stuff interesting and - compared to fg - kind of fun.Right on the recessed lights - just remove those suckers and surface mount or indirect light off your nice new, air tight ceiling.
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Joe and Vic,
My ceiling is similar to the first picture in Fred's reply, except the rafters are not exposed. It is also a "true" cathedral with no attic space. The ceiling (roof) also has two skylights roughly 4'x 8' made from plexiglass. They span over two rafter bays at 24 oc. They are well built,let in much needed light into the house, and accent the stone chimney running up the center of the house. I know that they are probably major source of heat loss, but it would be difficult to part with them. New skylights that size are outrageously expensive also. As far as the chimney goes, it has been retrofitted with a metal flue, to which we have a woodstove connected to. Our other source of heat is forced hot water. To answer your question about recess lights: No, there are none. I hope I have answered all of your questions. I look foward to yor response.
Thanks, Britt
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Hi Joe,
I would take exception to your assessment that your prescribed approach would be the easiest. I would choose an approach that doesn't require removing the T & G ceiling, building vent channels, hand cutting three layers of foam to fit, hanging poly then re-installing the T & G.
How about just removing one row of the T & G boards, stuffing a hose up and down each bay, blowing with cells, and putting the row of boards back in place.
Or if you want to avoid the mess inside, pull up a few rows of shingles on the roof and rip a strip out of the roof sheathing, blow the bays, and replace the strip of plywood and the shingles.
Regardless of how you feel about which solution provides better performance (I assume you feel the foam would perform better and some others would argue that the cells would perform better), the cells approach sure sounds easier and less expensive to me in this instance.
Steve
*You make a good point Steve, but the original post says:"Over the years the roof has leaked, damaging the celing and insulation. I would like to replace both of these"It sounds like he is planning to pull the ceiling out anyway.Rich Beckman
*Joe, The worst use for foam board insulation is "fitting it between" wood framing. And the amount of foam you are proposing in all those fitted layers, so technically exacting, relying on seals that can't work, that would take an inordinate amount of time and skill and special materials, then putting poly over foam....What the heck? Is this a trick post? How many layers of water trapping materials are you proposing, four? I love your ability to flame us all and I like your curving walls, but I must refer Britt to Fred L as the resident insulation/moisture expert with his simple, inexpensive solutions that make sense when referenced against the likes of the single best source of relative information,Building Science CorporationJustWarming The Grill Up For, Joe's Barbecue!Jack : )
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I'm with you, Jack. 100% right on the money. Keep up the great posting. Now I must go and find my flame proof suit.
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Greg,
Get with the program and put your suit on before login!...
OSHA Mandate Section 101 "when
Dealing with incendiary objects"
All suited up and just waiting for my Joe's
Barbecue Invitation,
Jack : )
*Its hard to listen when I'm laughing so hard!...Read your own posts...You have15 years experience with FG and poly but you don't mention 15 years ofexperience cutting triple layers of foam to fit in frame bays. If you hadto do what you off handedly recommended you would know that it's not simple likeinstalling FG as you or your crew or subs do...far from it. And I standfirmly behind the statement that multiple layers of foam, along with poly thatwill have some holes in it somewhere, (impossible not to have holes,) is a watertrap, waiting for driven interior moisture. Your mentioning roof sourceexterior water leads me to believe even more that you aren't understanding thesystem you proposed.We could go on and on, but I just don't understand your recommending systemsand then referring to your years of use of an entirely simpler, inexpensive,system of FG & Poly.In every sandbox, there's gotta be...Jack : )
*Peace, love & understanding?
*Andrew,I was counting on you to put a good end to this.Thanks,Jack : )
*Hi Joe,Yes, I missed the part about replacing the ceiling. If he's going to have it open, foam is a valid option, Though I have to opine that cutting and fitting it tightly is indeed tedious, though not beyond the scope of average abilities. If Britt's desire to replace the ceiling is fueled by really wanting a new clean ceiling, so be it, but perhaps he's thinking that the only way to rectify the insulation problem is to tear out and start over. That could be avoided and the insulation problem effectively dealt by blowing cells, should he decide the ceiling isn't too badly stained to live with. Plus I think there is an advantage to cells in the near complete air seal you get that is not dependent on puncturable barriers. And you get better dryability to both sides of the roof structure if you don't fill it with impermeable stuff.Maybe we all ought to try for a little more civility. In the words of the immortal Rodney King, "Can't we all just get along?"Steve
*Joe, How would I know that you would eventually turn to direct "verbal barbs" at me, verses defending your insulation proposals that I question?I enjoy discussing our individual building methods and I often save on firewood just by leaving one of your neat"flame posts" on my screen for the evening,...but lets stick to thefacts...my mouth is closed while I type these posts...and you're guessing again when you say"I flame myself"...You don't have that knowledge.Stick to the Barbecue Tricks, I like them!...Really!Jack : )PS- You may have the last of this, as I'm pretty"well done!"...Keep the heat on low if youcan...Thanks Joe Buddy!
*Joe,Your best advice is your little witty shots at others turned back to you!...so"I'm writing a list and checking it twice" just for you, buddy.Lets see, I did forget to mention drywall and paint in my prior post...so according to my"checking your post twice"...you're proposing not four moisture trapping layers...but six!...three layers of "easy to install perfectly" foam, poly, drywall, and paint.Have you ever unbundled a lumber pile that had been rained on and found the interior haddried out? Moisture likes to go into trapped,"almost" sealed areas but doesn't like to "get out" easily. Think of a sponge...Put a dry sponge on a wet counter...it sucks up the water!...Put a wet sponge down on a dry counter...You've still got a wet sponge! Capillary action is one way...like a lobster trap...like a roach motel...Voting for less layers, flame broiled,Jack : )
*Hi Joe,Well, I've used cells precisely one more time than you have. Since you've never used them at all does that mean you have no right evaluating their effectiveness? Of course not. I simply want to be able to build things that work well. In Minnesota there has been a rash of lawsuits about moisture damage in houses less than five to ten years old, They were built by the rules, yet failed to perform as promised. I too have built poly-batt-vented cavities that don't prevent all the problems they claim to be able to solve, so I'm looking for better alternatives in terms of both performance and long term-integrity of the system.I've got one client who has ceilings just like Britt's (T&G ceiling, 2x6 rafters, FG insulation). They develop severe ice dams most winters and have horrible condensation stains. They have had to repair the interior walls twice. In adding a second story over part of this house, I built the same style roof, but with poly, 2x12 rafters, R-30 batts, and vent channels with soffit and ridge venting, assuring them that the vapor barrier and the vents would prevent the same thing from happening in the addition. Well, it's better, but not as better as it ought to be. Ice has built up more quickly on this roof than I had hoped, and there are a few condensation stains appearing on the t&g already. I'm not a sloppy builder. I'm very meticulous. The poly was well sealed, the batts fit tight, the vents are properly installed, the cellar has a moisture barrier down.I think the main problem, especially with T&G ceilings, is the difficulty in creating and maintaining an unbroken air barrier with poly. Every little nail or electrical box allows inside air past the poly. And what happens when the tape at the seams in the poly drys out and lets go in a few years? What appeals to me about cells is the apparent simplicity with which they form a self-healing air barrier, and their alleged ability to more effectivly dissipate what moisture they do aquire. Also, it seems to me a plus to be ability to eliminate impermeable layers on both sides of the cavity, which ought encourage more rapid drying of any moisture that does get into the cavity than a wall with impermeable layers would allow.I don't like to build things that are based on plastics and sealants and complicated systems that competent builders and well-intentioned clients alike can easily induce failure in. If they perform as many people claim, cells appear to me to offer a valuable alternative to the complexity of multi-layered cavities stuffed full of synthetic materials. What is so wrong about trying to critically evaluate whether that is so? It's not like Poly-batt-vented cavities have proven to be the magic bullet. This summer I'm going to try blowing cells into this clients old 2x6 ceiling, because, based on the limited blowing I have done, and what I've learned about cells here and elsewhere, I think it will work. I'll let you know next winter if it does.There are plenty of things that I haven't built that I hope I'm capable of making an educated decision about using. I would hope I wouldn't have to personally install every single building product made before I could come to a conclusion about when using it might or might not be useful.The key is making an educated decision. To do that I try to look open-mindedly at all the information available, evaluate the credibility of the sources and independently verify results wherever possible. We don't all have to evolve from cavemen in the 75 or so years we have. That's what pasing on knowledge is all about.All the best,Steve
*Joe,When reasoning leaves people, I guess it's time to kick some sand in their face, and say "so there!"How did my tennis business end up in this sandbox "insult me when all else fails" debackle?Fred, it's kinda fun to tease the alpha male, he's flipping!Jack : )
*Steve,Thank you for reading my mind! I am in exactly the same situation here. All six homes in my neighborhood were built in the late 80's and all six have roof leaks and ice dams! In a fifty house subdivision nearby most of there roofs have problems and they have gone so far as to start a class action suit. I don't know about the rest of the people around breaktime, but I like to avoid lawsuits with a passion. There are no winners and lifes too short.Less lawyers, less layers, more Building Science Corporation,Jack : )
*Hi Patrick,I think the key is your effective air sealing. What is acoustical sealant? I've not heard of it. I've seen a lot of "50 year" caulk joints that have failed after a year or two. I just hate relying on sealants for anything permanent. The whole notion of a solid organic (cells) panel really appeals to me in its simplicity.We have very bad ice dams here because of heavy snowfall with daytime temps that are often right around freezing for most of the winter.Steve
*Steve I too have built cathedral ceilings much the same way you describe above i "with poly, 2x12 rafters, R-30 batts, and vent channels with soffit i and ridge venting,. . . "with both d/wall and T&G ceilings, with vented metal and asphalt shingle roofs above, and haveb NEVER had leaks, condensation stains or ice dams on these roofs,(10 yearsand counting) although virtually all neighbouring houses often look like ice sculptures!!!The two thing I do differently are: I construct vent shutes out of Glasclad, with the paper side up, and I caulk the joints in the poly with 'acoustical sealant'. Having always been suspicious of the "venting philosophy" anyway, I recently tried the no-venting scenario on a renovation job I'm doing; using the previously installed f/g, I added 2" XPS on the interior slopes and multi layers of F/g in the flat section. No ice damsi this winter. This Spring I will build an addition to this house, also with finished attic space (slope & flat ceiling) and intend to check out blown in cells if I can get the equipment. It remains my observation that I can, and do, a 'thermally excellent' job, no matter which combo. of materials I use. I freely admit that I have never had any of my jobs tested with some of the equipment that has been mentioned on this board, but neither will any of my customers be looking for a weatherization contractor to fix theiri unliveable/unsellabledwellings either!! My customers are happy with the thermal performance I deliver, but then it can always be argued that i ignorance is bliss. I have yet to achieve heating costs of $35/year as Gene L. does but then again I've never tried to build i 'thermally seperated' R35(?) walls either.Now that that standard is on the table, I plan to study it. You're right. . .b reinventing the wheel is a waste of time. Hoping to make 75+, got a good ways to go. -Patrick Seems I was editing while you were posting, and I ran out of the 30 min. time limit.. . had to repost and delete the other version hence the out-of-sinc posts. Acoustical Sealant is a 'goopy' caulking that perportedly never 'dries'. It is used as abed for d/wall and other coverings ini acoustical isolating applications and also to seal"vapour barriers" (old terminolgy acknowledged). It is part of the " unbroken envelope of poly" approach to interior "vapour barriers" that is a big part of the Canadian R2000 energy efficient homebuilding program started about 20 years ago. All joints in the poly are lapped at least one stud/joist space and double sealed with Acoustical Sealant and attached through the beds of sealant. It's truly ugly stuff to deal with, not at all like typical cauling, but very effective. . . I can't vouch for it'si real plastic lifespan. I've never wanted to build a true R2000 house but have borrowed some techniques from their approach
*I had an identical problem with my bathroom ceiling. T&G ceiling was water damaged due to skylight leak, old FG insulation was thin, wet, & useless, roof sheathing had water damage, and carpenter ants were having a field day. I tore out the old ceiling to locate the source of the leak, and had the roof repaired & reshingled. The bathroom was always the coldest room in the house. The reasons became obvious when I tore down the old ceiling & walls. The bathroom had been remodelled, probably in the late 60's or early 70's, and the T&G concealed "GAPING" holes in the walls open directly to the attic. Added to the "virtual" insulation in the ceiling, I might as well have installed an outhouse, for all the heat it retained.I built up the rafters w/ 1" furring strips to add some depth. Then I installed foam channels between the rafters leading up to the ridge. Then came R-19 FG batts fitted snugly between the rafters. After that, I put up 1" thick foil-faced (both sides) foam, w/ an R value around 10, I think. The foam boards were screwed OVER and PERPENDICULAR to the rafters, and all the seams were sealed with foam (where they contacted the drywall) or metal tape (at joints between boards). I then installed drywall (moisture resistant) over the foam, screwed into the rafters with 2" fasteners. In effect, I went from a R-5 (maybe) ceiling to a R-30 ceiling, and now have a nice cozy bathroom. If you are going to re-install T&G in your cathedral ceiling, you would have to use longer nails, or perhaps add an additional furring strip over the foam to give yourself something to fasten to. Although some of the previous posts recommend blowing in cels, I would suggest tearing out the ceiling to find the true source of the roof leak. Since you don't have access to the interior of the roof through the attic, the only reliable way to find the source of the problem is to go through the ceiling. And keep a can of ant spray handy in case you find a thriving colony of carpenter ants chewing on your rafters (like I did). Good luck.
*Hi Joe,Thanks for your thoughts. I'm sure your systems do indeed work for you, and I expect it is the precision of the air-sealing that makes it or breaks it with Poly/batt/vent systems. I'm going to loop this back around to the question originally brought up in R-Values thread. While a poly/batt/vent setup can be built to effectively insulate and take care of moisture problems (as you, Patrick, and countless others have done) is it cost effective? Will R-20 of DPC give you comparable performace to a poly/FG/vent setup theoretically R-rated higher?Does the venting that is incumbent in a FG system rob it of much of its insulating value? Unless you build the vent chute in such a way that it is air-tight to the FG, yet permeable to moisture (Tyvek was invented for this very purpose way back when for use over loose fill FG--see Gene's book), won't you get some convection currents going in the batts that will rob it of a sizeable chunk of its R-value? Gene, in his book, says research hasn't borne that out, but I think others here have said that it has. What do you think?That's one reason I think it will be interesting for me to blow cells in the 2x6 section of the roof mentioned in my earlier post. I'll be able to compare at least the ice-dam prevention effectiveness of R-20 DPC to R-30 FG/vent on the same house with the same ceiling. Unfortunately, the two roofs have somewhat different sun exposures, so it may not be a completely equal comparison.Steve
*Hi Stevei "Does the venting that is incumbent in a FG system rob it of much of its insulating value?"I don't think venting is 'necessary' with f/g, it was just part of the "received wisdom" (Gene L.)that insulated attic assemblies required venting. I, for one, have stopped venting my roof assemblies, subject to my usual careful air barrier installations. It remains then, to establish whether R 20 of dp cells is equal to or superior to R 30 f/g. Bring on the test results, no anecdotes please!!!-Patrick
*I guess this proves that consistency is a minor virtue.
*Hi JoeNope, first I ever heard of it was here on breaktime.
*
To whom it may concern,
I must say that I'm glad I stayed away from this conversation for the week, as it got a little too steamy in here for my tastes. Thank you to all for your advice.
Joe: I also believe that persistance is important. As far as the system you recommended, it seems to me that if it is not PERFECTLY installed it quite possibly could be a moisture trap. The reason I question it is because of the hundreds of nails that will pierce the 6 mil poly. Granted, I think I'm capable of installing the foam with ease. If the foam is not tight, I could always spray foam into the voids, right?
Jack: Your use of color and keen words of wit are mesmerizing, and down right inspiring, but I don't believe I recieved your opinion on my situation. You seem to lean towards cells without venting, but I'm not sure. I am quite interested in your opinion, and the believed merits behind it. I too am curious about convection and the loss of R through venting.
Steve: I must say I like your ideas and mild manner. I'm not sure I like the idea of cells because I do not have the equipment. Is it expensive or possible to rent? I'm intersted in the test results as well. Please keep us posted.
Patrick: You said you've "NEVER" had any problems with your system of insulating. So now I'm interested. Although, I'm not sure I understand what you use for vent channels. Is it like Tyveck, and do yo have to seal the joints? And again, what about the nail holes in the poly?
Fred: Since your the "expert". What's your opinion?
Andrew: "Peace is not merely a distant goal, but a means by which we arrive at that goal." -Martin Luther King Jr. I thought you might like that.
Per Swede: Of all the advice given here, I lean toward yours. I do this for a few reasons. The first is that I'm not convinced on the merits of not venting. Second, you achieved a good R value with minimal labor. Most of all though, you live with and are happy with your system. How long has it been though?
Again, thanks for the advice, info, and cheap entertainment.
Britt
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From the grill,
Britt...Breaktime is very entertaining at times
that's for sure...your post was such a nice surprise to see and well appreciated
by me and I'm sure by most others as well...
As to what's a good final solution, you've chosen well also. For years
I have built cathedral ceilings in log homes and I can tell you standard
methods are very leaky...The warm air
leaks can be seen where the snow melts on the roof where I build.
Patrick is big on sealing, that's good....per swede seals
with foam over the rafters, not fitted into them, that's good, venting
is good where one expects problems, good.
If you printed out per swede's post and followed his advice, I think you
should be very happy. Remember to make sure you get some idea what your
moisture situation was that started this whole project.
I have an idea for what I think is my "ultimate
cathedral ceiling building system" but am not ready to talk
about it till I really understand it more myself.
Back to the grill for me,
Jack : ()
*BrittYou've caught me in a transition stage. The system that I described, in reply to Steve's post, was one that I used sucessfully, many times in the recent past, but I have recently moved away from it. The vent chutes that I described were formed of continuous strips of Glasclad (R 4.5/"), a 1" compressed f/g board with a Tyvek type facer on one side,cut slightly oversize to fit snugly between the rafters, and only sealed with a bead of acoustical sealant when the fit was sloppy. The ceiling T&G or D/wall is nailed or screwed carefuly into strapping or rafters, with the d/wall being marked for centres before hanging to eliminate damaging the poly.I used to vent my roof assemblies, not because I expected problems as Jack suggests, but because it was thei "received wisdom"at the time, and also a code requirement!!Having long been suspicious of the logic of venting when so much effort is put into sealing the warm side from moisture, I was happy to read the info. that has come from this board which supported my 'secretly held belief' that there had to be a better way. Fortunately my provincial building code has caught up with this change in thinking.My most recent cathedral type ceiling (similar to what you're looking for)involved a retrofit of an attic living space ( knee walls, slope & flat ceiling planes) that already had poorly installed 6" f/g batts and poly. No venting had been attempted in the previous install,(DIY) and no water damage was apparrent, so I removed and recut/replaced the batts in the slopes, and added 2" XPS (R 10 -shiplap)to the underside of the slopes with acoustical sealant on all joints for a total of (R30 theoretically). The flat section received several layers of batts right up to the sheathing,(R 40) and carefully installed 6 mil poly to the warm side, and d/wall. It has survived it's first winter without ice dams!!My next cathedral will be an addition to this same building, with more of ther same type of attic living space and I hope to try out blown in cells for the first time.Whatever system you use, pay strict attention to thei detailsb success is in the details!!!
*Britt, yes, i am all for civil disobedience is the tradition of Thoreau, Gandhi, Dr. King (though his had some wrinkles). Or, more frankly, I'm a pacifist not a wimp. In any event, the ad hominem (personal) attacks are just a waste of time -- leave that stuff to the politicians.Many places that sell cellulose -- here, Hechinger's -- loan the blower for free with purchase. And theirs was in better condition that the one with flash-bang faulty wiring that I rented the other day.I have no idea where to post this, I just like it. Proof that toddlers (mine, Julian, 2 1/2 y.o.) are good for something:View Image
*Pitch a tent!Blue
*The new bathroom ceiling/insulation I described has been up about 6 months. I have not detected any moisture problems (although 6 mo. is not exactly a rigorous test), and I have been watching closely. I just have to re-emphasize that you have to be very meticulous with sealing all seams & joints, both in the foam board (I think I used Typar, by the way) and the sheetrock. If you are putting new T&G boards in the ceiling, I would maybe think about sealing the joints & edges with some kind of clear silicone caulk. Not sure if that would be overkill, but I found a lot of moisture had been trapped behind the boards (and not from the sky light leak) when I tore the old T&G ceiling down.
*And a fine looking young man he is too! Is this photo from a Diet Coke promo shoot?Rich Beckman
*Yeah, Coke for Toddlers. Actually, it would have to be a spot for OJ or lemonade.He thanks you for the compliment! (I missed a better shot -- he tried using a screwdriver. JD is fascinated by screws, bolts, etc. & adores the cordless DeWalt driver. Gotta hide the Sawzall.)
*
Britt: Pat's re-insulating technique sounds good but labor intensive.Possibly a simpler approach would be to simply fill the rafter bays with foam in place polyurethane. No poly. needed. You would have to run a cost comparison.
*
I have an unvented roof with 2x6 rafters and a finished cathedral wood ceiling. Over the years the roof has leaked, damaging the celing and insulation. I would like to replace both of these, and vent the roof in the process. I've considered 2 layersof 2" high R ridgid foam (leaving an 1 1/2" of vent space) or blocking down the rafters atthe expense of losing head room, so that the rafter bays could accept R-38 fibeglass. My question is should I go with either of these souloutions, or any other such variations, or go with R 19 in the roof?