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I am considering closed cell foam insulation in my attic renovation. I live in a 85 year old home that has 2×4 roof rafters that bear directly on the top plates making the introduction of soffit vents very difficult if not impossible. The limited depth to the 2×4 is also problematic for the introduction of batt insulation; even if I were to scab on a 2×2 the introduction of a baffle and 1 inch air gap would only allow for a R-19 batt. I am reluctant to believe the claims of the spray foam insulation manufacturers (i.e. no need to ventilate or introduce a vapor barrier) however; my problems would be solved if their claims are true. Can you please help me obtain the answers to these questions?
1. Is spray foam with out ventilation an acceptable insulation strategy in Wisconsin?
2. Is the foam sufficiently moisture resistant to prevent indoor humidity from reaching the dew point internal in the insulation?
3. Is there a potential problem with moisture migrating from the exterior and being trapped by the insulation leading to rot of the roof framing materials?
4. The manufacturers claim that the temperature difference between ventilated and unventillated roof is 2 – 4 degrees (I find this hard to believe), but they report the test in their literature; what do you think is the affect on shingle life?
5. Do you know any differences / preferences between the Icynene and Corbond manufactures of spray foam insulation?
6. Has FHB written an article on this subject? I am aware of the FHB issue #100 article which is excellent but does not address the ventless application of spray foam.
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Doug, There are many others on this site with strong feelings about venting and not venting, I'll let them weigh in. But I'd point out that in a 85-year-old house, you should only pursue low-hanging fruit in terms of energy savings. Going whole hog one or two components won't pay because air and heat will just leak out else where. But the roof IS the most important place to put insulation. There are foam spacers for providing an air gap before spray-in-place foam is applied. I don't know how thin they are available. Maybe there a 1-inch spacers and that would leave you 3 inches for spayed in place foam. Could you place strapping on the 2x4's "inside" the room and fill the bays with 1 or 2-inch rigid foam? Can you afford to lose 3 inches of head room?
Shingle life: Cool shingles live a lot longer. North roofs last longer (less heat and UV). Look at a 10 to 15-year old house with its original roof. But the biggest shingle killer is walking on them when they are hot or cold. You can sometimes see the paths taken by workers or kids years later if it was done above 80F (plus solar heating) or below 20F.
*Doug,You seem to have a bit of a unique situation with your house.I lean very strongly towards David's call on this one. Another option, seeing that you are maybe resigned to the idea of dropping the ceiling by 2", maybe to use rigid insulation, SM. But not the regular shiplap type, what I'm thinking of is the one designed for finishing basements. It has a recess in it for wall strapping. so that you have a continuous wall of SM, the strapping that fastens the whole assembly to the ceiling, sits neatly into the SM giving a smooth flat surface. Then you just drywall over the whole thing.This would allow you to add another 2" of SM between the rafters, if you wished, and still maintain good ventilation for your roof's membrane.
*I agree Fred,Often a well sealed and insulated addition will lower a houses energy usage even tho there is more square footage.By the way, do you have the R value for DP cellulose? I can only find it for regular fill.Ron
*For more information to fuel the controversy on ventless attics check out the links. It is OSB bonded to rigid ISO-board that celotex says can be directly shingled over with their shingles. NOTE - I could not find one reference to a reduction in shingle warranty when using their shingles installed in this manner.http://www.sweets.com/index/mfg/2356/P11501.htmhttp://www.celotex.com/products/commercial/hythermnailfr.htm-Rob
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Fred
What is the RV of SPRAYED on celulose? Loose-fill cels?
Even at 3.8 per inch, you don't get much RV in a 2x4 (3.5") wall. I understand the pluses of cels packing every nook&cranny, esp compared to the dreaded fibreglass. But 13.3 RV (3.8 * 3.5") doesn't sound like much.
Aren't there minimum requirements for RV in a wall, something like R19? Or does that include the sheathing, siding and gypsumboard?
Curiously,
islander
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Fred,
Funny you should mention backplastered houses. I was restoring the exterior of my parents' 1893 Victorian in Mpls. last summer. I was suggesting we do some weatherizing at the same time. My mother said that the weatherization folks had been out and said the walls weren't worth doing anything with, but she couldn't remember why. So I suggested we look at the attic. She said no, the weatherization folks said that was already fairly well done.
Well I had the walls open, and they were backplastered, leaving, as you said, spaces no deeper than about 1 1/2 inches. And the the attic had some kind of wooly type of stuff under the laid floorboards. My mother said they spend about 1200 bucks/yr on oil. This is for about a 300 SF house where the design temp is -20 I think.
It seems the "low-hanging fruit" has been plucked, but I would think they could do better than 1200 dollars worth of oil. More air sealing maybe?
Steve
*Doug,I have experience with Icynene but not Corbond. Do you have any info such as a web site for Corbond? I may be able to give you an educated guess of the differences.
*Fred,Yes indeed, they have triple track storms. Recently replaced about 25 percent of the windows with double paned. (I tried to talk them out of that, but mom wanted the operability and easy cleaning, not just fuel savings).The location is indeed sheltered. Last summer when there were horrific winds and tornadoes and record amounts of trees down all over Mpls. I had left ladders up against the wall, stacks of siding laying around, etc. that went completely undisturbed by the storms.There are two chimneys that breach the attic floor. I neglected to check them for seals. The access door to the attic is weighted and sealed.Steve
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Ron,
The Corbond site is located at
http://corbond.com/
I look forward to your comparison!
P.S. Thanks to all that have replied thus far; I admit I am as confused as ever. My greatest concern is applying a foam product in direct contact with the underside of the roofing deck. I would fear that any moisture that migrates through the shingles would be trapped in between the impermeable surface of the foam and the wood of the roofing deck leading to rot. One idea suggested to me was to insert 1-inch wood spacers and a one-inch layer of extruded polystyrene between each set of rafters. Then the foam can be applied to the underside of the polystyrene. This will necessitate the addition of a 2x2 on to each rafter to create a depth of about 3 inches for the foam.
I would appreciate any comments on this proposed solution. Thanks to all!
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Doug,
Corbond seems to be a polyurathane closed cell foam. It has about 50% higher R value than Icynene. It is stiffer than Icynene. I would suspect that it would take longer for the initial smell to go away. It can't be blown into enclosed cavities. Icynene can be blown into enclosed cavities. Both seem to bond well to wood. If you use a spray foam of any kind, have the contractor shoot the band joist in the basement while he is there. Get prices from both and from another PUR contractor before you do anything. Since you have a thickness issue, I would lean toward Corbond or one of the other polyurathanes. I would also think about a 2x2 or 2x3 if you have the headroom. If there is a flat part to the ceiling, you may want to have a thin coat of foam sprayed above the sheetrock to seal it and then put in loose fill insulation above it. Or if the cost isn't too bad, spray in 6 or 10 inches of foam.
Ron
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I am considering closed cell foam insulation in my attic renovation. I live in a 85 year old home that has 2x4 roof rafters that bear directly on the top plates making the introduction of soffit vents very difficult if not impossible. The limited depth to the 2x4 is also problematic for the introduction of batt insulation; even if I were to scab on a 2x2 the introduction of a baffle and 1 inch air gap would only allow for a R-19 batt. I am reluctant to believe the claims of the spray foam insulation manufacturers (i.e. no need to ventilate or introduce a vapor barrier) however; my problems would be solved if their claims are true. Can you please help me obtain the answers to these questions?
1. Is spray foam with out ventilation an acceptable insulation strategy in Wisconsin?
2. Is the foam sufficiently moisture resistant to prevent indoor humidity from reaching the dew point internal in the insulation?
3. Is there a potential problem with moisture migrating from the exterior and being trapped by the insulation leading to rot of the roof framing materials?
4. The manufacturers claim that the temperature difference between ventilated and unventillated roof is 2 - 4 degrees (I find this hard to believe), but they report the test in their literature; what do you think is the affect on shingle life?
5. Do you know any differences / preferences between the Icynene and Corbond manufactures of spray foam insulation?
6. Has FHB written an article on this subject? I am aware of the FHB issue #100 article which is excellent but does not address the ventless application of spray foam.
In upstate NewYark what is the depth of sprayed foam in an exterior wall enclosure to maximize the dollar value for the insulative properties?
I mean will 3 inches do the job to the reasonable maximum and any additional inches will provide little more worthwhile to the total R needed?
Hi rez,
In upstate NewYark what is the depth of sprayed foam in an exterior wall enclosure to maximize the dollar value for the insulative properties?
We certainly have a much milder climate in central NC but here is what I'm seeing done on a budget and still getting great benefit.....they're framing 2x6 24" oc, corners get 1/2" treated plywood with 1/2" closed cell foam board over that...the rest gets 1" closed cell foam board.
Once up they spray about a 1/2" of closed cell foam inside which seals every little crack and crevice.
Then they use unfaced fiberglass batts to fill the void.
Getting about R-28
View Image
You little devil - did you know how old this thread was?;)3"
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Ya, I did just stumbled on in in a thread search
and thanks Pedro.
Actually I was serious in the question in that a homeowner up here is sistering 2x6s to gutted exterior to be resheathed and then hardibacker claps.
I got to wondering about the benefit of the additional inches in a 2x6 wall remembering an old post with a spread sheet dealing with foam depths
but just found out that the sprayfoam insulation he will be using is Icynene with gives an R-20 in a 2x6 so no guessing on the practical side of a full bay.
and I did such a nice presentation there...Sigh.I just happened to be on the Tigerfoam site because I was making an order with them and they have some good direct answers too in their FAQ sheets.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
good questions Doug
1 - I don't know how acceptable it is to the local AHJ, but it certainly works well, if installed properly and if you use closed cell foam such as Corbond. Further, Corbond has domne extensive testing and documentation to be able to help many jurisdictions find it acceptable where before, they had been still requiring venting et al.
2 - closed cell foam is a VB. Open celled like the Icynene is not.
3 - if moisture gets in from outside, you have another problem that needs a solution, regardless what kind of insulation you use.
4 - There have been tests performed on shingle life with 'hot roofs' Actuall varies with your home's latitude and orientation, but the asphaltic shingles will suffer a shorter life by approx .2% or so, a few months in the expected lifespan is all.
5 - Definitely seek out closed cell polyurethene like Corbond. It has nearly twice the R-value as open celled types and has the VB aspect included.
6 - try the advanced serach button here at BT for far more discussion and information than you will find in the print issues.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
I would say fill the cavity. While there is the concept of the law of diminishing returns as we often refer to it and which you ellude to, it's not what should guide you. One inch of foam insulation may cut heat loss by e.g. 75% and the next 2 inches may cut heat loss by say only another 15%, you don't stop at 1 inch. The long term economics are worth it. It takes less $ to install the second and third inch. So that is why we install e.g. R-21 batts in walls and R-38 in ceilings ... because the installed cost is reasonable for the long term performance.
With foam, conceptually, I'd install the same R-value in the ceiling and in the wall, I think. The economies of the install probably have that limit (arguably). The reason we've generally gone from 2x4 to 2x6 construction is that economically, it works even though an R-11 batt cuts heat loss by say 90% while the R-21 cuts it only another 3-5%. But we don't just stick R-11 in a 2x6 wall ... we fill it ... because economically it works.
Ah...never mind.