Spray foam aginst roof acts like vapor barrier
I have a new construction house in Wyoming, that we sprayed 2″ foam against the roof and walls. Then in the ceiling filled up the 12″ Trusses with blown-in insulation. After drywall and texture we noticed moisture against the foam, and with winter, it freezes. We have been waiting for it to dry out for 2 months.
I opened some air holes along the soffit , and there is a vent on the ridge of the roof. Its drying slowly but I was wondering what to do.We need to put the tongue and groove in but wondering what will happen. I was told to put plastic up before the tongue-and-groove for a vapor barrier, but others tell me it will cause more problems. This is a great resource, and thanks in advance for any advice.
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It sounds as if your problem is indoor moisture passing freely into the assembly and condensing on the face of the sprayfoam which, because it is behind 12 full inches of cellulose is still quite cold (closer to the exterior temperature than the interior). A good rule of thumb (I think it's been stated on this site a few times) is that, in heating climates, doing a "flash and batt" system like this, you want to have about 2/3 the r-value on the cold side of your vapor barrier (which in your case, is the sprayfoam). So, it's kind of not fair. You did such an awesome job insulating that you created a condensation problem on the face of the sprayfoam.
Nice to know now, you say, now what? First I think I'd run a dehumidifyer in the top floor to drive down the humidity. That will help dry the assembly and alleviate the problem somewhat. Then, gulp, I think I would take down the tounge and groove and put up drywall and use the Air Drywall aproach aspoused by the building science corporation. Then if you want the look of the T &G you can put it back on over the drywall.
ONe could argue that a sheet of poly will do the same but I'd argue that that sheet of plastic will trap teh moisture inthe asssembly. I don't have enough experience to reccommend those smart membranes. Maybe someone else can chime in on that. Good luck
Bill,
You might be interested in reading the comments to this thread:
https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/15859/i-have-built-home-lower-new-hampshire-about-hour-north-concord-us
In my January 10 post, I include a long quote from an EDU article that documents problems like yours — condensation on the interior face of spray foam installed as part of a flash-and-batt job.
Caveat emptor.
Bill,
Are you saying that you spayed the underside of the roof sheathing with foam and then filled the flat of the ceiling below with cellulose? I assume that's the case or you wouldn't be able to see the condensation on the foam. If the flat ceiling is the thermal boundary, then insulating a ventilated roof deck contributes nothing but problems.
And do I understand correctly that you have a ridge vent but, until you "opened some air holes along the soffit" there were no intake vents? If so, the high vents will create a negative pressure in the attic and draw moist air through any openings in the ceiling of framing. There has to be balanced (equal net free vent area) low and high vents to prevent that.
Oh, didn't see the assembly like that. Thanks Robert. I had assumed you could see the frost because you opened up and investigated a rafter cavity. Clarification here might help others help you. In this case Robert is right, I think you need to vent that attic well (high and low) se if that fixes it. Then, if the problem persists or you'd like to improve the situation, a vapor retarder should be installed under your attic insulation. This could be in the form of sheetrock or 6 mil poly (meaning that you'd probably have to have the insulation company vaccuum out the celulose then put it back in after) but that depends on exactly what you have for your ceiling assembly.
Thanks for the input. Let me clarify were Im at. 2" of foam was sprayed against the top of the roof and walls. Then the trusses were filled with cellulose, and is held there with netting. we were getting ready to put up the T & G when we noticed moisture against the foam. There is a vent at the ridge, and I put holes in the soffit end to get some air moving thru. I figure theres a little space in there from setteling? recently I have been able to open some windows and let some air flow thru and it seems to be thawing and drying out. Thats were Im at, and hopfully this will let me finsh the T & G . I dont think there will ever be that concentration of moisture again but I will have to monator it. Another thing some one mentioned was to put a vapor barrier in the crawl space and Insulate under the floor. Thanks again for the help .
Bill,
You've only muddied the waters further. What do you mean "foam was sprayed against the top of the roof"? The roof deck was insulated from above? Or was it foamed underneath the roof sheathing?
And what do you mean by "the trusses were filled with cellulose"? Roof trusses are generally big triangles with the top chords being rafters and the bottom (horizontal) chord serving as ceiling joists. Perhaps you mean attic trusses or scissors trusses with two more or less parallel chords forming a slanting roof and a slanting (cathedral) ceiling.
I'm guessing, now, that you have some kind of cathedral ceiling truss and have roof sheathing then foam then cellulose blown tight to the foam and then you're planning to install tongue and groove boards as the ceiling. Is that correct?
If that is the case, then there should be no roof vents, since the only thing worse than a "hot roof" is an insulated roof with vents to let moisture accumulate in the insulation. There should be no settling of dense-pack cellulose and, even if there was, putting ventilation air between the foam and the cellulose eliminates the value of the foam as insulation (all the foam can do in that case is to keep moisture in the roof decking until it rots).
And, if you're installing a T&G wooden ceiling under fibrous insulation with open vents, then you absolutely have to install a perfect air/vapor barrier before installing the wood.
It sound like someone's got you in trouble with that assembly.
Ok Bob , Let me try again , The roof was sprayed w/ 3" of foam from underneath to the sheathing. The trusses are a cathedral Ceiling truss , 12 - 14" deep that was filled up with a blow in FIBERGLASS insulation held by netting ( sorry for the wrong info). Then to put T & G up under this.
What would you recommend, any info is appreciated
Bill,
If I understand you correctly, you have no air barrier between the conditioned space and the fiberglass insulation. If you have any small air leaks near the ridge — most roofs do — then air is passing continuously through the cracks between your T&B boards, carrying moisture with it. The moisture is condensing on the first cold surface it hits — the inner surface of the cured spray foam.
You'll have to remove all of the T&G boards and install an air barrier (for example, taped gypsum wallboard) under the T&G boards. If the T&G boards are securely nailed, it may be cheaper to abandon the boards, put the new air barrier on top of the existing boards, and then install a second layer of T&G boards.
The part about the ventilated space has me boggled....
Are you ventilating the space between the foam and the "blown-in"?
This is very confusing.
Can you post a link to some photos?
What I am understanding Bill to say is that the T&G boards are not yet installed, and that he has a ridge vent. Whoever designed this system didn't know what they were doing. I would suggest bypassing the foam and creating a ventilated attic with plenty of soffit vents and the existing ridge vent.
With a ventilated attic, simply installing 6mil poly on the underside of the attic floor joists (below the existing fabric) will function as a vapor barrier and the T&G boards can go right over that. There is a chance that water vapor could condense against the poly and stain the T&G boards, but with good air sealing, no recessed lights, and an additional layer of fiberglass blown into the attic, it should not be a problem. If it is a problem, the source of the moisture should be addressed. Drywall primed with vapor-retarding paint could be used behind the T&G wood instead of the poly film, and may perform slightly better.
Hello Bill,
I am a spray foam contractor in California, http://www.sprayfoam.biz , please contact me directly at (916) 608-1847 and after a few clarification questions, I can help you correct the problem or understand how completing the assembly if not already done, can eliminate the issue. We do not work in your area, But I want to make sure the spray foam does not get a bad name, as the stuff really work well if correctly installed...