Sealing Rigid Foam Board


Old 01-14-09, 08:17 AM
Thread Starter
Join Date: Oct 2007
Location: West Quebec - Gatineau Hills
Posts: 23
Sealing Rigid Foam Board

I have a 40' x 8' room under my front veranda. It is constructed of poured concrete, like a house foundation and in fact is part of the basement. The exposed floor of the veranda is a 4" poured slab, which is the ceiling of this room. A few years ago I finished this room to make cold storage, by glueing 1.5" rigid foam boards to the walls and ceiling. I then studded with 2x4's and filled the cavities with fiberglass batts and a 6 mil poly vapour barrier. MISTAKE!! Now due to trapped moisture between 6 mil poly and foam board, I have to tear it all out to remove the 6 mil poly and fibreglass to remove a moldy smell caused by the double vapour barrier. In one section of this room, the studs became moldy, so they have all been removed, exposing the foam board.
I am about to add spray foam to the top of the foam boards and Tuck Tape seal all of the joints of the foam panels. My question is: do I spray foam the joint of the foam panels where they meet the concrete floor, thereby sealing the top and bottom? I kind of think I should, to completely seal any warm moist air from getting behind the rigid foam panels, but want to confirm my thoughts before I do something that cannot be undone.
Thanks for any input. Need to get this room done as it is 40 below zero today where I live. Brrrr.
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Old 01-15-09, 11:44 AM
Join Date: Dec 2008
Location: michigan
Posts: 30
I'm doing roughly the same thing in my basement, minus the 6 mil poly. The thing is... you want your room to breathe, if you use something 100% impervious to moisture, moisture will get stuck behind it, as you unfortunately found out.

If you use spray foam to completely seal all the cracks/edges of the rigid foam, you are making it near imprevious to air flow, which means moisture can condense behind it... BUT, since you appropriately do not have anything organic behind it (the foam is slapped up against the concrete), thats okay.
Old 01-19-09, 01:24 PM
Join Date: Sep 2008
Location: United States
Posts: 10
Aspen or anyone else,
I'm currently getting ready to do the same thing in my basement. I'm having trouble deciding the best way to do the insulation. I have a poured concrete basement and I live in Kentucky. I was thinking of attaching Dow Tuff-R rigid foam directly to the walls with foam board adhesive, then stick building my frame with 2x4s and lay them directly up against the foam boards, attaching the top plate to the floor joists and bottom plate to the floor.

I don't think I'll be able to add any fiberglass bats between the studs because I am turning them on their side to gain room around existing fireplace, french doors, etc. I'm not planing on using any plastic over the studs and will just put up drywall. Do you see any issues with doing it this way? Would it be better to frame against the concrete, then attach foam boards to studs, followed by drywall? Do I need breathing room behind all of this?

Old 01-20-09, 02:47 PM
Thread Starter
Join Date: Oct 2007
Location: West Quebec - Gatineau Hills
Posts: 23
I am not an expert, but have had some experience with doing what you are planning to do. I glued rigid styrofoam panels to the concrete walls and ceiling of a cold room under the front veranda of our house. The floor of the veranda is a 4" thick poured slab, and the walls are 8" poured concrete. I then added 3 1/2" fibreglass batts plus a 6 mil poly vapour barrier. This I have found out is a big NO-NO! Use as much thickness of styrofoam as required for insulating R value, 1" = R5, 2" = R10 etc. Do not add fibre glass batts, as this changes the cold air/ warm air interface, where the foam is the vapour barrier. Basically, the vapour barrier must be on the warm in winter side. If you add fibre glass batts, the vapour barrier (the foam board) is now somewhat buried inside the wall construction, no longer on the warm air exposed side. The fibre glass is now on the warm in winter exposed side.
I am now going through an expensive and time consuming removal of my wall construction. I had lots of condensation on the fibre glass side of the 6 mil poly vapour barrier, causing the fibreglass batts to become wet and moldy smelling. All of the 2x4's became moldy and had to be removed, along with all of the 6 mil poly and the fibre glass batts. Another thing I have learned through all of this, is to spray foam (from a can) all of the edges of the foam board where it may be exposed to cold air to prevent warm moist air from getting in behind the foam board and condensing. Use Tuck Tape to seal all of the joints between panels to ensure that no moist air can get by and touch the cold cement walls. Tuck tape is what we call the tape used to seal house wrap. I also spray foamed all of the bottom perimeter to be absolutely certain that no moisture can get in contact with the cold cement walls, thereby virtually sealing the room from moist air contacting the cold cement. I live in a much colder climate than you do, so this step is a must for me. Last week it was 40 below zero F, and that is cold. I have also just been though a $15,000 mold remediation in the rest of my finished basement in an 8 yr old bungalow, where I removed all of the wallboard, insulation, laminate flooring, beds, clothes, ceiling tiles, and virtually anything else stored in the finished basement and carted it ALL to the dump. Had a company apply spray urethane foam to the exposed walls, added a heat recovery ventilator with a HEPA filter, changed the grading outside, treated for mold contamination, had indoor air tests done, and refinished the 2 bedrooms in the basement. Not hard to spend big bucks correcting a mold problem, far better off preventing it. Hope this helps.

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