Posted By simonzeke on 23 Aug 2010 03:54 PM
Hi everyone,
Somewhat new to the forums and learning a lot! I am about to start insulating the interior of my basement in a 1956 house in Nebraska. I have two questions. What is a good adhesive to use to glue unfaced polyiso to a concrete block foundation wall and should I use the same adhesive to glue two panels together (is there a spray adhesive to get better thermal connection between panels)? Where do you buy these products? I have 1 1/4" panels and will have two layers. I have heard some adhesives will eat away polyiso. Also, at the floor of the basement, I have old tiles glued to the concrete floor that have asbestos in them so I am not going to disturb them. My question is if I leave a narrow gap at the floor (maybe a half inch or so and fill that in with sprayfoam to get a better air seal, will that be better than having the panels directly on the floor? I am going to be taping seams and staggering the seams of the second layer to minimize thermal bridging. The foam board will be covered by a traditional non-load-bearing stud wall with little or no insulation in the wall cavity - just drywall to finish the wall and meet fire regulations. So, I guess that is 4 questions but thanks for your input!
Simon
I'm assuming you mean fiber-faced, not
UNfaced iso(?).
Only use foam-board construction adhesive- others are likely to have solvents that degrade/dissolve the foam. Walnut-sized blobs on a ~18" grid are sufficient to get it to stick. You can either try to glue the butted up sides with the adhesive, or leave a ~1/4" gap between them to be sealed with single-part foam (Great Stuff, or similar) and skip the taping- just plane the excess flush before applying the second layer. Stagger the 2 layers by at least a foot, and use smaller blobs of adhesive to minimize the air gap between the layers. (Foam board construction adhesive is available at the big blue & orange box-stores.)
You can either hold it permanently in place in a fire-code compliant manner with furring through-screwed in to the concrete on which to mount the code-required ignition barrer, or lock it in place with your studwall. 2.5" of iso is ~R15- adding un-faced batts to the studwall may still be worthwhile if you go that route vs. furring. A 2x3 studwall & unfaced R8 batting would bring you to over R20 at low cost with minimal intrusion into floor area. Even kraft facers are too vapor-impermable to be used here. If unfaced R8 isn't available, the cheap unrated batting used for sound control in 2x4 partition walls would still deliver ~R8 when compressed into a 2x3 cavity (~R11-ish in a 2x4 studwall.)
Before gluing anything up though, seal the foundation with a silane or acrylic masonry sealer to minimize the capillary draw of ground or rain-moisture through the concrete into the interior. This will enhance the bond of the adhesive. Use only vapor permeable sealers though- nothing with a perm rating less than 2. (Most are quite permeable, but a few aren't.)
DO leave a gap between the iso and the floor. Bare iso is somewhat hygroscopic and it CAN wick up water. Filling the gap with one-part polyurethane foam (which ISN'T hygroscopic) gives it a capillary break. (Use a sill gasket under the bottom plate of the studwall too, for good measure.)
If you're going to paint the wallboard, stick with vapor-permeable latex/acrylic. The foundation needs to dry toward the interior, otherwise the moisture level in the concrete can get high enough to cause efflorescence/spalling conditions on the exterior above grade and/or cause the foundation sill for the first floor framing to rot, as well as create mold & rot conditions in your basement studwall.