Request info on Dow Corning's FORMULAR rigid polystyrene insulation (any
Thu, 13 Feb 1997 11:45:49 -0500 (EST)

Arnie wrote "1)I don't know much about how much FOAMULAR or other rigid foam
insulation boards outgas, or over what period of time. I am confident,
however, that neither concrete blocks nor the mortar between them
provides a very effective air barrier, and that the air barrier properties of
that assembly will deteriorate over time. So, if the goal is to seal the
space from the foam, you need to install an air barrier. Properly sealed
drywall, for instance, is a good air barrier."

I agree that concrete block is not an air barrier. In field testing the air
permeability of concrete block I have found that it varies substantially from
batch to batch and by type. I have frequently found that a single 8x16 block
has an air flow leakage that behaves about the same as a 3/8 inch diameter
hole. A coat of paint or two reduces that by an order of magnitude. Check
with Bruce Henshel at the EPA indoor environment group at RTP, North Carolina
- they did some pretty rigorous lab testing on the permeability of block
walls a few years back.

I do not like making an air barrier in a plane that is separated from the
insulation layer. Over the years I have noticed that many energy, moisture
and air quality problems are the result of unintended airflows through
passages of the type created by this situation. In the block-foam-block
wall, I'd make the foam layer the air and thermal barrier. That can be done
effectively and durably enough using polyurethane caulks (both contain
compounds which may trigger chemically sensitive folks) and canned foams.
Tape on the interior surface of the foam would work well if moisture is kept
out of the cavity (moisture barrier to keep soil moisture from wicking up the
wall, inner finishes highly permeable so the cavity can dry to the inside). -