Search the forums


Basement vapor retarder

graphein's picture

We live in Zone 5, a cold climate, so expect our walls to dry, mostly, to the outside.  Vapor drive is from warm to cool, that is, generally from inside to outside in the colder months.  In summer, this can reverse, presumably, but infrequently--we don't require air-conditioning often--and without serious consequences.  Below ground, however, the story is different.   We excavated to the basement footings, installed drains, ran them to a pair of sumps, applied surface bonding cement (SBC), backfilled with washed 2B stone, then regraded the site to establish a minimum 6 inches in 10 feet slope away from the foundation.  Bulk water has thus been dealt with and air infiltration through the concrete block is negligible.  What remains are problems associated with vapor and with temperature. Ordinarily, I would assume the basement could only (or mostly) dry to the interior.  However, our top five courses of block, or about 40 inches, of half the foundation is exposed to the outside.  SBC is vapor permeable to a degree, so some drying to the exterior occurs.  Where the foundation is not exposed to that degree, it is exposed to some degree, at least the code minimum of 8 inches.  Moreover, I now know that few, if any, of the block cores were filled with concrete when the foundation was laid.  Air circulates freely through these empty cores.  (A chipmunk taught me that.) Question:  I wonder whether the application of a nearly vapor impermeable layer, i.e., FoamGlas, on the inside surface of the block could create adverse conditions.  (Like, could I rot my mudsills at hyperspeed?) Such a layer would (1) reduce the intrusion of vapor and would (2) prevent movement of interior vapor into the block wall while (3) providing a measure of insulation, now lacking.  I understand that additional insulation might prove necessary to prevent condensation. Or am I about to get whacked on the side of the head with something I haven't even considered?

You're on the right track. (post #207234, reply #1 of 2)

You're on the right track. Your mud sills are above grade, so the block wall that they rest on is not going to transfer ground moisture into them.

If you ever furr out the basement walls to finish rooms, you probably would need extra insulation to prevent interior moisture from comdensing on the foam glass.

Thanks. I follow you. The (post #207234, reply #2 of 2)

Thanks. I follow you. The block cores act like chimneys: I can feel the air (carrying, no doubt, moisture) coming up through them. But I'm capping the cores and considering adding a layer inside, Foamglas, that will prevent exterior vapor and whatever little bulk water gets into the block from drying anywhere but to the outside. In winter, Foamglas will stop interior moisture from migrating into the block, so, overall again, less moisture in the block, but the moisture that remains has only one one way to get out. Fingers crossed it's enough of a way out.