Replies

1. GBA Editor
   Martin Holladay | Dec 19, 2012 02:46pm | #1

   Brian,
   No, you don't want to install "an electric crawl space vent in the exterior wall to vent the air in the space between the foundation wall and the interior wall."

   I don't recommend that you install any fiberglass batts in a basement. I recommend that you install a little bit more spray foam. Then you're done. If the interior air is warm and dry, as it should be, you're all set.

2. Expert Member
   Dana Dorsett | Dec 19, 2012 06:05pm | #2

   With the membrane as a vapor barrier and capillary break against ground moisture it's not clear if there's a good rationale for using closed cell foam here.

   With and all open cell solution you might get a modest amount of moisture accumulation in the exterior over a colder than average winter, but it would dry quickly. If there's a significant chance of a flood it could be an issue, but it's less of an issue than with the fiberglass-batt portion of your proposed solution.

   Anywhere the spray foam would hit bare concrete (rather than your dimple-mat or whatever) you'd be better off with close-cell foam, but since there's an air gap it's OK to use the open cell over the seams. So you may want to seal the foundation sill to the top of your membrane with FrothPak or something before doing the rest if there is some bare concrete showing there.

   The 1.5" of closed cell foam alone is going to run ~$1.50-2 per square foot, which would be enough to buy you 5" of open cell foam and you could skip the batts entirely.

   Be sure to put some rigid EPS or EPS foam under the bottom plate of the finish studwall to avoid wicking up moisture that might find it's way to the slab and to keep the framing closer to the room temp than the slab temp, for ultra-low mold potential.

   If you stuck with the cc foam and batts plan, the batts have to be snug to both the foam and the gypsum to be effective, and "...finished walls will be set off of the foundation walls by anywhere from 5 to 10" leaving an air space behind it..." sounds like that would be impossible.

   Continuous coverage of 5" of open cell foam would buy you ~R18 clear-wall R, whereas 1-1.5" of closed cell would be half that. With R13s in a snugged-up 2x4 wall you'd be back at R18 after thermal bridging, but it would be more expensive, and less reliable, since any air leakage or compressions/gaps or air spaces between the fiber and foam would result in a performance hit.

   To get to R18 with closed cell foam would be over $3 per square foot. It would be more flood resistant, but beyond that there's little advantage. If there's a high-water mark you're sure never to exceed, you could install 4-5" of rigid EPS and seal the top edge of the EPS with the open cell foam- it sticks to EPS fairly well.

   As long as you don't use vinyl wallpaper or something ridiculously vapor-retardent on the finish wall it will dry to the interior just fine without venting.

   The siding and sheathing type might matter- I'm assuming it's a typical PNW stackup of doug fir or hemlock ship-lap or straight plank sheathing (or worst, case, plywood, not OSB), with wood shingle or clapboard siding. In the unlikely event that it's stucco or brick you may need to flash-foam the wood with 1" closed cell rather than going with an all open-cell solution. If it's vinyl siding or fiber-cement versions of clapboard/shingles it won't need cc foam.

   This is a Seattle-specific recommendation. In colder climates there's greater risk of performance-reducing levels moisture accumulation in the open cell foam or the wood sheathing in the pony walls from interior moisture drives.

   Monitor the relative humidity in the basement- at Seattle's outdoor dew points it can be brought to 45% @ 70F or less almost any day of the year by outside ventilation. If it's staying above 40% RH @ 70F in winter due to your ground moisture issues it's worth dehumidifying to that level with a 70 pint room dehumidifier set up to drain into a sink (or your perimeter drainage system) to bring it under 45%. Yes it's a plug load, but a dehumidifier converts a latent cooling load (humidity) into a sensible cooling load (heat), which is exactly what you WANT in your basement in winter, and it does it at heat-pump efficiencies. With an open cell foam solution letting the basement RH linger at 55-60% all winter it could get ahead of the ability of the sheathing to dry toward the exterior. It's not very likely that the uncontrolled basement humidity would run that high in winter, but it's a good idea to keep an eye on it, "just in case", given your known groundwater conditions. (A ~$10 battery operated AccuRite does a perfectly adequate job of monitoring humidity, available at some box stores.) From March through November you can let it run at 50% if you like, but under 45% would be better in winter. (Most homes in Seattle will run under 45% RH year round without mechanical dehumidification.)

   In real-world field tests open cell does just fine even in unvented attic roof decks in that climate:

   http://www.ornl.gov/sci/buildings/2012/Session%20PDFs/36_New.pdf

   Barring a masonry or stucco siding, it would also do even better in your pony-walls, given the better drying potential toward the exterior.