GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Crawl space solution – Joist rot and drying-in questions

tynanch8 | Posted in Energy Efficiency and Durability on

Recently purchased a home in Connecticut with a completely unconditioned, vented crawl space.
No evidence of mold or rot, but some musty odor, and a huge drag on energy efficiency as a fair amount of forced air ductwork travels through the space and air seeps in through the rim joists and vents and stacks upwards into the conditioned space. Space has what I wold describe as a poorly poured rat slab (meaning you can see dirt in certain areas of the crawl that are untouched by the rat slab).

We’d obviously like to improve air quality and energy efficiency and would lean towards making this conditioned space as most seem to recommend.

Primary conundrums are these:
While there is no standing water or evidence of any significant moisture issues in the crawl there is certainly reasonable efflorescence and we don’t want to create any moisture issues by actions we take.
-The joists have no capillary barrier between them and the concrete foundation walls. If we spray foam the joists with closed cell, are we opening the door to rot?
-One provider has pitched what appears to be an impermeable rigid foam wall insulation from floor to joist once floor is encapsulated. Given efflorescence present on foundation walls, are we asking for trouble by limiting the amount of “drying-in” that will take place?

Thanks!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Chris,
    Q. "The joists have no capillary barrier between them and the concrete foundation walls. If we spray foam the joists with closed cell, are we opening the door to rot?"

    A. If you do as you describe, you will definitely be increasing the risk of problems. Whether the risk is small or large is a judgment call. In most cases, it's worth insulating this area. The risk increases if the sill is close to grade, or if the area is shady. If the sill is well above grade, and the exterior gets some sun, the risk decreases.

    It should be noted that you can jack up the building and insert a rubber or metal capillary break between your foundation and your sill. The work is easier than you might think. Here is a link to an article that describes the work: Rubble Foundations.

    Q. "One provider has pitched what appears to be an impermeable rigid foam wall insulation from floor to joist once floor is encapsulated. Given efflorescence present on foundation walls, are we asking for trouble by limiting the amount of 'drying-in' that will take place?"

    A. In general, limiting moisture migration from your foundation to the interior is almost always a good idea. That doesn't mean that your foundation walls are solid and problem-free, of course. If you are worried about the integrity of your foundation, have it inspected by a knowledgeable local expert.

    Here is a link to an article with information that may help you: Building an Unvented Crawl Space.

  2. Expert Member
    Dana Dorsett | | #2

    The joists are resting on top of the foundation, without a foundation sill plate?

    Believe it or not, Martin is correct that it's usually not a very big project to jack up the house 1/16"-1/8" a section at a time with hydraulic jacks to slip in an EPDM membrane or sheet metal capillary break between foundation sill & concrete. If it's joists resting on bare concrete (no foundation sill) it would be even easier.

    But if you have at least a foot of above grade exposure of the foundation on the exterior, and there is no efflorescence visible above grade, the risk to the timbers is pretty low anyway, since the concrete will dry toward the exterior. Deep roof overhangs to keep the drip line splashback off the concrete helps too, but isn't absolutely essential.

    IRC 2012 code-min for CT / US zone 5 crawlspace foundation wall would be R15 continuous insulation or R19 if between joists. (see: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm ). To do that 100% as closed cell spray foam would be fairly expensive at 17-19 cents/ R-ft^2. Most ccSPF needs to be installed in lifts no deeper than 2" to avoid adhesion, shrinkage or fire hazard issues as it cures, and you'd need about 2.5" to hit that. Doing it with all- EPS would be bit more than half the cost, but you'd still want to use spray foam to air-seal the band joist to your wall foam. Rather than spray-foaming the walls It may be cheaper to put 1.5" EPS (R6 ish) against the foundation and put a non-structural batt-insulated 2x4 wall to the interior side, with a gypsum interior covering as an air barrier. At 1.5" you would have sufficient foam-R to be able to use high-density R15 batts to fatten out the R without creating a condensation problem at the foam/fiber boundary in your climate. The economics of it may depend on how much of the insulating portion is DIY vs. contracted out (and how much you discount your time.)

    An all-rigid-foam solution can be cheaper still, if you use reclaimed roofing foam (at about 2-4 cents/ R-ft^2). There are several vendors of reclaimed foam operating in southern New England.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |