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Q&A Spotlight

Details for a Closed Crawlspace

A homeowner ponders insulation and ventilation specs that will help keep his house comfortable and the crawlspace free of problems

Upgrades to a crawlspace often include installation of a moisture barrier and insulation, as is the case with this one in Lakewood, Colorado. A GBA reader in Climate Zone 4A is trying to sort out the insulation details for the crawlspace under his home.
Image Credit: Dennis Schroeder / National Renewable Energy Laboratory

Emerson has moved into a house in Climate Zone 4A, a region where humidity can be relatively high. As Emerson explains in a Q&A post, the crawlspace beneath part of the house has been closed to the outside, and now the question is how to insulate the space to help keep the house comfortable.

Emerson refers to an article on this topic posted by Carl Seville in 2011, and in particular to a comment from Ted Clifton who advocates the addition of insulation in the floor above the crawlspace as well as walls, and possibly the crawlspace floors. Clifton’s approach is aimed at keeping warm, humid air away from cold surfaces to prevent condensation and the moisture problems that would follow.

“I have read with interest Ted Clifton’s comments on closed crawlspaces in which he advocates insulating the walls and the floor (comment #9),” Emerson writes. “But I’m unclear on the recommended R-values for floors.”

Emerson has added a polyethylene vapor barrier over the concrete floor, and 2 inches of rigid foam insulation on the walls. In considering Clifton’s recommendations, Emerson is thinking of adding R-38 of batt insulation between the 2×10 floor joists above the crawlspace. Stone walls (R-3) on the first floor help make Emerson’s bedroom cold.

The crawlspace is adjacent to the conditioned and finished basement, connected by a 2-foot by 3-foot open grate. “The crawl is notably cooler than the basement, and our bedroom above is also chilly,” Emerson says. “So, we were thinking of double insulating, e.g. the crawl ceiling also, to keep heat in our bedroom. Note, there is a heat-pump water heater on the far side of the furnished basement. To our knowledge, no moisture issues in past many years, although we are new to the house.”

How should…

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  1. tnedelsky | | #1

    Who is Jon R. ?

  2. GBA Editor
    Martin Holladay | | #2

    Most of the people quoted in articles in our "Q&A Spotlight" series (including Jon R.) are GBA readers who posted comments in the thread under discussion.

  3. Jon_R | | #3

    It's important to get context by clicking on the first green link.

  4. John_OwnerBuilder | | #4

    I'm an owner-builder and we are currently working with our draftsman/architect here in Indiana on new home plans for a 3,000 SF one-story ranch. After getting some initial quotes from a few concrete contractors for a poured concrete foundation, we have decided that for cost reasons, we will need to build with a partial basement and partial crawl space (about 1,500 SF each). I've noted the concerns expressed on GBA concerning the challenges associated with crawl spaces, and would intend to proceed with a fully closed or encapsulated crawl space that would be partially conditioned with a supply and return duct in the trunk line running through the crawl.

    My question relates to the comment made by Peter Yost in this thread which was as follows:

    "Crawlspaces that connect through a limited opening to a full basement do not take care of themselves. That opening does not typically provide any driving force for significant air exchange with the basement".

    In our initial building plans, our draftsman/architect has designed the interface wall (poured concrete foundation wall) between the basement area and the crawl space area as a full height basement wall, which only allows access to the crawl space through a 3' x 3' access door. He has also shown a small opening in the concrete wall to allow the HVAC trunk lines to pass through the wall and into the crawl space area, since the HVAC equipment will be located in the basement area.

    My concern is (consistent with the above comment by Peter Yost) that designing the interface wall between the basement and the crawl as a full height concrete wall with only limited access to the crawl - as noted above - will not allow for a significant level of air exchange between the crawl space and the basement. My understanding is that this interface wall is not needed for weight bearing purposes. Nevertheless, my draftsman/architect maintains that this is how he has historically designed foundations that have a partial basement and partial crawl space.

    My impression is that a partial height interface wall between the basement and the crawl space would be more appropriate than a full height wall, since it would allow for much better air exchange between the two areas. As I continue to debate this topic with my draftsman/architect, I would appreciate any perspectives on whether a partial height interface wall (poured concrete wall) between the basement and crawl space would be most appropriate under the above circumstances (as opposed to a full height basement wall).

    Thank you.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      There are a couple of alternatives (which you should confirm meet your local code).

      One is to do as you suggest. As long as the wall remains open you should have adequate air exchange. The downside is you will always have the open crawlspace visible from your basement if you decide to finish it.

      The second option is to install an exhaust fan in the exterior wall of the crawlspace, with an supply air grill from the adjacent basement. This needs to be designed as part of your ventilation strategy for the whole house.

  5. GoodHealth | | #6

    Great article! We bought a home with a small crawlspace in basement. The floor is basically gravel and plywood. The walls are taped with foil by the previous owner.

    Would pouring a concrete floor constitute sealing? Some moisture can still rise up through concrete but will a concrete floor and plastic vapor barrier reduce the amount of moisture and stop any pests that may burrow through? I am on the right track?


    1. greenerliving | | #7

      Hi GoodHealth!

      This reply may be late to your crawl space party but thought it couldn't hurt. Pouring a concrete floor in and of itself would not IMHO constitute sealing from a moisture perspective. As you allude to, the concrete floor will allow moisture to enter the space and even act as a "wick" or conduit for water in the soil to make its way into the crawl space. Adding a vapor barrier to the floor would reduce that issue considerably. However, I see little reason to pour a concrete floor prior to adding 6 or 10 mil poly. I would, however, certainly recommend removing the plywood before installing a vapor barrier over it due to the fact that it is a food source for mold and other microorganisms.

      You may want to do some further investigating into what is below the gravel, as well as what material is actually on the walls. Who may find a slab beneath the gravel! I've seen a lot of "inappropriate technology" used in many different building assemblies and it's certainly possible that the foil you see could be foil-faced kraft paper or "FSK" batt insulation. Or even actual wrapping foil! (Although I highly doubt it.) I would not recommend either material for a crawl space (or basement) wall. Even foil-faced rigid foam board such as "R-Max" can allow moisture to come into a space if it is not detailed/sealed appropriately. With that said, you would want your vapor barrier to be sealed to your wall insulation material, as well as the top of the foam board sealed to the wall completely (if that is the material used). Alternatively, closed cell foam helps to alleviate some of these details. Hope that helps!

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