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Community and Q&A

Cold weather crawl space insulation

scrbagonias | Posted in General Questions on

First time home owner- I am planning on installing insulation into a crawl space in an older home in the 5-A Western NY area. It has 3 vented access points to the outside, I placed a thick vapor barrier down and around pillars. There’s no heat source except currently poor insulated pipes from the hot water furnace. (soon to be fully insulated). There’s no insulation at all going up to the floor currently.

I understand I’d need R-30 or higher but confused on fiberglass, Mineral wool, a layer of hard insulation first or faced or un-faced insulation. Also if I should plan on closing the vented access points. There’s no insulation on the walls currently, and i was thinking this would not be necessary due to insulating the floors.


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  1. GBA Editor
    Martin Holladay | | #1

    This article should answer all of your questions: Building an Unvented Crawl Space.

  2. fitchplate | | #2


    "Building an Unvented Crawl Space" is a good article and gives great detail on crawl space weatherization. I used your advice to do my own. But one point you made is a bit misleading and not an inherent problem to sealed and enclosed crawl spaces if they are built correctly.

    You said:

    "According to the previously cited Home Energy article, “Additional radon testing showed that radon levels in the closed crawls — with a relatively low dilution rate — were roughly 10 times the levels measured in the vented crawls.”

    The Home Energy observation is not applicable to the type of advanced crawl space designs you recommend. Radon cannot enter the crawl space if it is sealed out by either an inside or as is preferable, an outside contiguous vapor barrier.

    Radon can only enter if there is permeable material (i.e. concrete, block, unsealed permanent wood perimeter walls, etc) or a crack/opening -- there must be a pathway from the soil. Radon isthe only naturally occuing radioactive element in gas form. It can be kept out 100% if the vapor barrier is installed correctly. Another important reason for getting the air and moisture sealing details correct.

    And important to clarify so folks don't get the impression that a properly sealed crawl space has a potential radon risk.

  3. GBA Editor
    Martin Holladay | | #3

    My article accurately reports the findings of the Washington researchers. Here is a link to their paper: Vented and Conditioned Crawlspace Performance in Marine and Cold Climates of the Pacific Northwest.

    You contend that "Radon cannot enter the crawl space if it is sealed out by either an inside or as is preferable, an outside contiguous vapor barrier." But the reality is more complicated than that.

    If all that was required to prevent high indoor radon levels was "a contiguous vapor barrier," then there would be no need to install active radon mitigation fans on new homes. But some new homes do need these fans. The only way to determine indoor radon levels is to test the indoor air for radon.

    In any case, an air barrier is more important than a vapor barrier.

    The Washington researchers describe the homes with conditioned crawl spaces this way: "Two homes had conditioned crawlspaces ... Two homes had traditional vented crawlspaces... The builders varied in their approach to the conditioned crawls, as noted below. ... New Tradition Homes, a Building America partner builder, built four spec homes in Vancouver (Clark County), Washington (US DOE 2007). ... Conditioned crawlspaces had R-15 (0.38 W/m2K) interior extruded polystyrene perimeter insulation, were finished with concrete floor slabs, had passive radon vent stacks as required by the section 503 of the Washington State Ventilation and Indoor Air Quality Code (SBCC 2007b), and were power-vented according to IRC R408.3, except without an air pathway to common areas. During the study period, all the conditioned crawlspaces were continuously power-vented at a rate of 50 cfm (1.4 m3/min) exhausted from the crawlspace to the exterior. ...
    The Condict insulated crawlspaces used batts with a vapor barrier on the inside; the crawlspace floor was dirt covered with 6 mil poly. ... The conditioned crawlspaces in the cold climate houses were not power-vented."

    Clearly, the New Tradition homes appear to have been built with a more robust approach to excluding radon than the Condict homes. Neverthesless, the radon levels were higher in conditioned crawl spaces than in vented crawl spaces for both builders (see table below).

    The researchers concluded, "There appears to be a possible benefit for improved IAQ associated with decoupling the conditioned crawlspace from the house by power venting. The benefit would only continue with continuous fan operation and would require elimination of the IRC requirement for a return pathway to the house. Continuous fan operation, however, could have a significant energy cost. ... Recommendations for Conditioned Crawlspaces: ... Require passive or active radon mitigation, depending on the risk for the site."


  4. fitchplate | | #4

    You are right, I stand corrected. An effectively installed membrane is not going to prevent radon accumulation in an unvented crawlspace.

    I looked further and I see there seems to be no single layer, 100% radon impermeable barriers for sale around here; and so the best method requires multiple layers of the better quality air/vapor barriers to be effective along with ventilation. This study proves your point.

    The importance of the effect of ventilation on moving radon is shown by your reply and tables cited. Radon is moved by convection and air flow ... it has be entrained to be transported. Otherwise it lies still in the bottom of the well (i.e. cellar). That would be why a tight unvented crawl space would be a radon well.

    Radon gas is not magic. If radon is getting through an impermeable boundary, the material is obviously not truly impermeable to the gas or the installation is flawed. Radon found where there are properly installed, proven membranes would be the result of (1) infiltration elsewhere (air intake at or below ground level) and (2) the occupants bringing radon and radon's parent isotope (i.e. radium) into the home -- by various mechanical means (including on shoes, wind blowing into through openings, toys, gardening tools, etc). It even comes in the tap water from the well.

    Because radon it is heavier than air, it accumulates in the basement.

    High radon levels are a sign of high uranium and radium level in the soils, rock, concrete, fill aggregate - the site has to be previously contaminated for there to be a radon problem, and there are several explanations for contamination in a home.

    Radon blocker membrane solutions usually are combined with radon mitigation ventilation.

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