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Crawl space insulation climate zone 6B, 7000 feet elevation, low humidity.

Gary Macdonald | Posted in GBA Pro Help on

I have a question regarding an existing home in climate zone 6B located about 7000 feet in elevation?

Six months ago we bought a home with a vented crawlspace. Humidity in the crawlspace has range as high as 60 during the summer and as low as 55°. The temperature in the crawlspace varies between 50 and 60° With exterior temperatures ranging between the mid 20s and low 90s. We have two furnaces in the crawlspace both of which are vented to the exterior through PVC pipe with both intake and exhaust vents. All ductwork is insulated . The existing vapor barrier on the floor covering gravel is subpar at best with many exposed areas. Before changing the vapor barrier I would like to solve my insulation concerns first. The foundation walls in the crawlspace or approximately 4 feet high and there are several concrete partitions separating the crawlspace into three distinct areas. There Are unfaced fiberglass bats between the joists.

I have thought about closing the vents and insulating the perimeter walls but if I do this when I have to insulate all perimeter walls since there are three distinct partitions.

Two other ideas I had were to replace the fiberglass batt insulation with rigid foam insulation XPS with intumescent paint covering it for thermal barrier protection or using roxul comfortbatt R30 between the joists.

Given the climate conditions in the crawlspace, stable temperature and humidity without extremes what would be the best way to insulate this crawlspace?

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  1. GBA Editor
    Martin Holladay | | #1

    My usual advice is that sealed (conditioned) crawl spaces perform better than vented crawl spaces. If you want to read more on the topic, see this article: Building an Unvented Crawl Space.

    If your house is located west of the Rocky Mountains, and your climate is dry, your vented crawl space may perform well. If you decide to keep your crawl space vented, the best way to insulate the crawl space ceiling is with a continuous layer of foil-faced polyiso installed on the underside of the floor joists. Tape the polyiso seams and make sure that the perimeter of the polyiso is sealed to limit air leakage.

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