Insulating Unvented Crawl Spaces

How to Insulate Unvented Crawl Spaces

By Mark J. Donovan

If you are tired of having a damp and humid crawl space and having to open and close crawl space vents that never really seem to make a difference, then you may want to consider converting your crawl space into an unvented crawl space. The key components necessary to convert a vented crawl space into an unvented one is the introduction of insulation, a vapor barrier, and heat into the crawl space.

Insulating unvented crawl spaces enables the crawl space to effectively become an extension of a basement, or effectively a half-height basement.

Like a basement, an insulated unvented crawl space is not exposed to the outside air temperatures and humidity levels.

And when insulated and protected from ground moisture permeation, an unvented crawl space can become a useful and useable space for storing items.

Insulating Unvented Crawl Space Materials

Insulating unvented crawl spaces is a project that most do-it-yourself homeowners can tackle and the materials required are relatively inexpensive. To insulate an unvented crawl space, you will need; caulk, faced fiberglass batt or rolled insulation with an R-value of R-19, a vapor barrier (large sheets of 6 mil polyethylene plastic), duct tape, and some sand and gravel. In addition, you will also want to dress appropriately when insulating a crawl space. Make sure to wear clothes that cover your entire body and wear gloves, goggles and a mask to protect yourself from the fiberglass insulation.

Insulating Unvented Crawl Space Instructions

When insulating unvented crawl spaces it is better to insulate the crawl space walls versus the crawl space ceiling as this way there is no need to insulate plumbing pipes and ductwork.

First, caulk around any holes in the crawl space walls to prevent any air infiltration into the crawl space.

Next, apply a vapor barrier to the floor of the unvented crawl space. The vapor barrier should go up the sides of the crawl space walls by about 1 foot and should be taped with duct tape along the length of the wall. Cover the vapor barrier with sand and then gravel to protect the vapor barrier from being damaged when you crawl or walk on it.

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By installing a vapor barrier on the floor of the crawl space the humidity levels in the crawl space will be dramatically reduced and thus prevent the insulation from becoming damp and moldy.

Next, assuming a portion of the crawl space has framed knee walls, install the faced fiberglass roll or batt insulation in-between the wall stud bays by sliding it up snuggly against the wall sheathing. The faced side of the insulation should face inwards towards the crawl space. If the crawl space does not have framed knee walls then install pieces of batt/role insulation into the header and stringer bay areas, next to the band joists.

For the concrete, or masonry sections of the crawl space walls install faced batt/rolled insulation, or rigid foam insulation up against them, again with the faced insulation facing inwards towards the crawl space room.

Hold the batt/rolled insulation in place by nailing a narrow furring strip to the mudsills where the top of the insulation is sandwiched in-between. Make sure the insulation extends down onto the floor of the crawl space and overlaps the crawl space floor by a couple of feet. Use 2x4s to hold the batt/rolled insulation in place up against the wall.

Finally, introduce heat into the crawl space to allow the insulated unvented crawl space to have some semblance of warmth, similar to a basement.

For more help on Basement Additions, see’s Basement Remodeling Bid sheet. The Basement Remodeling Bid Sheet will help ensure that your hire the right contractor so that your basement remodeling project is done correctly and you get the project finished on time and budget.

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