How to insulate and ventilate a crawlspace

Answered by Brett ~ March 7, 2011 ~ Comments

My home has a crawl space which is damp and cold. I want to insulate the floor for better comfort inside the house, how is the best way to do this?

Kyle ~ Rawlins, WY

Brett Kulina

Kyle, insulating your home's crawl space can improve the comfort level within your home, but keep in mind that there are two other components needed for a properly functioning crawl space, namely adequate ventilation and an air-tight vapor barrier covering the ground. Your unheated crawl space should have a minimum of two vents which allow the outside air to freely flow into the crawl space, and you should research your local building codes to determine if the size of your home's crawl space vents is adequate and code compliant. Although the required size for crawl space vents differs from locale to locale, in general, one square foot of net free vent area is needed for every 1,500 square feet of floor surface area (presuming that the floor is covered with an approved vapor barrier).

Before you insulate underneath your home's floor, you should install a 4-6 mm vapor barrier (more accurately referred to as a vapor diffusion retarder) over the dirt floor in your home's crawl space. Even if the dirt floor appears to be dry, it is crucial that it is completely covered by a vapor barrier, which needs to be sealed to your home's foundation walls to ensure that no water vapor penetrates the barrier. Moisture that is able to leak into your home's crawl space can condense on the foundation walls and on the underside of the sub floor, which can lead to mold growth and wood rot. A durable double sided tape can be used to attach the vapor barrier to the foundation walls and seal around any footings or piers.

Once you have successfully vented your home's crawl space and protected it from moisture infiltration, then you can install some insulation between the floor joists. You can choose blown-in insulation, spray-in foam insulation, or rolled batts. If you choose to install rolled batts that have an attached paper or foil surface, make sure that the paper faces up toward the heated side of the house. It is also important that you avoid leaving any large gaps between the floor joists and the insulating batts, which commonly occurs when the floor joist spacing is wider than the rolls of insulation you are installing. If your home's floor joists are spaced at odd sized intervals, then blown-in and sprayed-in insulation may be the better choice.

Hopefully you will feel more comfortable in your home once you have installed some insulation between the floor joists. At the very least, your crawl space will be better ventilated and have fewer moisture problems once you finish the job. Good luck with your project!

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