Crawl space

Published on August 29th, 2012 | by Henri

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Cold floor over crawl space

Q.    I have an extension on the rear of my home that is built over a cement slab with a 3-foot crawl space. The space between the floor joists seems well insulated. However, the floor in the room stays very cold which makes it difficult to keep the room warm in the winter.

We have forced air heat with ductwork running through the crawl space. I was thinking of making a hole in an area of the duct within the crawl space to allow some warm air in. I would likely plug the hole in the summer months to keep from cooling the space with the air conditioning.

Do you think this is a good idea? If not, do you have any other suggestions for warming this back area of the house?

A.    The room above the crawl space is not receiving very warm air, as the air is cooled while running through the crawl space duct. Instead of cutting a hole in the ductwork to heat the crawl space (which would not help much as the heat would be wasted in a cold, uninsulated foundation), insulate the ductwork.

First, tape all the joints in the ductwork with duct tape and wrap it with aluminum-faced duct insulation. This will help in the summer and winter alike. If you are ambitious, consider insulating the foundation walls of the crawl space with 1-inch thick rigid extruded polystyrene.

You can adhere the insulation panels to the foundation walls with dabs of StyroBond, polyurethane caulk/sealant or equivalent after cleaning the areas where you will put the adhesive. This should help condition the crawl space, but it will take a long time before you recover the cost of the installation.

You should also check the insulation installed between the first-floor joists. It is possible that those who installed it stapled it to the bottom of the joists (as is commonly done), leaving a space between the top of the insulation and the floor sheathing. Heat from the room above is drawn out through this empty space and the floor feels cold.

The joist spaces must be completely filled with the insulation. You may need to pull the vapor retarder off the existing insulation (or flip it over so the vapor retarder is against the floor) and push it up against the floor sheathing. Then add another layer of insulation without a vapor retarder and hold it in place with string stapled in a zigzag fashion or by nailing lattice strips to the bottom of the floor joists.

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