Insulating a Crawl-Space

If you properly insulate your crawl space--in addition to air sealing and controlling moisture, you will save on energy costs and increase your home's comfort.

Before insulating or deciding whether to add insulation to your crawl space, first see our information about adding insulation to an existing house or selecting insulation for new home construction if you haven't already.

How to insulate a crawl space depends on whether its ventilated or unventilated. Traditionally, crawl spaces have been vented to prevent problems with moisture; most building codes require vents to aid in removing moisture from the crawl space. However, many building professionals now recognize that building an unventilated crawl space (or closing vents after the crawl space dries out following construction) is the best option in homes using proper moisture control and exterior drainage techniques. There are two main reasons for this line of thinking.

Insulating an Unventilated Crawl Space

If you have or will have an unventilated crawl space, then your best approach is to seal and insulate the foundation walls rather than the subfloor. The advantages of insulating the crawl space are as follows:

* You can avoid the problems associated with ventilating a crawl space.

* Ventilation in the winter makes it difficult to keep crawl spaces warm

* Warm, moist outdoor air brought into the crawl space through foundation vents in the summer is often unable to dehumidify a crawl space. In fact, this moist outdoor air can lead to increased moisture levels in the crawl space.

1. Review plans for this method of foundation insulation with pest control and local building officials to ensure code compliance.

2. Eliminate or seal the foundation vents.

3. Ensure that combustion furnaces and water heaters located in the crawl space are sealed-combustion units equipped with a powered combustion system.

4. Seal all air leaks through the exterior wall during and after construction, including the band joist.

5. Locate the crawl space access inside the home or install an access through the perimeter that will remain airtight after repeated use.

6. Install rigid foam board or batt insulation--exterior foam, interior foam, or interior batt--to achieve complete insulation coverage. Insulate the band joist with batt insulation, as well as the crawl space access if it's located in the wall.

7. Install a continuous termite shield between the band joist and masonry foundation wall that covers the wall insulation and extends completely outside (or leave a 2- to 4-inch insulation gap at the top for termite inspection).

8. Install a supply outlet in the crawl space, relying on the leakiness of the floor to provide the return air path.

* Less insulation is required (around 400 square feet for a 1,000-square-foot crawl space with 3-foot walls.)

* Piping and ductwork are within the conditioned volume of the house so they don't require insulation for energy efficiency or protection against freezing.

* Air sealing between the house and the crawl space is less critical.

The disadvantages of insulating a crawl space include the following:

* The insulation may be damaged by rodents, pests, or water.

* A radon mitigation system will require ventilation of the crawl space to the exterior. Not planning for radon-resistant construction may necessitate air sealing the floor to mitigate the radon through ventilation.

* The crawl space must be built airtight, and the air barrier must be maintained.

* The access door to the crawl space must be located inside the home through the subfloor unless an airtight, insulated access door in the perimeter wall is built and maintained.

Steps for Installing Crawl Space Wall Insulation

Here are some guidelines to follow for insulating a ventilated crawl space:

1. Carefully seal any and all holes in the floor above ("ceiling" of the crawl space) to prevent air from blowing up into the house.

2. Insulate between the floor joists with rolled fiberglass. Install it tight against the subfloor. Seal all of the seams carefully to keep wind from blowing into the insulation. Also, adequately support the insulation with mechanical fasteners so that it will not fall out of the joist spaces in the years to come. DO NOT just rely on the friction between the fiberglass and wood joists to secure it in place.

3. Cover the insulation with a house-wrap or face it with a vapor barrier.

4. Install a polyethylene vapor retarder, or equivalent material, over the dirt floor. Tape and seal all seams carefully. You may also cover the polyethylene with a thin layer of sand or concrete to protect it from damage. Do not cover the plastic with anything that could make holes in it, such as crushed gravel. Be sure the headroom of the crawl space meets local code regulations if you are considering pouring a concrete slab.

5. The orientation of the vapour barriers depends on the home's location or climate.

Insulating a Ventilated Crawl Space

Other Considerations

As mentioned above, when properly insulating a crawl space, you also have to consider moisture control measures and air sealing.

Finally, you need to consider radon resistance or control when installing any type of foundation.