If you properly insulate your crawlspace, 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 crawlspace, first see our information about adding insulation to an existing house or selecting insulation for new home construction.)
How to insulate a crawlspace depends on whether it’s ventilated or not. Traditionally, crawlspaces have been vented to prevent problems with moisture, as dictated by most building codes. However, many building professionals now recognize that building an unventilated crawlspace (or closing vents after the crawlspace 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: 1) Ventilation in the winter makes it difficult to keep crawlspaces warm; and 2) warm, humid outdoor air in the summer brought into the crawlspace through foundation vents makes it difficult to dehumidify the space. In fact, this air can increase moisture levels in the crawlspace.
Insulating an Unventilated Crawlspace
If you have or will have an unventilated crawlspace, then your best approach is to seal and insulate the foundation walls rather than the subfloor.
The advantages of insulating the crawlspace are as follows:
* You can avoid the problems associated with a ventilated crawlspace.
* Less insulation is required (around 400 square feet for a 1,000-square-foot crawlspace 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 crawlspace is less critical.
The disadvantages of insulating a crawlspace include the following:
* The insulation may be damaged by rodents, pests, or water.
* A radon mitigation system will require ventilation of the crawlspace to the exterior. Not planning for radon-resistant construction may necessitate air sealing the floor to mitigate the radon through ventilation.
* The crawlspace must be built airtight, and the air barrier must be maintained.
* The access door to the crawlspace must be located inside the home through the subfloor unless an airtight, insulated access door in the perimeter wall is built and maintained.
Installing Crawlspace Wall Insulation
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 crawlspace 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 crawlspace 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 crawlspace 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 crawlspace, relying on the leakiness of the floor to provide the return air path.
Insulating a Ventilated Crawlspace
Here are some guidelines to follow for insulating a ventilated crawlspace:
1. Carefully seal any and all holes in the floor above (the “ceiling"”of the crawlspace) 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 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 crawlspace meets local code regulations if you are considering pouring a concrete slab.
5. The orientation of the vapor barriers depends on the home’s location and climate.
As mentioned, when properly insulating a crawlspace, 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.
Information courtesy of EERE
