How to Insulate a Crawl Space

Insulation in a crawl space | Installing wall insulation
Image: CAZ Energy Services

There’s a right way and a wrong way to install crawl space insulation, depending on where you live. Do you know the difference? We do — read on!

If your furnace is going full blast and you still have cold feet, the culprit may be your crawlspace insulation.

Take a look under your house. Damp, dangling insulation is a sure sign of outdated or shoddy installation. If your house was built before energy-conserving building codes were standardized in 1990, you may find no insulation at all. The U.S. Dept. of Energy currently recommends insulation with an R-value of at least R-9 in floors.

To keep things cozy underfoot, you’ll need to select the right insulation approach for your local climate. Winter temperature is the continental divide:

In moderate or dry climates without the threat of sustained subfreezing temperatures, insulation between floor joists makes sense. 

Where winter temperatures are extreme, opt for insulating the walls and sealing off the crawl space entirely.

Floor Insulation for Moderate Climes

If your winter temps seldom linger below freezing, you’re in luck. Six-inch-thick, R-19 fiberglass batts installed between floor joists -- along with careful moisture control and mold prevention --  gets the job done. Best of all, at roughly $1 per square foot, it’s easy on the pocketbook. 

Here’s what it takes to do it right.

Support: Fiberglass batts should be unfaced and installed so they make contact with the underside of the subfloor. Wood lath placed every 18 inches or a crisscross webbing of wire provide the best batt support. Avoid stay rods (aka tension rods). They compress the fiberglass, lessening its insulation value, and can pop loose.

Ventilate: An insulation contractor can calculate the ventilation your crawl space needs and will cut in new vents as required.

Seal the subfloor: Holes for electrical wiring and plumbing should be sealed with spray foam insulation.

Insulate plumbing pipes and HVAC ducts to prevent heat loss and freezing.

Closed-cell spray foam combines thermal and moisture protection, but at $5 per square foot, it's too pricey for most budgets. However, it might be your only alternative for filling the webbing between truss-type joists.

Avoid open-cell spray insulation -- it soaks up moisture like a sponge. 

Related: HVAC Maintenance Checklist

Enclosing Your Crawl Space: The Cold Climate Choice

In a cold climate, the most efficient technique is to insulate the walls of your crawl space and close it off from the elements by sealing all air leaks. That way, plumbing pipes and HVAC ducts are protected from freezing temps, helping to conserve energy.

The best method is to insulate crawlspace walls with rigid insulation. At about $5 per square foot for professional installation, including materials, it comes at a cost but offers a permanent solution. You can do the job yourself for about half the cost, but it’s a challenging, time-consuming DIY project.

A thorough job also includes:

Nixing the vents: Simply closing the vents in your foundation won’t do the job. Vents must be removed and the holes sealed.

Insulating the rim joist: Use closed-cell spray foam to insulate the rim (aka band or perimeter) joists — the joist that rests on top of your foundation walls.

Insulating the foundation: Glue rigid foam insulation board to the inside of foundation walls, using waterproof construction adhesive, and seal all seams with waterproof tape. A 4-by-8-foot sheet of 2-inch-thick expanded polystyrene insulation (R-value 7.7) is $26. A double layer is recommended.

Add a vapor barrier: Whether the floor of your crawl space is bare earth, gravel, or concrete, it is going to exude moisture. A 6-mil polyethylene plastic vapor barrier covering the ground keeps the wet at bay.

Get rid of moisture: Moist household air is bound to cause condensation in the crawl space. In addition, any slight plumbing leak can build up over time. A dehumidifier or sump pump eliminates the moisture that mold loves.

Make moisture control a priority, warns Chuck Henrichsen, owner of Clean Crawls, a Seattle insulation firm. “If you don’t, your crawl space becomes a Petri dish.”

With crawl space sealed off from cold and moisture, your crawl space can be linked to your household HVAC system via vents. That way, warm air is circulating under your floors, warming them up and helping to keep you toasty. There's no need to cool off your crawl space in summer, however; close vents when your air conditioning is running.

Related: Save Money with Energy-Efficient Insulation

Dave Toht
Dave Toht

Dave Toht has written or edited over 60 books on home repair and remodeling for The Home Depot, Lowe’s, Better Homes & Gardens, Sunset, and Reader’s Digest. He's a former contractor with decades of hands-on experience.