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Read expert recommendations on insulating crawl space to bring this often understood area of many American homes into the 21st century.

Insulating Crawl Space - Moisture Problems Photo: tchaffordbasementssystems.com I grew up in a split-level, the house style that dominated suburbia in the decades immediately following World War II. The middle floor—the one that “split” the upper and lower levels—was built upon a crawl space. You could get to it from a hole in the wall that was covered by a plywood panel, but as children we rarely ventured through. When my elderly parents sold the house a few years ago, it fell to me (with some help from my step son) to clean out the space before the new owners moved in, and I was reminded of what a creepy place it was. Batts of soggy fiberglass insulation hung haphazardly from the joists. Dim light filtered in from vents in the walls. There were some unpleasant signs of rodent activity and what looked like mold covering some of the joists. The damp concrete walls were as bare as when they were poured more than 50 years prior. In those days, common building practice was to insulate the floor above the crawl space and to leave the crawl space’s wall vents open, so any moisture buildup would vent to the outside—a monumental design flaw, as it turned out. Instead of expelling moisture (at least in climates with humid summers), the open vents allowed moist air in. As that air passed over the cool surfaces of the crawl space, condensation was left behind. At my parents house, fiberglass batts had degraded to the point of sagging and no longer kept cold air from reaching the floor above. It was no wonder Mom was always fretting about mildew ruining her rugs. Find trusted local pros for any home project HomeAdvisor Logo + BobVila.com Logo Insulating Crawl Space - Encapsulation Crawl space vapor barrier, wall insulation, and water drainage system. Photo: capitolcleanspace Today, energy experts have a different prescription for crawl spaces based upon the idea that they should be part of the home’s conditioned space (the area that is heated and cooled). If conditioned, then condensation is eliminated, which in turn minimizes the chance of mold and mites. Energy loss from air ducts is reduced, and first-level floors become warmer in winter. Drafts are also minimized.

Eliminate sources of water in the crawl space before doing anything else.

Extend downspouts, maintain gutters, and regrade sidewalks, patios, and garden beds so that they slope away from the house. If necessary, install a basement waterproofing system or sump pump.

Insulate the walls, not the ceiling.

Rigid boards may be installed with construction adhesive or mechanical fasteners. You may also choose to lay an insulating mat over the crawl space floor.

Use an insulation that resists damage from water (not fiberglass or cellulose).

Any one of the rigid board insulations will work, but polyioscyanurate has the best R-value (6 to 7 per inch). Remainders and seconds are sometimes available from commercial roof insulation manufacturers. You just have to pick it up. Insulating Crawl Space - Vent Covers CleanSpace Vent Covers. Photo: BasementSystems

Seal all vent openings.

Vent covers, installed from the exterior, are available in standard sizes. Or make your own from plywood and caulk them in place.

Ensure that hatchways to the exterior are sealed.

Use heavy-duty weatherstripping to ensure a tight seal or purchase a pre-manufactured crawl space hatchway.

Seal rim joists and sills.

Fill the ends of joist bays with rectangles cut from rigid board insulation. Use foam sealant, such as Great Stuff, to seal the joints. Use caulk or foam sealant to seal the joint between the top of the crawl space wall and the sill.

Install an air and vapor barrier over the floor and tape it to the insulation.

Plastic vapor barriers are available in sheets of various thicknesses. If you plan to use the encapsulated space for storage, choose a barrier that will stand up to foot traffic. Many companies offer encapsulation services; in my opinion Basement Systems, Inc. has the most completely engineered system. Find trusted local pros for any home project HomeAdvisor Logo + BobVila.com Logo

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

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