Insulating a Crawlspace

After attics, unheated #“1 crawlspaces are the next vital places to insulate. You have two options: drape batts or blankets around the perimeter walls, or suspend material between floor joists. Draping the walls creates a sealed air chamber, which gains further insulation value. With some houses, however, you have no choice but to insulate between the floor joists.

Wrapping a crawlspace is a dirty but straightforward task—as long as you have enough space to move around. You’ll need unfaced batts or blankets, 6-mil polyethylene sheeting to cover the ground, 1x2s and nails, and rocks or bricks. Wear goggles, gloves, knee pads, and protective clothing. Close off unnecessary openings, but don’t seal vents permanently; a crawlspace needs these for “breathing” in hot, muggy weather. A crawlspace must have at least two vents, located at opposite corners, that can be opened in the summer. A crawlspace that can’t breathe turns into a giant moisture chamber in hot, humid weather.

WARNING: If you live in an extremely cold region, such as Alaska, the Northern Plains, or northern Maine, don’t use the technique; it could cause frost heaving that might damage your foundation. Check with local contractors or your building code department for techniques used in your area.

Tools: Utility knife, hammer, saw, drill, safety goggles.

1. Insulate between joists. For walls perpendicular to joists, place insulation against the rim joist. Cut pieces oversize so they fit snugly. Next unroll enough insulation to overlap the sill plate and cascade down, covering the earth at the wall’s base. Secure the insulation by nailing or screwing on a 1x2.

2. Insulate along the rim joists. Where a wall is parallel to the joists, let the material drape over the rim joist, down the wall, and over 2 feet of the ground. Secure the insulation with a 1x2.

3. Add a vapor barrier. After insulating the walls, roll up the batts and lay a polyethylene vapor barrier on the floor. Lap the joints by 6 inches. Attach the vapor barrier to the foundation with duct tape. Also tape the joints between the strips. Make sure you don’t puncture the plastic.

4. Weight the insulation. Weight the polyethylene and insulation with rocks or bricks. Don’t use wood—it could rot or attract termites.

Insulating a Floor

Floors built over crawlspaces, enclosed porches, or other unheated areas need insulation too. If the joists are covered underneath, your best bet is to have a contractor blow in loose-fill insulation. If you have open joists, install batts, blankets, or rigid planks, using one of the methods shown here. Rigid insulation planks can be easily and quickly attached to the underside of joists if no obstacles, such as pipes or ducting, get in the way. However they don’t provide as great an R-value. It’s usually worthwhile to cut blankets or batts to fit around obstacles.

Unless you’re simply adding another layer to existing insulation, get faced material and install it with the vapor barrier side facing the heated area. Foil facing works well here because it reflects heat back into living areas. Pay special attention to achieving good coverage at joists and headers around the floor’s edges. Cold slab floors present a special problem because you can’t get material under them where it would do the most good. It may help to hire a contractor to wrap the outside of your foundation. Keep in mind you’ll probably end up with a higher R-value by insulating the floor itself. To do this, glue down wood sleepers, place rigid insulation planks between the sleepers, then lay new subflooring and finish flooring.

Tools: Utility knife, straightedge, tin snips, stapler, hammer, saw.

Fold to seal the joist. Start each insulation strip by folding it so that the insulation covers the rim joist. You can staple chicken wire to the joists to support the batts.

Seal around obstacles. If you encounter a pipe or other obstacle, cut one or two slits in the insulation at the opening. Slip the insulation in place, then cut the insulation to fit snugly around the obstacle. Seal the slits with duct tape.

Support the insulation. Or you can support the insulation with friction-fit rods that slip between the joists. Leave some airspace between the insulation and the floor.

Rigid foam. To install rigid planks, apply adhesive to the bottoms of the joists and press the lightweight foam strips in place.

Insulating Ductwork and Pipes

As much as 20 percent of your JT\ heating and cooling energy can escape via ducts. If you insulate the ducts that run through your attic, garage, and other unheated areas, you can prevent most of this loss. Purchase 2-inch-thick duct blankets or standard insulation, which cuts heat loss by about one-third more than 1-inch material. You’ll also need several rolls of duct tape to seal all duct joints before you insulate. If your ducts don’t have dampers to balance airflow, consider adding some prior to insulating.

Tools: Utility knife or hacksaw.

Insulate around ducts. For ducts that run between joists, cut blankets to fit and staple them to the floor, as shown. Install the insulation with the vapor barrier facing out. Avoid compressing the insulation—the fluffier it is, the better it works. Seal any joints with duct tape.

Insulating a Partially Finished Room - In a partially finished space, you may be depending on radiated heat from the ducts to keep things warm. In this situation, you would be better off to insulate the walls and forget about the ducts. If the area receives only occasional use, consider installing a couple of basement registers and insulating the ducts and floor above.

Seal the end. Take special care to seal the end of a duct. Extend the wrapped insulation several inches beyond the end of the duct. Cut an insert with a flap, as shown above, so the insulation fits snugly into the opening and the flaps extend several inches in all directions. Fold and tape the flap as you would a package.

Wrap rectangular ducts. For other ducts, wrap insulation around all sides, and seal the joints with tape. Install with the vapor barrier facing out and avoid compressing the insulation. Take the time to cut carefully around obstacles so the insulation seals tightly without compressing.

Insulate hot-water pipes. Foam pipe insulation comes in various lengths. A lengthwise slit lets you fit the sleeve on the pipe. Cut pieces to length using a utility knife. Wrapping hot-water pipes saves energy costs. You can wrap cold-water pipes, too, to reduce condensation and prevent water damage to structures. At elbows, cut the pieces at 45 degrees for a mitered joint.

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