Insulation—silently hidden in your walls, no moving parts to fix—is a material you probably spend precious little time thinking about. Then along comes subzero (or sweltering!) weather, a three-digit utility bill or chilly drafts, and you start thinking about it a lot! At home, you ponder whether it's worth the time and expense to add it to your ceilings, walls and basement. At the store, you ponder which type, thickness, width and density to buy. And when you install it, you wonder just how good is “good enough.”
We asked insulation manufacturers and installers which questions they field most—and which blunders they see most. Following are six things they (and we) think you should know.
Attics are one of the easiest and most cost-effective places to add insulation. But you'll increase the effectiveness of insulation substantially if you first seal up air bypasses (Fig. A) around chimneys, plumbing vents, wires, interior walls and exhaust fans—places where warm, moist interior air escapes into the attic.
Heat has a natural inclination to both rise and migrate to colder areas. Combine these two tendencies and you can see why air bypasses can reduce the effectiveness of attic insulation by 30 to 70 percent. You can track down bypasses by lifting existing attic insulation and checking for dark patches of moisture or dust. Or head to the attic on a cold day and feel for pockets of warm air or use a stick of incense to check for drafts.
Before installing (or adding) attic insulation, take these steps:
- Use caulk and expanding foam sealant to close air gaps around pipes, ducts and electrical wires where they enter the attic. Use the fire-blocking type.
- Cut and fit strips of 24-gauge sheet metal between the masonry chimney and the surrounding wood framing. Use high-temperature caulk to seal the flashing where it meets the chimney.
- Install weatherstripping around the perimeter of the attic access opening, then use screw hooks to pull the hatch tight against the weatherstripping. Glue rigid extruded foam insulation to the top of the hatch.
NOTE: After air-tightening your home, always test your carbon monoxide detectors, or install them if you don't have them.
Leaving 5 percent of a wall uninsulated will reduce the entire wall's R-value (resistance to heat flow) by 20 percent. It doesn't take much more time to install fiberglass insulation properly (Fig. B). The most important steps you can take:
- Fill the stud cavity from top to bottom and side to side. To avoid guesswork when insulating walls built from standard 92-5/8 in. long studs, purchase and install precut fiberglass batts that are 93 in. long and 15 in. wide. When you have to custom-cut batts at wall corners and other places, cut the batts 1 in. higher and wider than the cavity you're filling.
- Split your insulation so half goes in front of and half goes behind electrical wires. Compressing insulation reduces its R-value.
- Around electrical boxes, notch, rather than compress, the batt—then tuck the cutout behind the box. To help prevent frozen pipes, insulate only on the cold side.
Kraft paper—the asphalt-impregnated brown paper facing available on insulation—is rarely called for these days. When insulation was first developed, it was only an inch or two thick and the attached kraft facing was stapled to studs to keep it from sagging. Insulation today is so “full-bodied” and fills stud and joist cavities so completely that it resists settling—even when walls have been purposely vibrated in tests.
Kraft paper does act as a vapor retarder to slow the movement of interior moisture through the wall cavity and insulation. But for a thorough job, especially in bathrooms and other high-moisture areas, a continuous 6-mil plastic sheeting vapor barrier is much more effective. Kraft paper still serves the purpose of temporarily holding insulation in place on horizontal or sloped surfaces. And when you're retrofitting insulation in the floor of a crawlspace (Fig. D), installing the kraft-faced insulation paper side up provides an adequate vapor retarder. Kraft paper and its underlying asphalt adhesive are flammable and should always be covered with drywall or other fire-resistant material.
Another important point: Except in hot coastal regions, insulation should be installed with the kraft paper (or plastic vapor barrier) on the interior side of the wall. In hot regions, vapor barriers are often eliminated or positioned toward the outside of the stud wall. If in doubt, consult a local building inspector.
Insulation is rated according to its R-value, or resistance to heat loss: the higher the R-value, the higher the insulating value. Standard fiberglass insulation has an R-value of about 3.5 per inch of thickness (Fig. C); this provides an insulating value of R-11 for 2x4 walls and R-19 for 2x6 walls. But if you're serious about energy savings, you can buy better-performing products. High-density types of fiberglass insulation, with more fibers and air spaces per square inch, offer R-values of up to 4.25 per inch. Some provide R-15 for 2x4 walls and R-21 for 2x6 walls. There's also high-density insulation for ceilings and attics. You'll wind up paying more and in some cases you'll need to special-order it, but high-density insulation delivers up to 35 percent more insulating value per inch.
You can also pack 5-1/2 in. thick insulation into a 3-1/2 in. thick wall to increase its R-value, but the compressed R-19 batt will only yield an R-value of about 17, and it will make it more difficult to get drywall flat against the wall.
The first 3 in. of insulation you add to a bare ceiling or wall will yield huge savings. Adding another 3 in. will increase energy savings, but not to as great a degree. For charts showing the Department of Energy's optimal amounts of insulation to install, check the insulation manufacturers association at naima.org. Their recommendations are based on climate, fuel costs and other factors. Adding more will give you a diminishing return on your investment.
The payback period for retrofitting insulation varies greatly, but studies show that added insulation usually pays for itself in saved energy costs within 5 to 10 years. In Minnesota, a typical payback period might be five or six years. Payback in more temperate areas takes longer, but remember that insulation reduces air conditioning costs too.
Since insulation changes the way attics and crawlspaces “breathe,” it's critical that you maintain or install proper ventilation (Fig. D). At first, it seems odd to add insulation for warmth and then purposely create ventilation “holes” for cold air to enter. But if you don't do this, you're setting yourself up for moisture problems.
When you add insulation to attic rafters and attic floors, it's important to maintain at least a 1-in. continuous air space between insulation and roof sheathing, from eaves to ridge. This air space flushes out moisture. It also helps maintain a “cold roof,” which prevents ice dams from forming in the winter and excessive heat from damaging shingles and increasing cooling costs in the summer. The biggest mistake homeowners make with installation is to install it so it blocks the flow of air at the eaves. The best way to avoid this problem is to install inexpensive air chutes to keep the space open.
Newly insulated crawlspaces also need proper ventilation. The standard procedure is to insulate the ceilings of unheated crawlspaces and the walls of heated crawlspaces. To reduce moisture migration if there's no slab, a 6-mil polyethylene vapor barrier should be laid on the ground in both cases. At least 1 sq. ft. of ventilation should be installed for each 1,500 sq. ft. of floor area. For more information, see How to Install a Vapor Barrier in a Crawl Space. There are regional differences and various codes and interpretations for insulating crawlspaces and attics. Contact a local building official to make sure your house stays code-compliant.