While insulation is a sure-fire way to reduce energy costs and make a house more comfortable, many homeowners are not sure if they have enough insulation in place. And some worry that there are places that should be insulated that are not.
An Important Definition
There are a variety of home insulation materials available, but their ability to insulate are all measured in the same way: the R-value, which stands for resistance to heat flow. The higher the R-value, the better the material is at stopping heat from moving through the material.
Fiberglass is the most common home insulating material in residential buildings, followed by cellulose and mineral wool. The materials all provide similar R-values in the range of 2.5 to 4 per inch of thickness. Fiberglass comes in batts and rolls that fit into standard construction openings, such as wall stud bays and the spaces between ceiling joists. The product's availability, cost and ease of installation make it popular among builders and contractors. Fiberglass' R-values are printed right on the packaging, so you might see an R-13 for a 3.5-inch-thick batt that is designed for wall studs. Cellulose and some types of fiberglass are loose-fill materials. They are blown into place with special equipment. There are also rigid foam boards and spray-on foams that have specific applications in the home.
Building codes specify the minimum R-values homes should have in their attics, walls and floors. Energy Star, a program of the Environmental Protection Agency, recommends R-value levels based on climate zones that are more stringent than building codes. These recommendations provide a good target to shoot for.
Increasing Attic Insulation
Most insulation is covered by a finish material, so the easiest place to start gauging insulation levels in many homes is in the attic, which is usually unfinished. Chances are there is already some insulation in the attic, but if you can see the tops of the ceiling joists or there is only an inch or two of insulation covering the joists, you probably don't have enough.
Fortunately, attics are failry easy places to increase insulation levels with either fiberglass or cellulose. Additional batts can be placed perpendicular to existing batts. Loose-fill insulations can easily fill-in around obstructions. There are a few things to keep in mind when insulating or reinsulating an attic.
- Air sealing is just as important as insulation. Common insulation materials retard conductive heat loss or the flow of energy from one molecule to the next. R-value means resistance to heat flow. Think of a cast-iron skillet put on a flame. After a while, the pan's handle becomes too hot to touch—that's conductive heat flow. But many insulation products do little to stop heat flow through moving air. As hot air rises, it looks for openings to escape into the attic or to the outside. There are dozens of possible openings where conditioned air can get into the attic, including wiring and plumbing penetrations and openings for flues and chimneys. Those openings need to be plugged as part of an overall insulation project.
- Recessed lights need special attention. Some recessed light fixtures have housings that protrude into the attic; these cannot be covered with insulation and require a three-inch space separating the fixture from the insulation—a situation that wastes a lot of energy. An alternative is to choose lights rated for insulation contact (labeled IC), which means the fixture can be in contact, or even covered, with insulation. Those fixtures rated ICAT go one step further, using housings that are also airtight.
- Pay attention to attic ventilation. The space above the attic insulation needs to be ventilated to the outside so that moist air does not condense on the building components. An effective strategy is a combination of ridge vents and soffit vents. Outside air is drawn in through the soffit vents and exits the ridge vents. The soffit openings need to be kept clear of insulation. Baffles installed between the roof rafters provide a pathway for the air.
Increasing Wall Insulation
Aside from drilling a series of holes into the exterior walls of your home, it is difficult to judge the quality of the wall insulation. One option is to work with a contractor or energy auditor who uses a thermal imaging device. These are hand-held tools that detect radiation in the form of heat loss through solid objects. They can help pinpoint gaps in or the absence of insulation.
To add insulation to a finished wall, a contractor can remove sections of exterior siding and drill holes in between the wall studs. He will use a loose-fill material to fill the stud bays.
If you plan on residing in the house, use an insulated sheathing under the siding. The sheathing is usually less than one-inch thick, but it does beef up the wall insulation. Perhaps even more important, the sheathing helps prevent thermal bridging, which is heat loss through the wall studs. The wooden or metal studs conduct heat much faster than the insulation between the studs; the insulated sheathing helps stop that heat loss.
In an energy-efficient house, the living area that is heated and cooled should be encased in insulation. The attic and exterior walls are obvious starting points, but there are others:
- Basement walls. The wall of finished basements should be insulated. One option is to adhere rigid foam insulation to the concrete walls and then cover the insulation with drywall. Another is to apply the foam and then build a 2x4 stud wall that can be filled with fiberglass or cellulose insulation. The 2x4 wall comes in handy for running plumbing and electrical lines.
- Rim joists. Sometimes called perimeter joists, these are the joists that create the frame for the flooring joists. They usually connect the house's foundation with its framing. Insulate them by adhering rigid foam board cut to fit between the floor joists. Seal the edges with the expanding foam that comes in a can—this technique will insulate and air seal all at once.
- Unheated crawl spaces. There is some controversy about how to approach this project. Traditionally, crawl spaces were ventilated to the outside, and the floor above the space was insulated with fiberglass batts. But many people now believe that crawl spaces should be sealed and the exterior walls and rim joists insulated, usually with a rigid foam board. And the crawl space should become part of the conditioned space of the house. Discuss this project with a professional contractor. He or she will spot the problems, if any, that you have now and suggest ways to correct them.
Insulation installed properly and in the right places can help make a house energy efficient.