The importance of insulation
Standard fiberglass blanket/batt insulation being installed in a wall. The kraft paper vapor barrier is stapled to the wall studs. (photo: Owens Corning)
It's generally understood that adding insulation to the walls or ceiling of a house will reduce monthly utility bills. The actual amount of savings for each home depends upon several factors — the current level of its insulation, climate, utility rates and the HVAC system's efficiency.
The current level of insulation is perhaps the most important factor in deciding whether or not to add more and how much. For example, doubling the amount of insulation in an attic not yet well insulated can cut heat loss through the ceiling by about half. A reliable contractor can help you determine the payback from the savings as compared to the installation costs.
If you double that amount again and super-insulate the attic floor, it will cut the original heat by only another 25 percent (half of half). This diminishing return is important to keep in mind when determining the amount of insulation to add.
Various types of insulation can be used to reduce conductive heat loss and/or radiant heat loss. Standard fiberglass batts, blown-in fiberglass, cellulose, rock wool, foam, all are used to block conductive heat loss. This is the kind of heat transfer that travels through materials, such as drywall, studs, bricks, etc.
Radiant heat transfer is the way the sun heats the earth, and your house also loses heat to the cold outdoor air and nighttime sky by this method. Radiant barrier insulation, often an aluminum foil film, is effective for blocking this heat loss. Some standard insulation batts include a foil facing to reduce both types of heat loss.
Insulation will make you feel more comfortable. If you are in a room at 70 degrees with little wall insulation, you may still feel chilly. This is because the exterior walls are cold and your body is losing its warmth by radiant heat transfer to the walls. During the summer, a hot wall makes you feel uncomfortably warm.
More important than its thickness is insulation's installed R-value — a measure of its ability to retard heat flow. Some types have twice the R-value per inch thickness as others. Also, blown-in insulation can be fluffed up when installed, not necessarily intentionally, resulting in less true R-value. Make sure your insulation contract specifies the final insulation R-value, not just the thickness.
You might consider an environmentally friendly insulation made of recycled materials, such as scrap blue jean material. It looks similar to chopped up blue jeans in batt form. It is treated for fire safety and has an insulating R-value similar to fiberglass batts.
Fiberglass is made basically from sand. Some manufacturers use 25 percent recycled glass, so check the packaging if you prefer recycled products. Rock wool insulation is made primarily from waste products. It and fiberglass have an insulation value of about R-3 per inch thickness.
If insulation space is limited, as in a masonry wall, injected foam is a good option. Look for foam that uses no ozone-layer-damaging foaming agents.
Another option to minimize voids is a blown-in-blanket method. Another similar system adds binders to the insulation to reduce settling.