Insulation and How it Works

You need insulation in your home to provide resistance to heat flow. The more heat flow resistance your insulation provides, the lower your heating and cooling costs.

Heat flows naturally from a warmer to a cooler space. In the winter, this heat flow moves directly from all heated living spaces to adjacent unheated attics, garages, basements, and even to the outdoors. Heat flow can also move indirectly through interior ceilings, walls, and floors—wherever there is a difference in temperature. During the cooling season, heat flows from the outdoors to the interior of a house.

To maintain comfort, the heat lost in the winter must be replaced by your heating system and the heat gained in the summer must be removed by your cooling system. Properly insulating your home will decrease this heat flow by providing an effective resistance to the flow of heat.

An insulation's resistance to heat flow is measured, or rated, in terms of its thermal resistance or R-Value.  The higher the R-value, the greater the resistance to heat flow.

Types of Insulation
 

There are four basic types of insulation: loose fill, batts and blankets, rigid board and spray foam. The most appropriate type of insulation to use will vary based on the type of construction, the extent of the rehabilitation planned, and applicable code requirements.

Loose-fill Insulation

Loose-fill insulation includes loose fibers or fiber pellets that are blown into building cavities or attics using special equipment. It generally costs more than batt insulation. However, it usually fills nooks and crannies easier, reduces air leakage better, and provides better sound insulation than batt-type insulation.  Loose-fill insulation typically has a value of approximately R-3 to R-4 per inch. Cellulose fiber has approximately 30% more insulating value than loose-fill rock wool for the same number of inches installed.

Cellulose fiber, made from recycled newspapers, is chemically treated for fire and moisture resistance. (Check that the bags are clearly labeled to indicate that the material meets federal specifications for fire resistance). It can be installed in walls, floors or attics using a dry-pack process or a moist-spray technique.

Batt and blanket insulation is made of mineral fiber -- either processed fiberglass or rock wool -- and is used to insulate below floors, above ceilings, and within walls. Generally, batt insulation does not form an air barrier and requires  careful installation to be effective.  The effectiveness of batt insulation is greatly reduced if there are any gaps between them.  Also, mice and other rodents like to live in fiberglass batts which further reduces its performance. 

This type of insulation is best suited to a standard joist, rafter, or stud spacing of 16 or 24 inches. Batts and blankets come in widths to fit securely between the wood-framing members. Some come with a radiant barrier backing. Batts generally come in lengths of 4 or 8 feet. Blankets come in long rolls that are cut to the desired length for installation. Both batts and blankets typically have an R-value of approximately R-3 per inch of thickness.

Rigid Board Insulation

Rigid board insulation is commonly made from fiberglass, polystyrene, or polyurethane and comes in a variety of thicknesses with a high insulating value (approximately R-4 to R-8 per inch). This type of insulation is used for reproofing work on flat roofs, on basement walls and as perimeter insulation at concrete slab edges, and in cathedral ceilings.

For interior applications it must be covered with 1/2-inch gypsum board or other building-code approved material for fire safety. For exterior applications it must be covered with weather-proof facing. 

Spray Foam Insulation

Spray foam insulation is a two-part liquid containing a polymer (such as polyurethane or modified urethane) and a foaming agent. The liquid is sprayed through a nozzle into wall, ceiling, and floor cavities. As it is applied it expands into a solid cellular plastic with millions of tiny air-filled cells that fill every nook and cranny. Spray foam insulation should be applied by a professional using special equipment to meter, mix, and spray into place. Spray foam insulation is commonly used for retrofits; it is good for irregularly shaped areas and around obstructions

Spray foam forms both an insulation and an air barrier, it can be cost competitive with batt insulation because it eliminates the steps for air-tightness detailing (such as caulking, applying house wrap and vapor barrier, and taping joints). Spray foam is often used in basements as it serves both as an air sealer and insulation. In attics it is mostly used for air sealing followed by a layer of loose blow cellulose.

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