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Once the walls, windows, and roof are in place, the next step is to maximize energy efficiency inside the house. The best way to do this is with proper insulation and air sealing, to keep the outdoor temperature out and the indoor temperature in.

Insulation—Another Barrier against Air Movement

Insulation is extremely important as a method of increasing energy efficiency. Adding layers of fiberglass, rock wool, foam, or reflective barriers prevents the movement of air through the walls, attic, and basement or crawl space of a house. This decreases the flow of heat out of the building in cold weather and into the building in hot weather.

Insulation is particularly important in framed houses, which usually have minimal thermal mass. In fact, installation of insulation in new buildings is mandatory under law in many places. Many new framed houses use two layers of insulation: one outside the studs, put in place before the exterior siding goes on, and the other between the exterior and interior walls.

	Digging Deeper Find out more about R-Value.

Insulation is rated in terms of R-value, which indicates its resistance to heat flow. Higher R-value numbers indicate a more effective barrier. The recommended R-value depends on the type of construction, location and climate zone, and type of heating and cooling systems, among other variables. The R-value of insulation depends on the type of material, its thickness, and its density. The value may be reduced if the material is improperly installed—if it is compressed, for example. In a framed house, if the insulation is laid only between the studs and not over them, this creates what is called “thermal bridging” through the frame. Heat movement through steel framing can be particularly severe.

Insulation comes in several forms, and most types work the same way, by limiting air movement. Different forms of insulation may be combined in buildings, and some forms are more suited for certain types of construction. In framed houses, blankets of mineral fibers, such as fiberglass or rock wool, come in standard-size rolls, or batts, which fit the spaces between the wall studs or beams in the floor or attic. The mineral-fiber blankets can also be cut as needed. Loose fill, consisting of fibers or pellets of cellulose, fiberglass, or rock wool, is designed to be blown into wall cavities and corners. It is often used for unfinished attic floors. Low-density foams can be sprayed into place. Some foams are open-celled (cushion-like), while others are closed-cell (hard and ungiving—some are even hard enough to walk on). Both fibers and foams may also be molded into panels of rigid insulation or pipe coverings.

Framed or masonry, a house needs insulation from top to bottom. The insulation of an unfinished attic is particularly important. In colder climates, the insulation in the attic prevents the loss of warm air from the house; in warm climates, attic insulation includes a radiant barrier. This usually consists of a thin sheet or coating of a highly reflective material, often aluminum, applied to one or both sides of some sort of heavy paper, cardboard, plastic film, or thin plywood. Ideally it should be suspended a few inches below the roof. Leaving an airspace between it and the roof is essential. The purpose of a radiant barrier is to reflect back much of the heat radiated from the hot roof. It can be placed directly on top of the insulation, and some batt insulation comes with a reflective layer built in. However, the barrier is much less effective in this location.

Houses with walls that have thermal mass, such as brick, concrete, or stone, also need insulation. However, the insulation for these houses should be on the outside, in the form of synthetic stucco or a similar type of material. Insulation can also be placed between the brick and the interior walls. Most foundations do not require insulation, because they are made from insulated blocks or concrete.

Concrete blocks are a popular building material. These blocks are usually hollow in the core, and the air in the core acts as an insulator. However, in colder climates insulation in the core—in the form of foam—is necessary to improve energy efficiency.

Insulation can also be used to protect water lines and heat or air-conditioning ducts that run through unheated areas of the house. This prevents water pipes from freezing in very cold weather and the air in the ducts from losing the predetermined temperature. Pipes and ducts can be encased in premolded coverings or wrapped with fiber insulation.

Air Sealing, Moisture Barriers, and Joint Wrap

Air sealing, moisture barriers, and joint wrap are essential complements to insulation. An important purpose of air sealing is simply to stop unwanted air entering the house. In a wood-framed house, the outer insulation sheathing (if it does not consist of closed-cell foam, which already does the job) is often wrapped in sheets of material that serve as an air barrier. The siding is installed over this wrap. All joints in the wrap—indeed all the joints and seams in the structure of any type of house—need to be sealed with special tape or caulking.

Air can still enter through all kinds of cracks and gaps: where walls meet, around electrical outlets and switches, behind cabinets that are set into walls, around attic hatches, behind bathtubs and showers, and places where plumbing and wiring enter the house. It is important to close these gaps before insulation is installed. The primary reason: to control moisture. Most moisture enters the house in the form of air leaks; sealing those leaks will minimize this problem. Moisture that condenses on the walls, ceilings, and floors—or worse, inside the walls, in the insulation—of a house can cause major damage.

A little bit of moisture comes by diffusion through the walls. Vapor-diffusion retardants are best used between the foundation and the walls. Some moisture enters the house from heat transfer. Since the purpose of insulation is to prevent heat transfer, it will also prevent moisture from entering in this manner.

Moisture is also generated by living in the house. Washing, cooking, doing the laundry, and even breathing all produce moisture. This moisture needs to be vented out of the house. The very fact that new houses are so effectively sealed makes ventilation all the more important. It also removes dust and molds, gases from fossil-fueled water heaters and furnaces, and other pollutants that might otherwise accumulate inside the house. And it helps circulate the air, thus improving cooling and heating. Exhaust ventilation systems work well, pulling air from around the house outside.