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Home Insulation Basics

Published on: August 30, 2012

In cold climates, insulation has been added to especially well-built houses for 100 years or more. In the past thirty years, new energy conservation regulations have made ever-greater amounts of insulation mandatory in new homes.

We can offer sound advice on how to increase wall and attic insulation energy efficiency in an existing home that was originally constructed with little or no insulation.

The Building Envelope

The building envelope is the skin of the house; it consists of the parts that are exposed to exterior air and weather. That usually means the walls, doors, windows and roof. These are the surfaces through which heat, moisture and air can move into and out of the building.

Insulation helps homes to retain heated and cooled air by slowing down its movement through the building envelope, and sometimes also by reducing air leakage. In general, a good insulation strategy will slow the movement of heat through the building envelope, prevent moisture from becoming trapped on vulnerable surfaces, and maintain adequate ventilation.

Slowing the passage of heat through the building envelope usually involves increasing wall and attic insulation energy efficiency.

How insulation works

Heat can move in three ways: conduction, convection, and radiation.

Conduction is the movement of heat through materials. Different materials have different heat conduction potentials, or R-values. In general, the less dense a material is, the better it will be at blocking the movement of heat.

Metal is a good conductor and is very poor at blocking the passage of heat. That’s why it’s used for cooking surfaces. Wood is better than metal for blocking heat transfer; it’s a medium-density material. Still air is even better, in fact, it’s what most types of insulation use to slow heat transfer.

If air is such a good insulator, then why do we need insulation?

We need insulation because of the other ways that heat transfers: convection and radiation. The air that occupies open spaces like the gaps between studs and joists doesn’t stay still. It circulates around the space. That allows any heat that the air has absorbed to pass between the inner and outer surfaces of the wall or loft. Heat can also radiate through air, passing directly from one surface to another.

Insulation is designed to trap air and prevent it from circulating and transferring heat through convection. It also slows the transfer of heat by radiation.

Types of insulation

The most common types of insulation are batts, loose fill, foam boards, and spray foam.

Batts come in rolls and are made of fiberglass and other types of fiber, such as cotton and wool. Batts are commonly used as both wall and loft insulation in stick-framed houses. They are inexpensive and are available in a variety of thicknesses. Their drawback is that they tend to sag in vertical installations, leaving spaces at the top of the stud cavities where air can circulate.

Loose fill insulation is made of similar materials and has many of the same advantages and disadvantages of batt insulation. However, it can be blown into existing walls, and it will neatly fill the gaps between joists.

Foams boards can be used to add insulation to the outsides of walls and roofs. It is usually added underneath the wall cladding or the roof membrane.

Spray foam can be injected into existing walls and will fill gaps in odd, uneven and difficult to access spaces. Because insulation is largely meant to prevent convection, the fewer gaps it has, the better.

How insulation can be added or supplemented

In a stick-framed house, one that’s constructed of wood or metal studs and joists, insulation is placed between the studs inside the walls and between the attic joists, roof rafters, or roof trusses. With added wall and attic insulation energy efficiency, HVAC equipment doesn’t have to work as hard.

Adding more volume can increase attic insulation energy efficiency, but it is important to take the passage of moisture into consideration as well. For example, in cold climates, a well insulated attic floor that is not air tight can cause chronic condensation on the underside of the roof, and that will invite mold and rot. Talk to your insulation contractor about the best ways to avoid condensation.

With increased wall and attic insulation energy efficiency, you can save a significant amount of money on your heating and air conditioning. If your house is currently uninsulated, adding insulation could make your heating and cooling costs a fraction of what they are now.

If you’re upgrading existing insulation, then the potential savings are not as dramatic. However, with the rising price of energy, it may still be cost effective. A qualified energy auditor can help you decide whether an insulation upgrade would be a sound financial choice for your family.