A material that inhibits the flow of heat. See also insulation. More below...

Above definition Copyright © The Home Ventilating Institute. Some content originally appeared in John Bower's book, Understanding Ventilation, published in 1995 by The Healthy House Institute.

More info from HHI:

Insulation in the walls, floors and attic is something like a blanket on a bed – the thicker it is, the warmer you’ll stay during the cold months while keeping heating costs down. The same principle works in reverse during summer, as insulation keeps heat out, reducing the air conditioner’s burden.

For this reason, all forms of insulation can be thought of as “green.” Reducing the need for gas, heating oil, electricity, or wood to keep a home comfortable at times of temperature extremes, means less carbon dioxide and other pollutants in the atmosphere – and a lighter burden on your energy budget. Even better, it keeps on saving energy and money, because insulation doesn’t wear out over time under normal conditions.

In addition to energy and environmental benefits, better insulation can mean better health. A well insulated and sealed home may have a higher relative humidity in winter than one without such improvements. A relative humidity between 30 percent and 50 percent is sufficient for most people to avoid sinus problems and vulnerability to illness from constant exposure to a dry environment, while being dry enough to prevent growth of mold in the home. In dry climates, this also may reduce or eliminate the need for artificial humidification, resulting in even greater energy savings.

Many existing homes were built with what’s now considered to be inadequate insulation, and may lack it entirely. Insulation wasn’t required as part of home construction until the mid-1960s.

Even in newer homes with better insulation, sloppy installation can still mean cold walls and drafts in unexpected places. That’s because most forms of insulation are not designed to block air infiltration around corners and gaps. In addition to better insulation, products like spray foam and caulk may be installed both indoors and outdoors to block drafts through gaps in the wall.

Indoors, components such as plumping pipes and electrical wiring installed through walls, subfloors and ceilings allow cold and warm air to enter or escape. Cold (or hot) air also enters the home from outside through gaps at corners, edges and between trim and siding, and around service entrances such as water pipes, gas mains, electrical wires and telecommunications cables.

The surest way to determine insulation and sealing needs is to hire an energy auditor, a contractor who uses specialized tools to find hidden air leaks in walls, ceilings and ducts. The auditor will analyze the test results and furnish a report with recommendations for better energy efficiency. The service costs about $300 in most major markets. Many private home inspectors also perform energy audits, as do some electrical and natural-gas utility companies.

Armed with this information, you’ll know what to ask for when considering contractors to beef up your insulation effectively while avoiding the expensive “extras” your home doesn’t really need.

A discussion of the pros and cons of the most common forms of insulation follows in the table below. Comprehensive information may also be found at the U.S. government’s ENERGY STAR Home Sealing page.