A well insulated home will create an effective weather barrier to keep indoor temperatures comfortable year-round and minimize your heating and cooling energy costs. Properly installed insulation will keep the heat outside in summer and inside in winter. The picture below shows all the places in your home that require insulation.

Heat moves through wall cavities, between roofs and attic floors or between floors and basements by a combination of conduction, and convection and radiation with radiation being the dominant method of heat transfer. Research shows that control of radiant heat transfer is the core of heating/cooling climate control.

Conduction

Conduction is the direct flow of heat resulting from physical contact of a warmer body with a cooler body. The transfer of heat by conduction is caused by molecular motion in which molecules transfer their energy to adjoining molecules and increase their temperature. An example is heat transferred from a hot burner to a pot through direct contact.

The denser a material is, the better it will conduct heat. Because air has such low density, air is a very poor conductor and therefore makes a good insulator. Insulation to resist conductive heat transfer uses air spaces between fibers, inside foam or plastic bubbles and in building cavities like the attic and the walls.

Convection

Convection is the transfer of heat caused by the movement of warmed air. Convective heat flow occurs whenever warm air contacts a cooler surface. Warm air rises, transfers its heat to a cooler surface, cools and then settles. The pressure from falling cool air helps push more warm air up. An example is warm air rising from a radiator - the air in direct contact with the radiator has first been heated by conduction. Heating increases the energy of the air causing its molecules to move about more rapidly and spread farther apart making the air less dense. Warm air is less dense (lighter) than cold air and so it rises.

Insulation to resist the flow of heat through convection uses small air spaces between fibers, foam, plastic bubbles, paper, straw, etc. to trap the rising air and slow heat loss through walls and ceilings. Common forms of mass insulation used to impede convective air flow include fiberglass or rock wool batts, rigid foam, spray foam, blown cellulose and straw.

Radiation

Radiation is the movement of infra-red energy through air or a vacuum. All surfaces above Absolute Zero emit radiation to different degrees including a stove, a ceiling and ordinary insulation. Radiant energy travels outward from a source in all directions at near light speed until it is absorbed by a body in its path, whereupon it is transformed into kinetic energy - or heat - within the intervening body. When this energy strikes a dense surface, it is absorbed and increases the temperature of that surface.

An example is radiation from the sun that strikes the outer surface of a house wall and is absorbed causing the wall to heat up. This heat flows from the outer wall to the inner wall through conduction and is then radiated again through the air spaces in the building to other surfaces. Radiation is the dominant method of heat transfer in a building accounting for 65-85 percent of all heat transfer through walls, ceilings, attic and floors.

While other types of insulation are made to resist or impede the flow of warm air, reflective insulation reflects back radiant (infra-red) energy from the sun so it does not penetrate the building. It can also reflect back radiant heat inside the house so it does not escape. The concept is simple: each unit of radiant heat energy that is reflected away from your home in summer and each unit reflected back inside during winter means less operation of your air conditioning and heating systems, less wear and tear on your equipment, and less money you pay in utility costs.

Reflective insulation is commonly made of either aluminum foil attached to some sort of backing material or two layers of foil with foam or plastic bubbles in between creating an air space to also resist convective heat transfer. The aluminum foil component in reflective insulation will reduce radiant heat transfer by as much as 97%.

Use reflective insulation in the roof, attic, walls, floor, basement, around the hot water heating system and around ductwork. Reflective insulation can be used by either by itself or in conjunction with other types of insulation where convective heat loss is greatest - particularly in the attic. Reflective insulation is impermeable to water vapor and will create a vapor retarder to protect mass insulation and wood from mold, mildew and fungus if installed properly.

There are two basic types of building insulation: Bulk Insulation (blankets, foam, or cellulose), and Reflective Insulation.

Most buildings use a combination of both types to make up a total building insulation system to effectively resist the three forms of building heat transfer: Conduction, Convection, and Radiation.

The following illustration shows how rigid foam and reflective insulation can be used together in a cathedral ceiling to provide a complete insulation system. Similar combination systems can be used in walls, attic, and floors to achieve the greatest insulation efficiency.

Radiant barrier

All new buildings should incorporate a radiant barrier as part of the insulation system. The concept is simple: each unit of radiant heat energy that is reflected away from your building in summer, and each unit reflected back in during winter, means less operation of your heating and air conditioning systems, less wear and tear on your equipment, and less money you pay in utility costs. Use either a foil radiant barrier or reflective insulation. Radiant barriers (aluminum foil with no center layer) become reflective insulation because they face an air space like an attic, wall cavity or crawlspace. While radiant barriers primarily impede the downward flow of radiation, using a radiant barrier in the floor will also impede the upward flow of radon gas.

Moisture control - Vapor barrier

A vapor barrier (or more accurately, vapor diffusion retarder) is an essential part of the moisture control strategy for a home. The primary purpose of a vapor barrier is to keep moisture from getting inside your walls. Condensation within the walls can cause wood rot, mold, mildew and fungus growth. A vapor barrier acts as a physical shield to repel moisture. In addition to its properties as a radiant barrier, reflective insulation can be used as a vapor barrier. When properly installed, waterproof reflective insulation can reduce or eliminate condensation. As reflective insulation is non-absorbent, it will not mildew or promote fungus growth.

Air infiltration - House wrap

One of the key reasons to use insulation is to keep both hot and cold air outside. Leaks in the exterior of the house will allow this air to migrate inside and cause your heating and air conditioning systems to work harder to control the inside temperature. House wrap can be used under metal, vinyl, wood or cement plank siding will seal the building against outside air infiltration when properly installed and taped. Reflective insulation can double as both a radiant barrier and a house wrap.

Proper installation is the key to achieving the goals of the insulation system in terms of temperature control, energy efficiency and moisture control. Insulation should be installed in the attic including the attic door, the ceilings, the roof over a heated attic or cathedral ceiling, walls, floors over a crawlspace or unheated basement, walls in a heated basement, under the crawlspace for a vapor barrier, around ductwork and water pipes, in the garage including the garage door, over and around the swimming pool and around the hot water heater.

Germer, Jerry,
"Quick Guide: Insulation & Ventilation",
Creative Homeowner Press, Upper Saddle River, NJ, 1995.

Nash, George,
"Do-It-Yourself Housebuilding, The Complete Handbook",
Sterling Publishing Company, 1995.

Barr, Jodie,
"Consumer Guide to Building Insulation",
www.insulation-insulation.com. October 2007.