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insulation insulation informationConsumer Guide to Building Insulation |
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| There are two basic types of building insulation - Bulk Insulation and Reflective Insulation. Many 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. | ||
Different Modes of Heat Transfer Require Different Insulation Strategies |
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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. Convection - Convection in buildings is the transfer of heat caused by the movement of warmed air. Convective heat flow occurs whenever warm air contacts a cooler surface. In a building space, 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. 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. Insulation to resist the flow of radiation uses aluminum foil to reflect back the infra-red rays so they do not penetrate into the building from the sun or exit the building in winter. Aluminum can reflect up to 97% of radiation that strikes it. Reflective Insulation - 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. |
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Reflective Insulation
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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%
Radiant barriers (aluminum foil with no center layer) become reflective insulation because they face an air space like an attic, wall cavity or crawlspace. Reflective insulation works completely differently than all the other types of insulation on the market. |
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| Click here for a comprehensive article on the Physics of Foil. | ||
Types of Reflective Insulation | ||
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Common constructions for reflective insulation are:
Foil-Foam-Foil - Two layers of polyethylene backed foil with a center layer of polyethylene foam. At R-14.5 (Details), Foil-Foam-Foil has more insulation value than 6 inches of common mass insulation, reflects radiant heat, acts as a barrier to convective and conductive heat transfer, is vapor-proof, and is unaffected by humidity. It is used in roofs, attic, ceilings, walls, floors, and metal buildings. Foil-Bubble-Foil - Two layers of polytheylene backed foil with a polyethylene bubble layer in between. At R14.1 (Details), Foil-Bubble-Foil reflects radiant heat, acts as a barrier to convective and conductive heat transfer, is vapor-proof, and is unaffected by humidity. It is used for new and retrofit construction in roofs, attics, ceilings, walls, floors and metal buildings. Foil-Fiberglass-Vinyl - Encapsulated fiberglass between polytheylene backed foil and white reinforced vinyl. At R14.1 (Details), Foil-Fiberglass-Vinyl reflects radiant heat, acts as a barrier to convective and conductive heat transfer, is vapor-proof, and is unaffected by humidity. White vinyl is aesthetic and is used in metal buildings or other places where the insulation will show. It is also used with concrete floors, above or below ground swimming pools and foundation walls. Foil-Foil - Two layers of aluminum foil backed by woven polyolefin. Foil-Foil is a radiant barrier. It is used to reflect radiant energy as a house wrap, under the roof, in the attic, and with radiant heat flooring. | ||
Reflective Insulation used as House Wrap
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| A note about R-value: R-Factor is a measure of the ability of insulation material to resist heat transfer per 1 inch of material. "R-Value" is the R-Factor of the insulation multiplied by the amount of the material. R-value is a laboratory measurement that does not effectively measure all 3 methods of heat transfer that occur in your home: convection, conduction, and radiation. It is only a measure of convective heat transfer while radiation is the dominant method of heat transfer in a building. R-value cannot be used to compare the indoor comfort level or energy savings potential of reflective versus bulk insulation. In addition, R-value cannot be used exclusively to compare the different types of bulk insulation either as each has different properties relative to sealing the building against air infiltration. | ||
Types of Bulk Insulation | ||
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Fiberglass Batts - Fiberglass is the most common type of building insulation. It is made of fine strands of glass layered in fluffy batts that trap air. The average resistance is R-3 per inch. It comes in various thicknesses and widths to accommodate the standard spacing of rafters, floor joists and stud walls.
Unfortunately, fiberglass has many downsides. If it was not so inexpensive, no one would willingly subject themselves to the danger and misery of installing fiberglass insulation given the option. Warnings on insulation rolls advise you to completely protect your self from coming into any kind of contact with the fiberglass - wear safety glasses, face mask, gloves, long tight sleeves, long pants. And still, the itchy fibers manage to get in. Click here to see the standard warning label on fiberglass insulation | ||
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| "To be effective, fiberglass must keep its pillowy loft and not be exposed to high-moisture conditions such as a damp basement where the material will be compacted if it gets wet, negating its insulation value". Fiberglass insulation requires an additional vapor barrier (or vapor retarder) between the warm surface of the wall or ceiling and the insulation. Vapor retarders are either aluminum foil (such as reflective insulation), treated paper, or plastic that inhibits the passage of water vapor and keeps moisture from migrating into insulation and building cavities. | ||
Reflective and Batt Insulation used Together in a Wall
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Alternatives to Fiberglass InsulationFortunately, there are many alternatives to fiberglass that effectively insulate against convective heat transfer and are equally inexpensive.
Blown cellulose - Cellulose is composed of recycled newspaper and small quantities of shredded cardboard. This is a loose material that is installed with professional air blowers through injection holes - usually into walls or attic from the building exterior. Cellulose insulation can achieve resistance of R-2.3 to R-3 per inch. It carries less health risk than fiberglass, but can condense as it settles and take on moisture, which will significantly degrade its resistance value and potentially grow mold if it stays wet. Spray Foam - Polyurethane foam insulation is ideal for filling small spaces - such as window jambs, small stud bays, rim joist areas, and for sealing around electrical boxes where cutting fiberglass batt insulation to fit can be difficult. Foam expands quickly and cures in place to become a closed-cell system that air cannot penetrate thereby providing insulation and air sealing in one step. Foam has an average R-value of R-5 per inch. Many sprayed foams are environmentally friendly, do not contain HCFCs that are harmful to Earth"s ozone layer, or off gas formaldehyde-a potentially harmful component of older fiberglass batt insulation. Rigid foam - Rigid foam is made of air-entrained plastic formed into sheets. Water-resistant rigid foam board has almost double the R-value of cellulose or fiberglass insulation. R-values for rigid foam range from 3 -7.2 per inch. It is more resistant to air and water vapor than batts because of its density. Rigid foam is divided into three types: polyisocyanurate (polyiso), expanded polystyrene (EPS) and extruded polystyrene (XPS), each varying in cost, R-value and environmental friendliness.
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References | ||
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Old House Journal Insulation from the Top By Noelle Lord, April 2006
Energy Efficient Rehab Advisor
ToolBase Services
Mother Earth Living
Mother Earth News
Reflective Insulation Manufacturer"s Association
Quick Guide: Insulation & Ventillation
Do-It-Yourself Housebuilding, The Complete Handbook | ||
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