The purpose of insulation is to slow the rate of heat transfer. This is true in both hot and cold climates. In cold climates, it is intended to stop the flow of heat out of the building. In hot climates, its purpose is to slow the movement of heat into the building. Fortunately, insulation works in both directions. For the many climates in North America where heating and cooling are needed at different times of the year, insulation serves both functions.
We emphasize insulation only slows the movement of heat. It does not stop it entirely. What’s more, it isn’t the insulation material that stops most of the heat. The trapped air pockets within the insulation do most of the work. The solid material of the insulation does reduce direct radiation. Conduction and convection are controlled by still air.
Not surprisingly, no insulation meets all these criteria.
The insulation value of walls is more than the value of the insulation itself. All materials have some insulating value, including the plaster or drywall, polyethylene air/vapor barrier, wood sheathing, building paper, siding and the air films. However, the majority of the R-value of the wall is attributable to the insulation.
Gaps in insulation can dramatically reduce the effective R-value of a wall assembly. If insulation batts do not completely fill stud cavities side to side and top to bottom, convective loops can be established as the air moves freely through the stud cavity. Voids can allow a great deal of heat through the wall. It is important that voids are filled. Mineral wool and fiberglass batts can lose up to 30 percent of their effective R-value if there is a 4 percent gap in the insulation, measured over the exposed, insulated surface.
Typically, insulation is not designed to prevent heat loss from air leakage. Air sealing, including caulking and weatherstripping, reduces air leakage.
By and large, insulation takes one of four forms:
We’ve talked about some of the desirable features of insulation materials. The chart below (PDF of the chart) sets out the properties of various insulating materials. Some of the information in the chart has been approximated and there are exceptions to these numbers. We understand there are variances with each material, depending on density and other factors, but this is a broad guideline. Now, let’s look at some of the insulation materials.
Fiberglass: Fiberglass is a commonly used, versatile, inexpensive insulation. The glass itself is not combustible, although the binders used in it may be. Fiberglass may have an air/vapor retarder fixed to the surface, which could be kraft paper, foil, vinyl or a housewrap.
Mineral wool: Mineral wool insulation is similar to fiberglass. It is made with slag or rock spun into a wool. Typically treated with oil to reduce the dust, and with binders to hold it together, it is non-combustible, water-resistant, rotresistant and inexpensive.
Cellulose: Cellulose is shredded newspaper, chemically treated to resist fire, rot and fungus and to prevent corrosion when wet. Cellulose is denser than fiberglass and therefore provides a better air barrier than fiberglass. Cellulose is less susceptible to wind-washing and convective air movement through the insulation itself. 44
Vermiculite or perlite: Vermiculite is expanded mica, and perlite is a rock similar to granite. Perlite, when expanded, takes on a popcorn-like appearance. Vermiculite, when expanded, becomes small, light cubes.
Polystyrene: Polystyrene insulation is a combustible plastic. It comes in two common forms:
Concrete Forms: Polystyrene also can be used as permanent forms for poured concrete, both above and below grade. This provides a concrete wall with good insulating value.
This type of insulation deserves special attention because it has been controversial.
Urea-formaldehyde foam insulation, or UFFI (pronounced “you-fee”), was a common retrofit insulation in the 1970s. In general, it was injected as a mixture of urea-formaldehyde resin, an acidic foaming agent, and a propellant such as air. For the most part, it was used in existing houses, most often injected into wood-frame wall cavities or other areas where
installation of conventional insulation was impractical. Holes in the wall may have been used to insert UFFI insulation in this brick home. UFFI has been studied extensively and there is no evidence of any health issues. From a real estate value standpoint, UFFI may have an impact in your working area; however, our research has not convinced us of any reason a homeowner should have any UFFI-related health concerns.
We have introduced the function of insulation, the forms, common insulation materials and their properties. There is much more to learn about this system of a home. Greater details on the material types, common deficiencies, strategies for inspecting and implications can be found in the ASHI@Home training program.
Conduction: The transfer of heat between substances that are in direct contact with each other. Conduction is slowed by insulation.
Convection: The transfer of heat by the circulation or movement of the heated parts of a liquid or gas. Convection can be interrupted by a physical barrier.
Radiation: Radiation is electromagnetic waves that directly transport heat energy through space. Heat energy transferred through radiation can be interrupted by a material that reflects it.
Air films: The air space that exists within the wall assembly that adds to the insulation value of the wall.
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