Insulation Materials
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Function
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The purpose of insulation is to slow the rate of heat transfer. This is true in both hot and cold climates. In cold climates, we are trying to stop the flow of heat out of the building. In hot climates, we are trying to slow the movement of heat into the building. Fortunately, insulation works in both directions. For the many climates in North America that need heating and cooling at different times of the year, insulation serves both functions.
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Insulation Doesn't Stop Heat
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We should emphasize again that insulation only slows the movement of heat. It does not stop it altogether. We should also remind ourselves that it really 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 the still air.
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Characteristics
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The ideal insulation material would have the following characteristics:
- High resistance to heat flow (a high R-value)
- Inexpensive
- Durable (lasts the life of the home)
- Completely fills cavities
- Air barrier (stops air leaks)
- Vapor barrier (stops vapor diffusion)
- Moisture and rot resistant (because all houses eventually leak)
- Noncombustible
- Chemically inert
Not surprisingly, no insulation meets all these criteria.
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Measuring Insulation
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Resistance to thermal conductivity is referred to as an R-value. The metric equivalent to this is RSI. To convert from R to RSI, multiply the R by 0.1761. To convert from RSI to R, multiply the RSI by 5.679.
- RSI = R x 0.1761
- R = RSI x 5.679
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All Materials Add To R-Value
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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.
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Thermal Bridges
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Thermal bridges occur at studs, top plates, sill plates, etc. These are points where the insulation is not continuous. Solid, high-density materials like wood studs conduct heat readily though walls. That's why these are called thermal bridges.
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Wall Marks Due To Bridging
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Thermal bridges not only cause heat loss, but can lead to localized cool temperatures on interior surfaces. Cool interior surfaces can result in surface condensation. The moisture on the surface attracts dust particles and, over a period of years, will create a permanent pattern on the wall. You may have seen dirt marks on walls in older houses that have not been painted for a long time. Very often, these dirt marks follow the pattern of the wall studs. In other cases, they may follow the wood lath. The dark spots are typically the areas where thermal bridging has resulted in cool interior surfaces and localized condensation on the plaster surface.
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Insulation Voids And Convective Loops
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Gaps in insulation can dramatically reduce the 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 R-value if there is 4 percent gap in the insulation, measured over the exposed, insulated surface.
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Loose-fill insulation that is blown or poured in can also settle, creating voids at tops of wall cavities, for example. Where insulation is poured or blown into wall cavities, voids may develop if the insulation gets hung up on obstructions partway down the cavity. Voids may also develop in insulations that shrink. Urea-formaldehyde foam insulation sometimes shrank dramatically after it was installed, reducing its R-value.
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Heat Loss Through Air Movement
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Insulation is not typically designed to prevent heat loss from air leakage. Air sealing, including caulking and weather-stripping, reduces air leakage.
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Wind Washing - Roofs
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We've touched on a type of air movement that can dramatically reduce the R-value of insulations such as loose-fill fiberglass and mineral wool. Depending on the density of the insulation, it may be relatively easy for air to blow through the insulation. On a windy day, considerable air movement may be experienced in an attic, for example. If air blows through the insulation on the attic floor, the R-value will be dramatically reduced. Remember that the goal of insulation is to trap air pockets and keep them still.
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Wind Washing - Walls
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Similarly, the windward side of a wall may allow enough air movement into the wall system to dramatically reduce the performance of fiberglass batts in the wall. Dense insulation materials, including cellulose fiber and most of the rigid board insulations, are not as susceptible to wind washing as fiberglass and mineral wool.
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Housewraps
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Housewraps are designed to stop wind washing by acting as an exterior air barrier. These breathable fabrics form a tighter windbreaker or outer jacket on buildings than does traditional building paper. While some debate the merit of housewrap over building paper, reduced wind washing is one of the advantages of housewraps, according to their supporters.
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