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Table 9.1 R-Value of fiberglass batts compressed within various depth cavities.

Table 9.1 R-Value of fiberglass batts compressed within various depth cavities.

Outside Air Film 4" Brick

R-l 1 Batt Insulation 3-1/2' Fir Framing (23%) 5/8' Gypboard inside Air Film

Figure 9.4

comprise 20-25% of a typical wall (including window framing, sill, etc.). Wood studs installed 24 inches on center comprise approximately 15-20% of the gross wall.

The use of this method is appropriate for situations where the materials in the wall section are sufficiently similar that little or no lateral, or "sideways" heat transfer takes place. Of course a certain amount of lateral heat transfer does take place in every wall, resulting in some error in the above calculation procedure. The amount of error depends on how thermally dissimilar the various elements of the wall are. The application of this procedure to walls whose penetrating members' conductivities deviate from the insulation conductivities by less than an order of magnitude (factor of 10) should provide results that are sufficiently accurate for most analysis of construction materials.

Because the unit R-value for wood is approximately 1.0 per inch and fiberglass batt insulation is R 3.1 per inch, this approach is justified for wood-framed building components.

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