Exception F200 Wood Insert
Appendix A
Determining Thickness Requirements ofAlternate Hearth Extension Materials.
If a manufacturer’s specifications call for a materialhaving a minimum thickness of one inch with a certain k value, any material substituted for that specified materialmust have the same thickness and have the same or lower kvalue, or if the k value of the selected alternate material is higher, a greater thickness of that material must be used. In some cases, if the k value is lower, thinner materials may be used.
The followingformula gives the meansof determiningthickness of alternate materials:
k of the alternate material | x thickness of | = thickness of |
k of the specified material | specified material | alternate material |
Example 1: The manufacturer specified a certain material aminimum of one inchthick that is appropriate for use as a hearth extension with their fireplace system. The k value for that material is .84. You decide to use common brick for hearth extension protection instead of that specified material. How thick must the common brickhearth extension be to be equivalent with that which the manufacturer specifies? (Common brick has a K value of 5.)
Using the given formula to determine the equivalent thickness of common brick for the hearth extension, you will divide 5 by .84, which gives a thickness of 5.95 inches of common brick. This means that it would take about six inches of brick to provide the same thermalprotection as one inch of the specified material.
NOTE: Most of the hearth extension protection materials specified by manufacturer’s instructions
are much better insulators than common masonry materials. In this case, brick thickness must be about 6 times greater than the manufacturer’s recommended material.
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