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R Values of common hearth materials

Q: My new woodstove's installation manual calls for a non-combustible hearth (floor protection) extending so many inches beyond the rear and sides of the stove and in front of the loading door. This much I understand. The manual also states this floor protection must have an "R" value of at least 0.800. This part I don't understand: what is an R value, and how do I ensure that my hearth material meets the minimum requirement?

Sweepy R-value is a measurement of the insulating properties of a given material at a given thickness. Here's a chart showning the R-values of several common hearth materials:

Material Thickness R-Value
Mineral Fiber Board ( Micore 160 )1/2" 1.270
Ceramic Board ( Rescor 360 )1/2"1.110
Ceramic Board ( Fiberfrax Duraboard LD )1/2" 1.100
Mineral Fiber Board ( Micore 300 )1/2"1.030
Common Brick4" 0.800
Common Brick2-1/4" 0.450
Gypsum or Plaster Board1/2" 0.450
Cement Board ( Durock Next Gen)1/2" 0.390
Cement Board ( Wonderboard ) 1/2" 0.260
Cement Board ( Hardibacker 500 ).42"0.200
Limestone1" 0.153
Cement Board ( Hardibacker )1/4"0.130
Cement Mortar1/2" 0.100
Slate1" 0.100
Concrete1" 0.095
Marble1" 0.090
Ceramic Pavers1" 0.080
Ceramic Tile1/4" 0.020

Sweepy As the chart shows, you can achieve the required R-value of 0.800 with 1/2" of Fibrefrax or Micore, 8-1/2" of poured concrete, or one layer of 4" brick. You can also meet your requirement by stacking layers of the materials listed: ie, 4 layers (1.68") of .42" Hardibacker would give you the 0.800 R-value you require.

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