Calculating Minimum Thickness if

Multiple Materials are Used

At times it is important to know what combination of materials are acceptable for use as floor protection. The “R values” are used to determine acceptable combinations of materials because “R values” are additive where r and k values are not.

“R value” = 1/k = "r" x thickness of material used

Example: Given that the required “R value” for a suitable floor protector used must be equal to or greater than:

“R” = rL x TL = 2.33 x 1” = 2.33."

If it is desired to elevate a marble hearth extension to a level of 5” or more above the floor surface. What combination of noncombustible materials can be used to accomplish this?

If common brick is used so that the 3 1/2” dimension is the height, “R” for the common brick becomes:

brick

“R”M = rM x TS = 0.20 x 3 1/2" = .70

Using 1/2” of mortar to set the brick, “R” for the mortar is calculated as follows:

mortar

“R”M = rM x TS = 0.20 x 1/2" = .10

Next, a 3/4" marble slab set in 1/2" mortar covers the brick, “R” for the marble and mortar becomes:

marble

“R”M = rM x TS = 0.09 x 3/4" = .068 mortar

“R”M = rM x TS = 0.20 x 1/2" = .10

The sum of all “R values” is: .70 + .10 + .068 + .10 = .968

This would NOT be an acceptable combination of material for the hearth extension since the total calculated “R value” of the materials used is under the required “R value” of 2.33. An additional layer of insulating materials must be used.

Note: Also see NFI Certification Manuals for expanded explanation on calculating “R val- ues” when multiple materials are used.

22

WARNING

The crack between the fireplace and the hearth extension must be sealed with a noncombustible material.

AVERTISSEMENT

La fente entre le foyer et l’extension d’âtre doit être scellée avec un matériel non- combustible.

WARNING

When installing hearth extension in front of a non-circulating fire- place model, the fireplace must be raised if height of hearth extension exceeds 7" above the bottom of fireplace (Figure 54).

When installing the hearth extension be careful not to block the heat-circulating air inlet grill on circulating models.

AVERTISSEMENT

En installant la prolongation de foyer devant un modèle decirculation de cheminée, la cheminée doit être augmentée si la taille de la prolongation de foyer excède 7”(pouces) au-dessus du fond de la cheminée (Figure 54).

En installant l’extension d’âtre faites attention à ne pas bloquer la grille d’entrée d’air sur les modèles à circulation d’air.

NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE.

7"

Max. Thickness Of Hearth Extension When Non-Circulating Models Are On The Floor

Figure 54

Secure the hearth extension to the floor to prevent possible shifting.

FINISH TO YOUR TASTE

There are a wide variety of “finished looks” for these fireplaces, from formal wall decor with elaborate mantels to rustic wood paneling or warm brick facings.

Only noncombustible materials like stone, tile, brick, etc., may overlap the black front facing. Do not extend these materials beyond the black facing and into the firebox area. Be sure not to interfere with the installation and operation of glass doors or block the upper or lower grilles. Seal all joints between the black facing and wall surrounds to prevent air intrusion. Use noncombustible caulking material only to seal the black metal facing to the surround material on the finished wall.

Combustible materials may project beyond the sides of the fireplace opening as long as they are kept within the shaded areas illustrated in

Figure 55.

Combustible Ma-

 

 

terials Allowed In

1/2"

Fireplace

ShadedArea“Safe

 

 

Zone”

 

1"

7"

 

 

 

 

45°

 

12"

7"

Min. Distance To

 

 

 

Protected Side Wall

12"

Min. Distance To

 

 

Unprotected Side Wall

Figure 55