Hearth Extension for Floating Hearth - R value

[BurntEmber]

I have a question about hearth extension R values, specifically for “floating hearths” (i.e., slab hearths that extend out from the chimney, but that hover above the room floor by 10-12”). We are looking to install a Hearthstone Clydesdale wood burning insert into a fireplace with a floating hearth and the chimney guy indicated that we would need a hearth extension with an R1 rating to prevent errant embers from falling down onto the hardwood floor below. I can see why the hearth extension would provide combustion protection for hot ash and embers that might pop out onto the floor and bounce underneath the floating hearth, but I don’t quite understand why an R1 rating is necessary? Wouldn’t replacing the hardwood beneath and surrounding the floating hearth with porcelain or ceramic tile and durarock backing board be adequate for embers? I always thought that the hearth extension was primarily protecting against heat transfer that comes directly from the firebox or stove. Since our insert will be inside a fireplace and 12” above the floor, I am trying to understand if the R1 is truly necessary in this application. Thanks.

 

[Nick Mystic]

The manuals that come with most stoves and inserts are quite specific on such requirements. Your manual should tell you exactly what is required for the Clydesdale. As to the R1 rating for a hearth extension, as you mention, hot embers can fall out of the insert and bounce off the hearth landing on the floor below the raised hearth. An extension on this area of the floor would have to offer sufficient R1 value to keep the heat from this burning ember from igniting the combustible material below it. Remember, these values are set with a wide margin of safety in most instances, so you could be correct in thinking that a material like ceramic tile could suffice, but the regulators are looking for that 1 in a 1000 cases that could occur.

 

[begreen]

The R1.0 requirement is for when the hearth is at the same level as the fireplace insert. There is no definitive spec that I know of for continuing protection at the floor level when there is a raised hearth due to the location variables. If the hearth is high and has say half the required depth then perhaps only ember protection is needed at the floor level. Unfortunately the insert maker doesn't know the height of the hearth, how much extension at the floor level is needed, etc.., so they err on the side of caution and safety.

 

[BrotherBart]

R1001.9.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51mm).

Exception: When the bottom of the firebox opening is raised at least 8 inches (203mm) above the top of the hearth extension, a hearth extension of not less than 3/8 inch-thick (10mm) brick, stone, tile or other approved noncombustible material is permitted.

 

[begreen]

Thanks for the reminder. Can one fall back on NFPA 211 when the manual doesn't mention this as an option? That seems to be the accepted rule for clearances.

 

[BurntEmber]

Hearth.commers,

Thanks one and all for your thoughtful responses, they was very helpful. Looks like in this case, Maryland code defers to the International Residential Code cited by BrotherBart.