Is it safe to put a woodstove on a sheet of cement board?

Question

I'm looking at how to install a woodstove in a yurt. In this example, it looks like he put 1 sheet of cement board on the floor, and another against the wall.

(click for full size)

Is a single sheet of cement board enough to protect the wood floor from the heat of the stove?

(More pictures here: http://www.barefootboy.com/yurt/. This is not my yurt.)

The woodstove I'm looking at says this in the manual:

A combustible floor must be protected by a non-combustible material (like tile, concrete board, or certified to UL-1618 or as defined by local codes) extending beneath the heater and a minimum of 6" from each side and minimum 16" from the front face of the stove and minimum 6" (or the rear clearance to combustibles whichever is smaller) from the rear of the stove.

which doesn't tell me much.

EDIT: I just got this response from the manufacturer:

The woodstove does not require a R value, the pedestal or heat shield and legs raises the firebox high enough not to require the R value. You just require the floor protection like a piece of sheet metal and tiles etc.

Answer 1

Actually there are a few considerations you need to take into effect.

You first need to look up the "R" factor required from combustible materials for your specific stove. Every manufacturer has a safety insulation factor. This can be provided by materials such as concrete board, high density fiberboard insulation, steel sheets and air barriers. The factors can be different from the bottom, sides and back of a stove, so it is important to have the full specs. You should also check with the local building inspector to get the minimum distances from combustibles in your area. I believe most communities use the NFPA specs, but always check local requirements.

Practically speaking, using a single piece of concrete board under a wood stove is rarely enough insulation factor. I think 1/2" concrete board only has a R-rating of 2 or 3, which is much too low if attached directly to a wood floor without an air space. I would feel much more comfortable with a layer or two of brick or brick over high temp fiberboard insulation. High temp fiberboard can be rated as high as 800 degrees F per inch at 6 inches away from heat source. Available at any HVAC supplier.

After all the work you have done on your yurt, be absolutely safe and sure of you woodstove installation.

Good Luck my friend.

Answer 2

There's enough space between the firebox and the walls that the risk of the walls bursting into flame from the heat is non-existent.

The reason the directions tell you that the combustible floor must be protected with a deeper protection boundary in the front than on the sides is due to the risk of hot embers falling out of the firebox and landing on the floor which could start a fire. Especially on a finished floor or carpet.

Fire requires three things - fuel, heat, and oxygen. Remove one and you remove the fire.

Covering the wood with the non combustible material removes fuel from the equation - and if the concrete board DID ever reach temperatures in excess of 525F (the ignition temperature of wood), its presence over the wood removes fuel from the equation. (Plus if that happens you've got bigger problems anyway...)

Answer 3

I used cement board in that same fashion (under stove and as a back wall) for one year. It got extremely warm (almost hot) on the wood floor and the wall board in back of the stove got hot, but not much of a concern. The back of the stove was about 18" to 24" from the wall. There was no air space behind the wall board that was attached to the studs. It was screwed directly to those studs. After one year, the cement board hearth had to be discarded. It was all cracked up. The wall cement board was fine and then a second sheet was installed over that first sheet, then granite thinstone directly applied to that second sheet. No worries other than I was told it did not pass code whereas there had to be a 1" air space behind granite or sheet heat shield installed. I installed a steel heat shield. As far as the floor hearth goes, I had a 5" high hearth built from wooden studs, two layers of cement board and one layer of slate tiles. Works perfectly. What I don't understand is why does there have to be a 1" space behind stone on the wall but not on the floor? Sounds like a code type scam to discourage homeowners from installing wood stoves.