"Understanding Your Home" by Building Inspector Mark Visser
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Masonry heat shield behind
wood stove
See text to the left.





Pyrolysis and Heat Shields

 


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- Pyrolysis in a nutshell
- Clearances for wood stoves from combustible materials to prevent pyrolysis.

PYROLYSIS IN A NUTSHELL
Pyrolysis is derived from the Greek words pyro (fire) and lysis (separating). It is a process where the chemical composition in organic material changes when exposed to prolonged heat. When this happens it lowers the flash point required for wood to burn.
Wood stoves located too close to combustible materials can cause a phenomenon called pyrolysis. Over time, the heat from the stove causes a chemical change in the wood, lowering the temperature required to make it burn. For instance, a normal piece of 2"x4" will spontaneous ignite above 660F/350C.
If the wood stove or flue pipe is too close to the wood it will draw the moisture out from the 2"x4". When this happens repeatedly, the piece of wood can spontaneously ignite as low as
390-570F/200-300C. Generally this happens in 3-5 years. The wood stove can be used for years without a problem, then suddenly there is a fire.
NOTE: The process of pyrolysis has been known for centuries and is still used today to produce charcoal.

CLEARANCES FROM COMBUSTIBLE MATERIALS TO PREVENT PYROLYSIS
Always check the manufacturers listing plate for proper clearances from combustible materials. If there is no plate on the back of the stove it most likely is not certified. Contact your local building department in your area to find out what the minimum clearances are for non-certified stoves.
The minimum required clearances may be reduced if a heat shield is installed.
Heat shields can be mounted on the walls around the stove and on the ceiling above.
Heat shields are spaced out from the wall by 1" (25 mm) non-combustible spacers. Clearances may be reduced by as much as 50% when sheet metal or ceramic tiles are used, but again, check local building codes.
Hearth size and floor protection.
The local building department should be contacted about requirements regarding the hearth size and installation on combustible floors.

MASONRY HEAT SHIELD
The heat shield behind the wood stove, shown on the right, was constructed as follows:
- two layers of 5/8" (16 mm) drywall (also know as wall board or gypsum board) on 2x6 inch framed outside wall.
- 1/2" (12.5 mm) cement board fastened with 1" (25 mm) spacers to this wall.
- 2" (50 mm) manufactured stone.
The foundation for the raised hearth was made of 2x4s and plywood. The flagstone was laid in cement reinforced with wire lath.



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