Cement board for above a furnace?

[chopped]

I was wondering what people think of this idea.I presently have 5/8 drywall.I was thinking of adding some 1/4 inch of the cement board that is made for under ceramic .I would imagine this would add alot to the protection factor. Thanks .l this is an oil hot air frnace.

 

[SPIKER]

I would not use the 1/4" as it is rather flimsy, even the 1/2" might not hold up real well in overhead use. Lots of screws and then it still cracks pretty easy.

5/8" drywall is typically Fire Coated and has two layers would be standard for use in fire blocking applications between housing units and car garages. Install one layer (Screw & Glue) coat and tape it then install 2nd layer (Screw and Glue) and tape & coat it...

Mark

 

[dave1949]

A section of sheet metal mounted on ~2" tall stand-offs screwed into the ceiling framing will make a good heat shield if you think the ceiling is getting too warm above the oil furnace.

The heat shields for wood stoves that allow closer installs to combustible surfaces use this technique.

 

[ray66v]

I use a piece of 1/4" above my catalytic heater.

The 2'x3' piece is hung with 4 screws. And I used thin wall cut into 1" spacers between the cement board, and the drywall ceiling.

It doesn't sag, and it dissipates the heat really well.

 

[Raspy]

I've used it with complete success. But use the Hardy brand instead of Wonderboard and use the 1/2" stuff.

The trouble with using it is that it will still get very hot. It can take it, but it gets hot.

The absolute best thing I've found for the reflective surface is corrugated roofing in bright galvanized finish. Put this over the board with screws right through it. No standoffs are needed. I did this on the wall around my wood stove seven years ago and it works so well that I'm now doing it again in my new house. The wood stove can be at full temp less that a foot away and the metal will be at room temp. It's amazing.

I experimented on an older project, years ago, and looked for the most reflective surface. Brass, stainless, tile, bright paint, galvy, etc. Aluminum foil with the shiny side out worked very well, but it's impractical for interior finish use. Bright galvy corrugated roofing is durable and just as effective. It's cheap too at about 10 bux for a 6' X 2' piece. My new stove is going into a corner and there will be 5 pieces of metal behind it, with the middle one bent around the inside corner. They will simply be screwed directly to the wall.

The concrete board is really not needed with the metal, but often a fireproof material with an air gap is required by the inspector. Sheetrock is so flimsy and not waterproof, so a good solid fireproof wall is nice. I put two layers of 1/2" Hardy behind the roofing material and now think it was total overkill as it will never get warmer than room temp.

 

[Sodo]

adding some 1/4 inch of the cement board that is made for under ceramic.

I tried EXACTLY what you are suggesting; 1/4" cement board over drywall. The cement board got very very hot, too hot to touch, and it cracked from the heat. It did not stop the heat, only delayed it. Then I put sheetmetal on 1" standoffs in front of the cement board. Now the cement board stays COLD. Success.

Sheetmetal, with an airgap, is the way to protect against radiant heat. This was on a vertical wall behind a woodstove.

 

[ray66v]

I tried cement board over drywall too. The cement board got very very hot, and it cracked too. Not that I cared but it wasn't giving me the feeling like I was stopping the heat from getting into the wall. Then I put sheetmetal on 1" standoffs in front of the cement board. Now the cement board stays COLD. Sheetmetal, with an airgap, is the way to protect against radiant heat. This was on a vertical wall behind a woodstove.

The air gap is the key.

The cement board can dissipate heat better than the sheet metal. But, they both will only work well, if you put "an air gap" behind them.

 

[cqaigy2]

Careful, i found out some cement board has Styrofoam beads incorporated into it and not rated for that heat shield type of use. I guess it's because the beads would probably burn but also i think the ignition point goes down, the longer it's exposed to heat, kind of like wood does.

 

[sixdogs]

A section of sheet metal mounted on ~2" tall stand-offs screwed into the ceiling framing will make a good heat shield if you think the ceiling is getting too warm above the oil furnace.

The heat shields for wood stoves that allow closer installs to combustible surfaces use this technique.

This is the only way to do it. To butt two sheets of stuff together could easily just transfer the heat by conduction, if only through the screws. A piece of sheet metal allows an air gap and air is the best insulator of all. I used to have a thin piece of sheet steel--actually a baking pan--to disrupt the heat flow from my woodstove to my mantle. The mantle stayed cool as a cucumber.

Same way the white artic army boots use pressurized air to keep you warm. The only insulation is the air.

 

[crazyal]

Why are you thinking you need to add something to the sheetrock? Is the room getting too hot? The sheetrock used should have been fire rated. If it isn't then you should replace it. If your furnace is installed too close to a wall then a metal fire shield would be the easiest and most cost effective way. Like others have said, you will need an air gap behind it as well as a gap at the bottom to allow cold air to flow.