Aluminum instead of steel

[WayTooManyHobbies]

Building a temporary hearth and the install instructions say two 1/2 inch layers of dura-rock with a sheet of 24g steel over it. It's easier for me to find aluminum that is slightly thicker. Can I use that instead of steel?

Thanks,

John

 

[GENECOP]

I don't see any problem with that....what the heck kind of stove is calling for that? Why not just put stone or tile over the Durock?

 

[Dune]

You could but it is much more expensive.

 

[GeHmTS]

Just be aware that Al melts at ~ 1200F while steel melts at ~2500F. However, it's not necessary to reach these temperatures before you see noticeable changes in the metal. Generally, this can occur at 50% of the melting point. So it depends on how hot the hearth is going to get that determine the kind of metal you need to use.

 

[webby3650]

I don't see any problem with that....what the heck kind of stove is calling for that? Why not just put stone or tile over the Durock?

I'd like to know too.
Most call for durock and tile or 24 gauge sheet metal. Not both.

 

[WayTooManyHobbies]

The stove is a Vermont Castings Vigilant. When its in its permanent location I'll be using tile, but even with that I believe it may require a heat shield on the bottom, but I have to double check that. I'll call around again and see if I can find a place that sells sheet steel at a reasonable price. There are a lot of metal places around but they supply other companions and want a fortune to sell to "Joe Homeowner."

I'm not sure I understand the purpose of the steel without an air gap. Steel doesn't insulate so I'm thinking the steel transfers heat from under the stove to the area beyond the stove where air can then cool the steel. I could also put a third layer of durarock just under the stove, held above the bottom two layers with a few bricks, creating an air gap. Not pretty, but will that do the job of keeping the floor cool?

 

[begreen]

The Vigilant called for two sheets of 1/4" asbestos cement board with a 24 ga sheetmetal top. You are not bound to this construction. Asbestos board is no longer available. There are alternatives that would do the job. For example, you could tile the top layer of cement board. I don't think there is a need for overkill here by adding the air gap.

 

[WayTooManyHobbies]

Thanks for the clarification begree. I have another stove running while the house is under renovation. The wall behind that stove is built from aluminum studs with 1/2" durock over them. Since it's not all installed, I can reach inside the wall and feel the temperature. I can tell you this, the back side of that durock gets pretty hot. I know that's not two layers with tile, but I'm thinking that durock doesn't have much of an R-value. Since the temporary hearth I'm building now doesn't have to look good, maybe I'll just lay some lose bricks under the stove? So two layers of 1/2" durock and some brick and I should be good to go… correct?

 

[begreen]

The new Durock NexGen has an R=.39/.5 inch rating. the original Durock has a rating of R=.26/ .5". Most stoves radiate much more heat from the sides and top than on their bottoms. On a wall you are feeling heat radiated from the sides of the stove.

Have the stove's clearance requirements been properly honored? Will the wall with the metal studs be ventilated in each stud cavity top and bottom? If so, no worry.

 

[Dune]

Just be aware that Al melts at ~ 1200F while steel melts at ~2500F. However, it's not necessary to reach these temperatures before you see noticeable changes in the metal. Generally, this can occur at 50% of the melting point. So it depends on how hot the hearth is going to get that determine the kind of metal you need to use.

Well, it really should never get to even 600 under a wood stove, especially one with a layer of sand or ash in the bottom (which they pretty much should all have).
The real concern is that aluminum is a far greater heat conductor than steel, being second only to copper, but again, that shouldn't be an issue over 2 layers of durock.

 

[GeHmTS]

I'm glad to hear it's going to work.

 

[gholler]

I'm glad to hear it's going to work.

 

[gholler]

The metal is to cover areas that might be exposed when the dura rock cracks. The area under the stove will be cooler than the areas beside the stove because of the radiated heat coming off of the stove sides.

 

[elmoleaf]

The purpose of the continuous sheet steel is to prevent embers from contacting combustible material. More specifically, it prevents embers from getting into joints that may exist between abutting cement boards and possibly falling down to contact any wood that may exist in a lower layer.
This is one reason that---if installing two layers of cement board--that the joints between layers should be offset (offsetting also adds strength to the construction).
When used as an ember protection only in this manner(no R value to hearth), heavy aluminum is likely just a suitable as steel, but it is best to just follow the owner's manual.

 

[smokedragon]

I'm not sure I understand the purpose of the steel without an air gap. Steel doesn't insulate so I'm thinking the steel transfers heat from under the stove to the area beyond the stove where air can then cool the steel.

As others have stated, steel gives a continuous layer of ember protection, however, there is another reason.

Stoves radiate heat. Insulators absorb heat (like tile, durock, brick, etc.)

Steel will reflect radiant heat, which will reduce the amount of heat heat that your insulating material is absorbing (even without an air gap).

With an air gap, the rise of heat creats convective currents in the air, which helps cool the surface even more.

Heat transfer is fun stuff

Regardless of what the manual say, just make sure its safe. Even if you follow all their directions, check the temps when burning the first few times. That matters more than any install directions in the world.