Thread: Acceptable substitute for micore, for hearth extension

Does anyone know of an acceptable substitute for micore for insulating under a hearth extension. There is combustible material below the hearth surface (approx 1" natural slate). I have heard that 1/2 micore 300 is the best to achieve a .84 k-factor, but I can't find it any where in my location.

I know two layers of 5/8 drywall is approx. the right k-factor, but it seems like the paper backing would be combustible itself. I know you can get paperless drywall, and fire-rated drywall... does anyone have any thoughts on this? I have some room to play with (1 - 2 "), as far as how thick of insulating material I use.

Any help would be much appreciated.

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Originally Posted by Brian Puccini

Does anyone know of an acceptable substitute for micore for insulating under a hearth extension. There is combustible material below the hearth surface (approx 1" natural slate). I have heard that 1/2 micore 300 is the best to achieve a .84 k-factor, but I can't find it any where in my location.

I know two layers of 5/8 drywall is approx. the right k-factor, but it seems like the paper backing would be combustible itself. I know you can get paperless drywall, and fire-rated drywall... does anyone have any thoughts on this? I have some room to play with (1 - 2 "), as far as how thick of insulating material I use.

Any help would be much appreciated.

When you say "K-factor", are you referring to thermal conductivity?

If you are trying to keep heat from being transferred through 1" slate to a combustible surface below the slate, I think it is kind of a moot point. Just think how hot the heat source would have to be so that the slate could conduct enough heat before it could transfer that heat to an object so that it could reach it's flash point.

If I understand what you are wanting to do, I would say that it is almost overkill.

Maybe one of our fireplace experts (Bob Harper comes to mind) could chime in on this.

dur rock or any cement board
two half inch layers was the spec from a job this week

Brian:

There is a huge difference in the thermal protection characteristics between Micore 300 and slate or DuraRock. First step is to find out what the appliance manufacturer requires. If the .84 k-factor is required for that model that's what you need to achieve. Many manufacturers include a list that will show the k-value and R=value of common building materials in the installation manual. Find it for your model and follow their specifications. Both slate and DuraRock tend to simply transfer the heat to the combustible material below. But like so many issues, the requirements vary from model to model. Don't make generalizations, find out for sure.

Originally Posted by Brian Puccini

insulating under a hearth extension.

I know two layers of 5/8 drywall is approx. the right k-factor, but it seems like the paper backing would be combustible itself. I know you can get paperless drywall, and fire-rated drywall... does anyone have any thoughts on this?

I would not recommend gypsum board to be used on a horizontal surface where subject to heavy loading.

It is one thing to use gypsum board on the top of a half-high wall on which nothing of significant weight will be placed, but to use is where a person could stand on it ... no way. You would want something more substantial for that.

As Rick said "Both slate and DuraRock tend to simply transfer the heat to the combustible material below.", the way to stop that transfer of heat would be to allow for an air space and air flow behind it. With vertical surfaces, that air space is 1" minimum, with open bottom, sides, and top to allow for air flow. Creating a 1" air space horizontally below the surface above would not give the same heat separation reduction as you would not have the same air flows, plus, you would need to leave the sides of the air space open, which might not be pleasing to look at.

Look up what the manufacturers specifies and follow those instructions, otherwise ... *IF* ... there is problem (fire, charring, etc.) the manufacturer will void any and all warranties and they will not be the one standing behind the problem, you will be left holding the bag ... provided that bag did not burn down too. You would want no evidence left, none.

We are talking about a hearth here where *some* occasional embers *might* get on top of the *1 inch* slate
Is this hearth for a wood burning fireplace or a stove. A stove will radiate a lot more heat down to a hearth than a firpelace radiating the heat out the front and heating the slate.

You say extension to a hearth. How far away is this extension starting away from the heat source.

It sounds like you may be talking of way overkill here. To each his own but I cannot really judge it with out more details or pics.