Rigid foam insulation in Hearth pad construction

Hi All,

My first post here. I've been researching construction of a hearthpad for a hearthstone (homestead) stove i am about to acquire. As you may know depending on the setup, the hearth pad can require quite a high R value (6.6). I've been reading online quite a bit in the past few days and it's become clear that it is difficult to attain such values without a hearth pad that is rather thick.

Then, browsing home depot, i came across rigid insulating foam, such as supre TUFF foam insulator and or owens coarning foamular. (home depot links embedded). These materials have R values upwards of 5 for an inch of thickness.

In all my reading over the past few days, I've not seen any one mention these materials for constructing hearth pads. Now I suppose there may be a good reason for that so I apologize if this is a silly question, but is there a reason why this stuff should not be used as one of the layers in the hearth pad? And why would that be?

Thanks very much for your wisdom!

-Jakub

It melts.

Micore is the solution

I guess the earth pad reuirements are in case something(embers) fall out of your stove. It sure cant be cuz the stove is hot on the bottom, mine is always very cold.

According to the manual http://www.hearthstonestoves.com/assets/files/document_library/Homestead8570Manual.pdf you can get it with 6" legs which requires only R2.5, if 6" legs work for you. It can be done with mineral board panels like Micore.

Jakub, i would defenetly stay away from the rigid stuff. We insulated our basement with this + R14 batts on the walls.
The product is great but it is higly flamable and toxic.

Roxul, Micore / Cement board is the way to go here.

+1 on Roxul. HD in Canada carries it, seen it in a couple other building supply stores. They have a "safe and sound" version - essentially the same as their regular "comfort batt" insulation but in a more dense 3" batt. They also make a semi-rigid board (firestop) that comes in a 1" x 16" x 48" sheet. Not sure how easy it would be to get hold of just a couple pieces of that, but that would probably be great for a hearth if you didn't want to raise it too much.

Thanks everyone for the answers. Rigid insulation is clearly not the way to go.

The homestead i will have has the 4 inch legs, however it itself will be sitting on a proper fireplace hearth. That brings me to page 14 of the user manual as far as setup is concerned. All I need is the extension in front to be 2.5. My hearth is flush with the floor, so to not make it look funny, I wanted to make a 2.5 hearth to cover the entire surface... The stove would then be on a hearth pad which would be on the fireplace hearth.

My challenges are that I only have 2 inches of play. Raise anymore than that and the stove exhaust won't fit under the fireplace, which is exactly 24" high. So I have to get to 2.5 in ~1.75 inches.

Roxul sounds great, but 3.5 inches is too thick. If i could cut it across into an 1 inch piece, the r value would still be ok, but I'm wondering how it would not sag under the wight of cement board and almost 400 pounds stove.

I see air gaps are good too (r-0.9 for 1/8 of an inch), though i have no idea how to introduce air gaps between sheets of cement safely. Any ideas there?

Finally, micore seems very difficult to find, though i found one place online that has micore 300.

there's always the store solution, the shop say they have a 6.6 hearth pad that's 1.25 inch thick (how that's possible i don't know). but that's 500$. I'd rather not go that route if at all possible.

Ideas welcome and thanks!

-Jakub

Hi Jakub. I think a previous post (begreen)mentioned steel studs with roxul - basically framing the hearth with metal to support the hearth top layer,with insulation set between metal studs. Furring strips might also work? Just spit-balling there, as I've used metal furring strips as spacers for heat shields on the walls.

Check out my installation

Not sure what you have for a stove; if you haven't already done so, contact them to see if they offer a bottom heat shield. Bottom shields can reduce the requirements for the hearth.

A local building supply store ordered the Micore for me.

The tec. Data says not to exceed 85 f / 70% h. ? Not being a pain,but won't the hearth get hotter then that ?

It looks like you could go with steel studs, turned flat, then rockwool batt in between/inside. (split the batt) It gets an r value around 3 per inch.. you would be 1.5 inches thick.Use wonderboard or durarock under your tile or whatever to get up to the finished floor height you need..

Are you figuring to get under your lintel with a standard transition (oval to round adapter), or the close clearance adapter?