Hearth Pad Construction

[Goldbug]

I am new to the forum and have some questions regarding the installation of one of my two Hearthstone I stoves in my home. I moved last spring and brought my Hearthstone stoves with me when I moved. My house currently has a fireplace with a wood mantel surround. The room that the stove is to be installed in is about 16' x 30', with 3/4" oak flooring over 1" hardwood subflooring. The foundation wall runs under the floor joists (3"x12"s) directly under where I would like to install the stove. My intentions are to build a 60" deep x 72" wide platform in front of the current fireplace. The back of the stove would be placed 42" from the front of the mantel. The Hearthstone I is a real monster of a stove, it puts out over 100,000 btus and probably weighs over 1000 lbs when loaded with osage orange (my only source of firewood). My plan is to put down a layer of felt on the floor that I will paint with drylock. Then I intend to build a wooden form the desired size of my planned pad. Then I want to pour into the form a mixture of 8 parts perlite to 1 part portland cement with an air entraining additive to a depth of 4" with woven wire as a reinforcement. When the poured pad has cured I will put two layers of 1/2" permabase over the pad. Then I will cover the pad with either ceramic or soapstone tiles. This should provide a lightweight, insulative base for the stove, but it could be removed at a later date without too much work. I want to install a flexible, insulated stainless liner in the chimney, about 28'. I want to use a Saf-T-Thimble to breach into the existing flue, then use laser welded 8" stovepipe to connect to the stove. The ceiling in the room is 10' and I want to breach the wall at about 7' 6". I believe this should be a very safe installation. Does anyone see any problems with this approach?

 

[edthedawg]

Do you have any idea how hot the hearth surfaces used to get w/ your prior installs?

From the following page on Permabase: (broken link removed to http://www.unifixinc.com/products/pb-constr-guide.pdf)

"PermaBase cement board should not be exposed to temperatures over 220°F (105°C)."

Ceramic tiles will not provide any real insulation value for you, but still - 220F is pretty high. I don't think my Heritage heats any part of the hearth over 200, even if i try...

If you instead use Durock/Wonderboard over the 4" Perlite/Cement mixture, you probably have a little better than R1.2, which is probably sufficient - tough to gage w/ an old stove like that... Does your area have any "default" values for hearth requirements?

 

[Goldbug]

Thanks for the advice on the Permabase. I did not know it wasn't suited for high temps, I'll go with the durock. I live in rural Illinois, I can't find any guidelines regarding hearth construction for my area. I contacted the local fire department and they said they would send someone over to inspect it after it was installed, but couldn't advise on construction techniques. I just want to make absolutely sure that I don't burn (or char) or otherwise damage the flooring under the pad, and not burn my house down. I thought that 4" of perlite concrete would provide me with more than R1.2, I was thinking more like R3 or more. It is hard to determine what type of heat the floor under the stove used to get because it was sitting on a concrete slab, and the heat would get wicked into the ground below. The floor used to get too hot to touch in barefeet in the area of the stove. I want to be able to operate the stove without using the ugly heatshield underneath. My son suggested raising the platform to about 8" so there is someplace to sit next to the stove, but I don't want to go any higher than needed. Here is the tech sheet on the perlite concrete. (broken link removed to http://www.perlite.org/productuides/12) lightweight insulating concrete.pdf

 

[edthedawg]

I hadn't read up on the perlite / concrete properties. Impressive stuff! I'd think you will have some really good coverage there, but the math is making my head hurt. Figure out the math from the k-value tables widely available online:

http://www.perlite.info/hbk/0033016t2.htm
(broken link removed to http://www.perlite.org/perlite_info/guides/lightweight_insulating_concrete/general/perlite_concrete.pdf)

and/or contact perlite.org for help - you're planning a simple 4" thick slab mixed @ 8:1, so it shouldn't be that hard to define everything.