Gas fireplace: on concrete mass or framed floor?
I am renovating a pre-existing non-confirming ADU in a region with cool, wet winters.
The space will include a mini split heat pump, and a gas fireplace.
I am about to frame out the floor, but wonder whether to first form up a concrete mass where the gas fireplace can sit, and frame the floor around that?Code requires that I at least tile out a hearth on top of framing/subfloor. Would the concrete mass be optimal for heat distribution, though? Or would its connection to the foundation (and thereby exterior humidity/cold) be a problem? Would a sheet of vapor barrier between the foundation and hearth mass be helpful?
Looking forward to a little shared wisdom from folks with more experience than me!
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Replies
Having a larger radiator surface makes your fireplace more comfortable, but it doesn't make it more efficient. Having a large, uninsulated hole in your building envelope makes it less efficient and less comfortable -- whether or not the fireplace is on.
A thicker hearth won't do a better job of distributing heat. In fact a thinner one will respond more quickly. The quality you want is heat conductance, not heat capacity.
I would frame the floor normally and make sure the area under the fireplace is insulated and air sealed well.
First start by reading the instruction manual for your fireplace it will have very specific requirements.
If we are talking about new modern gas fireplace for new construction it seems likely to have a sealed front glass making a hearth requirement unlikely. Generally the glass of the fireplace is elevated above the floor to the point that a wood floor is not a fire danger. Be sure you select a vented unit and follow the instructions for venting and clearances.
If you have an insert or log conversion kit the requirements will be very different.
Walta
+1 on Walter's comments. It is very important (required) to follow the fireplace manufacturer's installation instructions. The good news is that most modern gas fireplaces are rated for zero- or nearly zero-clearance to combustible construction around the firebox, and most do not require a fireproof hearth. That said, you are welcome to include a concrete hearth and cover it with tile if you want. I'm not it would add any particularly useful performance improvements, but stone and tile make gas fireplaces seem more like traditional ones.