Insulating concrete slab under wood stove in my basement

I just built a house with insulation on the outside of the foundation. I debated putting the under-slab foam but decided not too since the foundation crew didn't seem to be up to doing something they hadn't done before. It would have been nice, but I think I'll be alright with all the other insulation I have.

I have a Jotul F55 on the basement floor.

I'll bet that heat is mostly radiative actually ... as I too notice my old Jotul heats the basement floor nicely. The legs don't get hot enough for that so I seriously doubt conduction explains it.
With this in mind, I don't think your concern will be valid: "However, I am concerned that if I put insulation on top, then the heat from the stove won't make its way around as easily since the heat in the floor won't be able to make it through the insulation."

That said, should be easy enough to test. Rig up a short panel of insulation and something on top and set it at a reasonably safe distance but "close" to the stove... then measure its temps on top via infrared thermometer. Also measure the concrete floor in the "shadow" of this insulation panel (assuming the stove is the "light source" so to speak), and measure the concrete floor temp next to it just outside of that "shadow".

iluvjazznjava said:

I don't have outside insulation on the foundation walls, but they are insulated inside (framed 2x4 with fibreglass batts). My main issue is that I don't like a cold basement floor and the stove seems to warm it up a lot (obviously more closer to the stove). I'm wondering if adding an inch or two of rigid foam on my floor will be a net gain in terms of the how warm the floor feels, or if I should just leave it alone.

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My prediction is it will help.

iluvjazznjava said:

That's a good idea - finally gives me a an excuse to pick up one of those IR thermometer guns.

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They're indispensable ;-) Also if it's primarily radiative heat transfer, lifting up that insulation panel should reveal a fairly cold concrete surface below relative to the uncovered concrete.

2" rigid foam under the slab is now part of the Energy Code in Mass. I'm sure it applied elsewhere.

How about something like this? Seem to see it a lot on home improvement shows up here: http://www.amvicsystem.com/amdry-insulated-subfloor

The physics of mass are pretty simple. Your concrete floor is a huge chunk of mass. Mass sucks up all the heat you can give it but won't 'feel' warm to your hand or feet until it's over 90 deg. Mass will store the heat as energy and give it off more slowly than wood or a lighter material.

If you don't have proper insulation under your slab floor, the concrete slab is grabbing a lot of heat from the stove and the room and much of it is heating the earth underneath. Even if you have 2" of rigid insulation underneath, the concrete will still always feel cool. Your best bet is to properly insulate all of the concrete so that it won't rob you of your heat. You will notice the difference immediately. Insulating just a section under the stove is completely worthless, you need to insulate the entire basement. Your home will be better for it.

I designed, built and live in a high mass house that has worked wonderfully for 30 years. In all those years, there is nothing new in technology that changed, just a lot more BS to dig through. However, even with a wood stove much larger than yours and with super insulation under the slab, the only way to successfully use mass is to gain energy from the sun and let it give off the heat in the evening.