Any experience with an unheated (but insulated) slab as a finished floor? Will it be uncomfortably cold?
We are building a 1000 s.f. single-story home in Northwestern Ontario (climate zone 7) on a very tight budget. It will be well-insulated (walls R-40, roof R-60), and have triple-glazed windows, etc. An energy efficiency consultant calculated our heat loss as 16,800 Btu/hr at -32.8 F. The foundation is a short ICF stem wall with a 4″ concrete slab (R-20 insulation underneath). Heating will be accomplished with the combination of a high-efficiency woodstove and electric baseboard, and we might add a mini-split down the road.
We would like to use the concrete slab as our finished floor for two reasons: 1) cost–we won’t need to pay for additional flooring, and 2) to act as a thermal mass, absorbing some of the heat from the woodstove and from the south-facing windows. However, a number of contractors have expressed concern that the floors will be intolerably cold, and have suggested we lay PEX tubing in the slab in case we want in-floor heat in the future. On the other hand, we have also been advised that in-slab heat will be too slow to respond to temperature fluctuations (especially with the wood stove) and therefore lead to overheating, and that the slab will feel cold even if heated (because the heating requirements are so low).
Does anyone have experience with an unheated (but insulated) slab in a cold climate? We do have a number of area rugs we can put down–that should help. My only point of reference is the mud room of our current home–it has a concrete slab (uninsulated) on top of bedrock, and standing on it without shoes for more than a couple of minutes is *not* enjoyable.
To summarize, our options are:
1) Unheated concrete slab as finished floor–simplest, least expensive option, but too cold?
2) Heated concrete slab as finished floor–adds complexity/expense we don’t want, slow to respond?
3) Unheated slab with a floating floor on top (i.e., wood, cork)–simpler, but do we lose the thermal mass benefits?
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