Should we actively condition our unfinished basement?
We’re in SW Virginia, building a 2 story, 1900 sq ft home with a full basement. 2 inches of XPS foam under the slab, and 2 inches of polyiso on the interior of basement walls. Heat pump/forced air and woodstove will heat upper floors. Future plans include doubling the thickness of polyiso all around in the basement. Was also thinking that if we had cold floors on the first floor, I could add some insulation under the floor.
We also could potentially be finishing about 600 to 700 sq ft of the basement, but that might be a few years off or might never happen. For that finished room, which might be used infrequently, was thinking we’d make it superinsulated, heat with either a baseboard heater or gas appliance, and ventilate with a small “spot ERV”.
I liked the idea of keeping the rest of the basement on the cool side for food storage and running freezers.
Now the HVAC contractor is telling us that by code we’d either need to condition the basement or insulate under the floors from the get go. (Our builder didn’t seem to know this, or just overlooked it.) He’s suggesting we force some air down their, but at amounts to keep temperature about 10 degrees F cooler than upper floors in heating season. A return would be installed as well, but he recommends to cover it up with cardboard during heating season, and open it up during cooling season. I’m wondering about pressure buildup if we do that.
Also, wouldn’t we need to increase our ventilation rate? We might often have the ERV running but not forced air heat or cooling (high solar gain days, or days when we’re neither heating nor cooling but are running dehumidifiers.) But when forced air is running, wouldn’t the basement air be mixed with the air from the upper floors? Shouldn’t we add the additional 950 sq ft in basement to our required ventilation calculations? We might have to get a higher capacity ERV in that case.
Any thoughts, suggestions? Does our HVAC guy have it right about the code requirements? Should I take the easy route and just condition the basement?
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