Basement Heating Options
I built a small home (18×26) in New Hampshire (zone 6). I have a walk-out basement and am getting ready to finish it. The basement has two planned rooms, a TV room and guest bedroom (bedroom will be on the walkout end of the basement and has an egress window). The basement slab has a vapor barrier and 2″ of EPS insulation underneath. The exterior walls were sealed and have 2″ of EPS up to the grade (so none on walkout wall but the frost wall on that side is insulated). The stud walls are 2″ away from the concrete and there is closed cell spray foam of 2-3″ on all interior concrete walls. I plan to fill the bays of the exterior fully wood walls (walkout end) with 4″ of rockwool but leave only the spray foam on the concrete walls. I will then hang moisture-resistant drywall everywhere.
I have a mini-split and wood stove on the first floor but no ductwork. In the woods and only have electricity (no gas/propane). Tankless water heater is in the basement.
I was hoping the basement would stay relatively warm but even with the spray foam the basement still dips into the 40-50s on very cold days. My thoughts are to go for a radiant floor option but I’m worried about the height of the system as the stairs were set on the concrete slab expecting a 1/2″-3/4″ floor to maintain riser heights. The simple option might be to just install two electric panel heaters but those don’t seem very efficient.
My question is what is the best option for heating the basement at this point? Thanks for reading and you’re help!
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Since you are giving thought to the electric resistance route, you might find this discussion useful; Expert Member Michael Maines recommends an Envi heater.
Getting to the "right" solution starts with a room by room load calculation (Manual-J or similar). For a first rough cut see what you can get out of online freebie tools such as CoolCalc or LoadCalc, neither of which are as fully featured as a "real" tool, but good enough to ball park it if you're sufficiently aggressive with the inputs. (Assume the air tightness is perfect, and the R-values are the maximum reasonably possible for the foam, etc to avoid oversizing. Even aggressive Manual-Js tend to oversize by at least 10%.)
At very low loads electric mesh radiant floor (losing less than a half-inch in total headroom) can begin to look attractive, despite the inherent inefficiency of resistance electric heat.
How is your domestic hot water being heated?
It's hard to say. Basement loads aren't that high in BTU/hr, but are higher in "heating hours per year" than the rest of the house. So be careful. My basement would need heat year round to be truly comfortable. I usually shut off my basement panel radiators in June since it seems silly to heat when it's 25C outside. I used to use electric heat in my basement and it worked great as long as I only used it when I was down there.
Depending on what your goals are, perhaps you could install a ducted fan to draw the hottest air near the ceiling of your mainfloor and deliver it to the basement thru a piece of vertical ductwork. (ideally far from the stairwell or whatever return path the air would take). You really need to situate the fan to maximize the temperature delta. Moving 70F room air to a 50F basement would take a lot more air.
If you can draw in 90F+ air from the ceiling somewhere not to far from your wood stove and deliver it to your basement at say 120cfm, you get about 5000btu/hr (assuming a 40degF delta). This would probably be less than 2000btu/hr with only the mini split running though.
BTU/hr = CFM x dT x 1.08