Basement Insulation Plan

After tackling deficient attic insulation (which made a BIG difference), getting a modern wood stove, and going all LED for lighting, we’ve lowered our yearly electric usage from ~30,000kWh to 20,000kWh in an all-electric home. Now we’ve got the time and enough funds to address the basement, which is the biggest remaining energy sink of the house. It’s uninsulated and has too many leaks.

We had a free energy audit done, and the company suggested spray foam for the entire band joist and basement ceiling. I was skeptical of that plan both because of the high price quoted, and because our basement houses not only our wood stove, but our heat pump, ductwork, water equipment, and my brother’s workshop area as well.

Here’s a map of our first floor, with the basement layout overlaid (blue is concrete-floored crawlspace, and green is uninsulated slab-on-grade, or tile for the greenhouse):


I’ve bought a foam gun and some 2” R-10 XPS foam board to start on the rim joist first of all, and we’re planning on replacing the electric water heater with an air-source heat pump model (good in all seasons, given the wood stove, and some welcome moisture control in humid summers).

Next, my brother wishes to focus on the larger crawlspace, as heat from the stove and inadequately-insulated ducts has a hard time reaching that end of the house, making things chillier in those rooms and the master bedroom above. After witnessing the original fiberglass batts fall from the joists years ago, he’s partial to replacing them with rock wool, then walling off the section beneath the family room so that fewer BTUs sink into the ground and more of the stove’s heat travels up the stairwell (we keep the door open in winter to aid heat diffusion).

I pointed out that insulating the crawlspace walls and floor instead wouldn’t take much more in materials, but could preserve airflow for a heat path and dehumidifying, while also serving as a barrier for air/moisture/radon if impermeable foam was used in place of rock wool. At that point he revealed he had a secret plan to keep the crawlspace as an extra storage area, and possibly dig out the core of the space for a fuller basement, which he feels floor insulation would interfere with. I’m not keen on either idea.

I know a very similar situation was discussed before, but wanted to hear your thoughts. I’m not sure how the walls facing the greenhouse and concrete slab impact the need for insulation, or if insulating the crawlspace floor is as great a concern, considering the largest heat differential will be near the stove and the exterior walls.

Unfortunately, the energy audit was done a couple years ago and I misplaced the quote. I do remember it seeming large in comparison to our yearly energy bills of $2600-3000, and it was out of our budget at that time. The space is also jam-packed with tools, engines, and other stuff, which would have to be mostly cleared out for a foam crew to come in. My brother worked in construction and now for an HVAC company, so we're receptive to either option, really. Just want the best results.

OK, that's good to hear, woodgeek. Less work as well.

I'm guessing then that the wall facing the 1st-floor concrete slab shares more in common with the crawlspace floor than the perimeter exposed directly to outdoor air? Whatever heat bleeds that way will still be sitting under living space. However, the greenhouse's location and shading doesn't allow for much extra winter warmth, so I feel it's more akin to an exterior wall with its temperature extremes.

For the concrete floor, masonry sealant was suggested in the previous thread. I like that idea more than attempting to create a perfect vapor barrier via plastic. Lower possibility of being compromised if heavy/sharp stuff gets moved on top of it, too.

What's the best way to take care of the stove's alcove insulation in a firesafe and space-saving manner? Due to the chimney position and recommended flue angle, we needed to place the stove nearly touching the cinderblock behind it (6" away for one corner, 11" in the other). I'd imagine blocking radiant heat transmission begins to make sense there. And would a blower attachment make sense to transfer more heat from the rear of the stove and out into the rest of the basement? Lowering the temperature gradient across the walls in that corner should allow the insulation to work better.

We also went to maintain that corner crawlspace shelf for wood storage and be able to chuck logs to the back with impunity, so the standard advice to cover with drywall won't do. However, for that eastern section we have to do some watersealing from the outside, so insulating the exterior for that small portion is an option.