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Partially Finishing a Basement

willnn81 | Posted in General Questions on

Looking to partially finish my basement with two unfinished spaces.  One will be where the furnace and water heater sits and the other will be where electrical box, oil tank and forced air systems sit.  My plan is to use 2″ foam, stud wall with rockwool against the concrete area of finished spaces.  Floor system will be foam, OSB and rubber (gym) or LVT (den / bathroom).  My concern is the transition interior walls between unfinished and finished parts of the basement.

I had planned to leave the unfinished spaces as bare concrete walls and floors as both cost and ability to cut around existing utilities make finishing those walls undesirable.  I can build the 2×4 walls and batt with rockwool, but can’t put foam on the unfinished side of the wall due to fire rating issues (note: Dow Thermax not an option I’m considering on those walls due to cost).  Should I put a vapor barrier on the unfinished side of the interior walls?  Other options to consider?

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Replies

  1. DCContrarian | | #1

    Basement walls should be insulated. Full stop. It makes no more sense to have uninsulated basement walls than to have uninsulated above-ground walls, there's nothing magical about being in the basement.

    You're wasting your time insulating interior walls. Figure out a way to insulate the exterior walls.

  2. willnn81 | | #2

    Perhaps more context. The house was built in 2003, has no exterior basement insulation and from what I can tell where a spot for roughed in bathroom sits, has no underslab insulation either. It's about as practical to figure out a way to insulate the exterior of the exterior basement walls as it would be to add insulation under the slab at this point. One full wall section abuts a 3 car garage slab, so I'd need to start tunneling to access the exterior wall.

    As far as insulating all exterior walls from the inside, I have the oil tank placed in a corner as well as the water heater and furnace tucked in another corner. Both are <6" from the walls, which means while I could probably slide a 1-2" sheet of foam behind them, I wouldn't be able to seal the edges of the foam and I wouldn't be able to cover it with a 15m fire rated covering. Unless folks have had success just adhering sheetrock or plywood directly to foam on a wall. But I've read that's a bad idea as well. Every house in MA I've seen with a "finished" basement is actually a partially finished basement. As such there has to be a practical recommendation for the best way to manage the unfinished to finished transition walls. Same is true for basement-level garages or raised ranches with a garage abutting 1st level living space.

  3. DCContrarian | | #3

    You're confusing finished with insulated. Space can be unfinished but insulated. It's OK for part of the basement to be finished and part unfinished. What's not OK is for part of the basement to be insulated and part uninsulated. It's like having part of your roof be rainproof or part of the bottom of a boat waterproof.

    If you have six inches behind the water heater and oil tank, that's plenty. You could build a wall assembly and slide it in behind. Or you could just put Thermax behind those two things, two pieces isn't going to break your budget.

  4. Jonathan Blaney | | #4

    I would build you walls just like exterior walls, Drywall, vapor retarder, insulation, siding. You could probably use taped drywall on the unfinished side (cheapest) or sheet siding.

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