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Basement ceiling insulation strategy

DyedInTheWool | Posted in Energy Efficiency and Durability on

We have a 1920’s house on a 1685 fieldstone foundation in Mass. We installed a hydro radiant system under the 1st floor subfloor 10 years ago during a remodel, and it’s been bliss. But: mice have gotten into the fiberglass batting that insulates the radiant PEX so we need to replace some of it, and it was installed without any support wires, or other fasteners, so portions are sagging.

Also, as a further wrinkle, there is a funky section of the basement — a separate room behind an ill-fitting wooden door — that is damp, with a sump pump. This room is mostly under a screened porch so 80% of its ceiling is concrete slab over wood and 20% is livingroom subfloor (and that 20% has radiant PEX and degrading fiberglass insulation). (Ah, these old houses…) The dank room doesn’t get all that cold, maybe 15-20 degrees cooler than the main basement area (which stays room temp thanks to the steam boiler that powers the 2nd floor radiators). 

Several questions:
1) Should we use faced or unfaced fiberglass batting to re-insulate the radiant in the ceiling of the warm/dry main part of the basement? (Or use a reflective lining under the PEX, then unfaced batting?) If faced, what kind and facing up or down? We want to stay within code for fire, of course. Also, we use this area as a shop, so do we need to think about covering the exposed fiberglass (or just be careful removing/reinstalling)? Also, it’s hard to completely eradicate mice from old houses with leaky old fieldstone foundations… Any suggestions on deterring mice from nesting in the ceiling batting going forward, aside from being much more aggressive with traps?  

2) What type of fiberglass batting should we use to cover the radiant PEX in the ceiling in the cooler damp room — the part under the livingroom floor? We’re concerned about moisture here.

3) Should we insulate the rest of the ceiling in that cool/damp room, i.e. the part that’s under the screened porch slab? We are concerned about condensation there, of course.

Thanks for any ideas!

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Replies

  1. Walter Ahlgrim | | #1

    Seems to me you should consider removing the ceiling insulation and covering the field stone walls with spray foam and bring the basement fully inside the conditioned space my guess is the house will be more comfortable and use less energy.

    Walta

    1. DCContrarian | | #2

      That will also go a long way toward keeping the mice out as well.

  2. Expert Member
    Zephyr7 | | #3

    I would also insulate the basement. I'd do a good job of air sealing it too, during which you'll probably find where those mice are getting in. Embed 1/2" mesh hardware cloth in any large areas of canned foam to make sure it's chew proof. This will keep your basement mouse free, and also warmer.

    I'd use polyiso under the PEX, not batts. I hate using batts for stuff like this. Once you've insulated the basement, you don't need much insulation under the floor. I'd probably use 3/4" or 1" polyiso, so that it would be strong enough to help keep the PEX tight to the underside of the floor above.

    Bill

  3. DCContrarian | | #4

    Those old houses had a fuzzy notion of where the boundary between the inside and outside is. Unless you have no plumbing in the basement you want it to be part of the inside of the house, which means the walls are air sealed, vapor sealed and insulated. As Akos notes spray foam is the best way to do that.

    With the basement insulated, it's still OK to insulated between the basement and the first floor. I also dislike fiberglass batts, I only tolerate it when it's completely encased. It's going to be a tough insulation job, I would expect a house from the 20's to have non-standard joist spacing and the joists and the space below to be filled with 100 years worth of mechanicals. I'd price open cell foam.

  4. DyedInTheWool | | #5

    Thanks very much for all the suggestions and advice. It's a little challenging to bring a 17th c. basement up to 21st c. standards. The fieldstone walls are quite attractive and the basement makes a very nice shop, so I think it would be a little dreary to foam them over, especially as our heating bills are pretty low (perhaps those 18" thick walls have some insulation value?). Getting rid of the mice entirely, however, would be great.

    The house is otherwise well-insulated. When we bought it ~25 years ago, there was no insulation at all in the walls -- pretty common in old New England clapboard frame houses.

    Foil-faced polyiso seems like a good option. And perhaps we can paint the exposed face, which would improve the aesthetics. I've also been reading about Reflectix and similar bubble-type reflective insulation. I'm a little leery of burying the radiant system in foam, in case we ever need to repair something.

    Thanks again!

  5. Expert Member
    Zephyr7 | | #6

    Reflectix is only going to get you about R1 or so worth of insulation. Not much.

    You can air seal the basement without fully insulating the walls. This should help with your rodent problem. It’s possible to insulate the exterior of a foundation wall to, but that requires excavation, and the rigid foam normally used might not work well if your foundation has a very irregular face.

    Bill

  6. DyedInTheWool | | #7

    Reflectix claims up to R-21 if you install two layers with two air pockets, like this: https://www.reflectixinc.com/applications/hvacr-plumbing-contractor/pro/radiant-floor-wood-joists/. The "thermal barrier" is the air pockets you create between the layers (as you note, the material itself has negligible thermal properties). I'm skeptical that the 2-layer approach really delivers that high an R-value, but in the (warm) main basement, R-21 would be overkill anyhow.

    Polyiso, if fire-rated, does seems a better option -- quicker install (1 layer, rather than 2), won't sag, better cosmetics (if we paint it).

    We've already done a pretty good job on air sealing, but need to do a better job insulating the steam pipes (and the radiant) as the basement is very warm.

    Thanks again for all the great advice!

    1. Expert Member
      Zephyr7 | | #8

      Reflectix claims a lot of things. There is an article on here somewhere about some lawsuits about misleading R value claims with radiant “insulation”. It has its uses, but don’t consider it to be a substitute for regular insulation.

      To insulate steam pipes, I’d use a proper split fiberglass pipe insulation. This material is readily available at commercial mechanical supply houses, and it’s not terribly expensive. If you use the white finishing tape and fittings, you’ll end up with a very professional looking installation.

      Bill

  7. DyedInTheWool | | #9

    Thank you!

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