TN Zone 4 basement ceiling insulation question
We are in northeast TN in zone 4 according to this map:
https://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table
House is brick two story, 2×4 walls with 1/2″ of foam sheathing, r19 attic insulation in finished attic areas (considering foam on roof deck if we can find a contractor to do it). The basement ceiling has r19 fiberglass batts which tend to rain down fibers if you sneeze near them. Basement walls are block and uninsulated, floor is concrete and I’m sure it’s uninsulated as well. Ducts for the main floor are in the basement, trunks are sheet metal with duct liner, flex to the registers.
Basement stays very comfortable, although we’ve only been here a couple of months, I was here in March and it was fine then too, probably 60 degrees and I’d guess in the winter it might dip to upper 50s worst case.
I’d like to remove the ceiling batts to make it a more pleasant and useable space. I’m sure this would affect the overall energy use negatively, but I guess the question is how much. With the slab being uninsulated does it really help that much to insulate the walls with polyiso? Should we just remove the fiberglass and leave it and see? I can repurpose the glass into the uninsulated attic spaces so it wouldn’t be wasted.
Thanks guys!
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With basements & codes it's usually an either/or thing- either insulate the ceiling OR insulate the walls. In general insulating and air sealing the basement walls has a bigger effect on overall heating/cooling energy use.
In zone 4A a continuous layer of R10 on the walls or R19 in the ceiling would meet current IRC 2018 levels. It's fine to have both, but if you want to get rid of the batts, insulate the walls.
One cheap way to do that is to use RECLAIMED 2" polyiso roofing foam strapped to the foundation with 1x4 furring through screwed to the masonry with 3.5"-4" masonry screws, hanging half inch wallboard on the furring. Another somewhat riskier way is to install a 2x4/R13 studwall tight to the foundation with only a polyethylene vapor barrier between the studwall and foundation. The risk with the studwall approach is that during the summer the temperature below grade at the vapor barrier can dwell well below the outdoor air's dew point, and if there isn't sufficient mechanical dehumidification condensation can collect to mold-inducing levels inside the studwall. Installing an inch of foam between the studwall and foundation would keep the studwall side of the foam at a high enough temperature even with R13s in the cavity, but that just adds complexity and cost.
These folks in Knoxville seem to be trading in new/used/surplus roofing foam at sort of reasonable pricing, and have a bunch of 2.25" thick goods (of odd dimensions, not 4' x 8')
https://knoxville.craigslist.org/mad/d/knoxville-polyiso-rigid-foam-insulation/6942450823.html
Running these search occasionally might find other vendors cheaper or closer to you:
https://knoxville.craigslist.org/search/sss?query=rigid+insulation
https://asheville.craigslist.org/search/sss?query=rigid+insulation
With the foundation insulated & air sealed the basement will maintain a fairly stable temperature year-round, even without actively heating or cooling it.
Hey Vince,
You can definitely take down the fiberglass insulation. Though it is a somewhat common insulation strategy, it rarely performs well and doesn't keep the basement comfortable. It is definitely worth insulating the walls as Dana suggests. Here's some more info on how to do that: http://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall
Thanks Dana, that is very helpful information - I wasn't aware of the reclaimed polyiso option. The basement has some areas with efflorescence where the grading above was poor, we are currently working on addressing the grading and running dehumidification and the atmosphere in the basement has dramatically improved. There wasn't mold or anything but the home inspection showed I think 40% humidity in a couple of areas. Given that we may or may not totally get the humidity in the wall resolved (maybe we need to test again) is that a concern for the polyiso against the wall? I guess the drywall covering is for fire safety, so this could be level 1 or 2 drywall finish. Would you also add foam in the rim joist area or leave it as-is? Thanks again!