Why not remove batt insulation from basement ceiling joist bays?

August 13, 2012 1:19 PM

I live in a 1993-construction modified cape-style house, in New England. Three floors, ~1900 square feet. The rim joists in the (unfinished, unheated) basement have been air-sealed with Great Stuff expanding foam. Do I really want or need fiberglass batting in the basement ceiling joist bays?

This weekend, I discovered that the fiberglass batt insulation in the ceiling joist bays in the basement have a significant amount of rat/mouse dropping detritus in them. This, I assume, predates our ownership of the home, as well as the air sealing. There are no signs of rodents in the basement now, and we have cats.

Is there any reason to not simply remove the batt insulation and leave the joists exposed? Putting more fiberglass up seems like it would add minimal R-value to the house, and I suspect it actually makes it harder for the boiler and hot water system to do their job in the winter, as it effectively cuts the basement out of the thermal envelope. Am I missing anything, aside from the minimal soundproofing the insulation adds?

posted by ellF to home & garden (4 answers total)

if the basement is unheated, having insulation in the ceiling of the basement means the floor above it won't be cold. If that's a correct reading of what you're asking, I'd leave the insulation there.
posted by k5.user at 1:22 PM on August 13, 2012 [2 favorites]

Cold floors are no fun. It doesn't make any sense to heat the basement (by leakage) to make it "easier" on your boiler and hot water system. If it is so cold in your basement that your boiler uses significantly more energy to heat, then your house will become much colder without the insulation between the basement and the floor. You'll just end up heating your house even more to make up for the cold from the floor.

There's also code compliance to consider. In many places your floors are required to have an r-value.
posted by oneirodynia at 1:43 PM on August 13, 2012 [1 favorite]

In the winter, do the basement walls feel cold, or just a bit cooler than ambient?

If you have your furnace in the basement and a properly insulated foundation (which means you have the insulation on the outside of the foundation), insulating the basement ceiling won't gain you very much, so you may as well take it out.

If, however, you have a drafty basement, uninsulated foundation, or all Monitor-style heating rather than central, taking that insulation out will noticeably increase your heating bills.

FWIW, just about all New England houses have mice to some degree (whether you see them or not), so you may just want to pretend you never saw that - And honestly, if the mouse droppings will ever make you sick, they'll do so when you disturb the insulation to try to clean up after them.

Personally, I'd leave it alone until some other major remodeling job required ripping a significant amount of that insulation out anyway.
posted by pla at 1:48 PM on August 13, 2012 [1 favorite]

Not sure about the basement walls, either in terms of relative degree of cold or external insulation. We purchased the home this past April, and have yet to see a winter in it. Longer term, the plan is to put 2" rigid foam up along the walls, and then run studs up against them, with drywall over the studs. When I do so, I expect to either put in electric basement heat, or a gas stove to keep my woodworking shop toasty.

For now, however, we're stuck dealing with a basement that was very humid (85% RH) when we moved in. There is mold on the ceiling sheetrock in a few of the (partially, poorly) finished spaces, and signs of it on the paper backing of the existing insulation. That was what prompted us to first pull down a bit of batting, at which point we discovered the crazy amounts of rodent poop. We'll be wearing HEPA respirators and appropriate protective gear when we take it down.

The floor point is a good one; I'm curious as to how cold we're really talking about. Previously, a cast-iron oil boiler kept the basement fairly warm, but we've switched to a high efficiency natural setup, and I am indeed concerned that the heat loss through the floor will be noticeable. Heat obviously rises, but I'm aware of the potential loss of heat downwards. The issue is that replacing the batting would cost about $1000; that would take a very long time to repay itself, vs. living with a cool floor for a year and then moving forward with insulating the interior walls.

Thanks to everyone for the advice; I'll look into the local building code and see what we're obligated to do.
posted by ellF at 4:29 PM on August 13, 2012