Insulate below a floor or not?

[Steve Barker LT]

I've mentioned in other posts about our 135 yr old house we're working on. At this point I'm rebuilding the floor on the lower level. This house basically sits on the ground. My new joists will be about 4" off the dirt. I've been through the "to vent or not to vent" threads about crawl spaces. I've decided not to ventilate. It's been closed for 135 years, it can keep being closed. My question is, IS there any reason why I shouldn't fill this space will insulation? Whether it be fiberglass or blown cellulose? In a few days I'll have all new joists and proper center support and will probably insulate unless there's some reason I shouldn't.

thanks!

[RicodJour]

4"? Did you use treated lumber for everything within 18" of grade? I would never stuff the space entirely with insulation no matter how tempting that might be. It would end up being a perfect nesting place for vermin, you wouldn't be able to inspect underneath to make sure everything is copasetic, and it'd end up being a mold breeding ground. Rigid insulation would be a good choice in your situation. Cut it to fit with some expanding foam as needed to seal the edges. It's a little more expensive, but the higher R-value will give you whatever level of insulation you want and keep it within the depth of the joist bay. R

[longshot]

use KRAFT FACE R-19 between the joists

[M]

hmmm... I too am doing something similar. I am between 12" and 24" above grade however. I was thinking of making a 2" foam skirt around the perimeter and make it removable so as to let it breath in warmer months. I was also going to stucco the exterior faces. You mentioned solid insulation. Would that be foam or lumber?

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[Matt Whiting]

Insulating between the joists would be a good idea, using rigid foam board, but you'd have to do so before you install the flooring as it sounds like you don't have clearance to work under the floor.

If you are talking about just blowing insulation on top of the dirt and up under the floor, I'd be hesitant about that. I'd fear that this would just provide a path to conduct ground moiste up around your floor joists.

Personally, I'd insulate between the studs and then ventilate the space under the joists. Then again, if you live in the desert of AZ, never mind the ventilation.

Matt

[bowgus]

If you seal around the perimeter (not vent) then that air space is insulation. A filled space can become a home for humidity, snakes, mice, and all those other insects and critters that will crawl into insulation. Since you're insulating against ground temperature which is not as cold as outside air can become, if it was me, I think I'd just put maybe 2" or so solid insulation between the joists just to make the floor above a little bit more comfortable on the feet. And that's why I would not fill the space with insulation.