basement ceiling insulation thread (hey, why not?)

[mrbigstuff]

I'm more interested in what type of material I should use: plastic to cover the floor joists or some other material or fill in with fiberglass. My main concern is flammability for which plastic would not be the best choice. However, the practice of fitting fiberglass insulation in between all the piping, conduit and heating is, well, not a rosy picture. Plus, the plastic has the added benefit of hiding some of that mess.

So, what have you done to insulate a basement ceiling or what have you contemplated doing? and why?

[Acer]

Is this over a crawlspace or a full basement?

I've heard that over a full basement, even unheated, insulating the basement walls is a better bet...similar energy savings, plus improves the basement.

[ajakeski]

Use fiberglass insulation wrapped in plastic. You can find it at any building center.

[OWC Jamie]

I'd spend money on getting the basement airtight before I insulated the ceiling of a basement.

Some areas code requires living area insulation materials to be covered with sheetrock or equivalent.

Sound insulation I might understand, I don't think you'll get any money back from heat savings.
(unless you live in the basement and you don't want your heat going up) :-)

[RonT]

Is your basement heated or not? If heated why insulate it at all? Hot air rises, cool doesn't so both would seem to work to your advantage with an un insulated ceiling in a basement that's heated.

OTOH if the basement isn't heated then you'll want to use a faced insulation with the kraft paper toward the upper floor and the insulation facing the unheated basement.

Either heated or unheated I don't think I'd even consider using plastic. With a heated basement the plastic would serve no purpose except to inhibit the warm air moving up to some minor degree. In an unheated basement putting up plastic could conceivably trap the warm air filtering down from above and the cold air rising from below could potentially cause condensation which in turn could cause mold, mildew, rot or what have you.

My 2¢

[mrbigstuff]

[quote Acer]Is this over a crawlspace or a full basement?

I've heard that over a full basement, even unheated, insulating the basement walls is a better bet...similar energy savings, plus improves the basement.
basement walls is what I am working on. the floor of the basement is still dirt (old house) so I'm looking to prevent the outside air from coming in and up through the house from the basement level (it will look for an escape route through the upper floors once inside). However, I agree, insulating and working on the exterior walls is the first route, but that is pretty cut and dry for me.

[MacArtist]

I would not put plastic on the basement ceiling. Why? Because I would not want to trap moisture in the basement. You want moisture to travel up through the house and out of your attic through a properly vented roof system.

[OWC Jamie]

I'd insulate between the joists, but only at the sill over the concrete (or brick/rock) basement wall itself. I'd also look for leaks there first and try to seal (foam ?) them up. Amazing amounts of air infiltration there, even in new houses.
Every penetration. Water pipes, cable and electric wires. Everyone drills a bigger hole than what they need.
Windows. The (old wooden) frames are often horrible.
I'd do everything I could to keep the dirt around the foundation dry.
(carry the roof drain water far away and slope the groung to shed water well)
Dry dirt supposedly is a better insulator than wet dirt.

[davester]

What exactly do you mean by "plastic"?

Plastic is not an insulation product as far as I know. Assuming the basement is unheated and vented, there are two things needed: 1) a airblock to stop cold air from circulating up through any breaks in the floor (i.e. along pipes and the like); 2) an insulation product that provides R-value.

The usual way to do this is to detail all the breaks through the floor with expanding foam and the like, then use either rigid insulation or kraft-faced batts (or perforated plastic encapsulated batts) to jam up against the floorboards. The plastic/kraft can act as an airblock for old leaky floorboards but is insufficient for pipe pass-throughs.

If you use rigid styrofoam insulation panels you can cut them to fit and jam them up between the joists against the floorboards. Alternatively you can put in a continuous layer attached directly under the joists, though this will be a bit less effective because of the large air gaps (joist spaces) above them (note that that the joist spaces absolutely must be hermetically sealed all around or the insulation will be utterly useless. The styrofoam panels will give you much more insulation per inch than fiberglas, are easy to install, and are not so susceptible to gaps which can occur around fiberglass batts and render them virtually useless.

Did I understand correctly that you are insulating the basement walls? That makes no sense if the thermal envelope is the basement ceiling. You need to concentrate on the ceiling. Forget the walls unless you are going to live down there.

[Filliam H. Muffman]

I was going to say some of what davester said.

Is the basement going to be finished? You usually need ventilation in a basement until you get some sort of floor that is either a sealed slab (and walls too if they are dirt) or a joists suspended on concrete footings over the dirt.

Most of the expanding foams are really good at sealing. There are two types, one is a high density closed cell polyicynene that you could have a contractor just apply as a skin to seal the gaps, but it might be expensive to do that. It usually costs something like $500 just to have a truck show up to spray any sort of large volume.

There are small "contractor packs" of low density open cell foam that can be cheaper than buying a dozen or more aerosol size cans of expanding foam. That would be good for part of one joist bay