13 posts / 0 new
Last post
tonyc56
insulating crawl space

Question regarding insulating crawl space floor. Is it wise to add insulation directly on the crawl space floor since the ground is a constant 55 F and would radiate heat keeping the crawl space area somewhat warm during the winter months.

MLB Construction
Re: insulating crawl space

insulating between the floor joists should be enough, do not insulate the cement floor unless you want to use rigid foam insulation. absolutely do not put fiberglass insulation on the crawl space floor.

tonyc56
Re: insulating crawl space

MLB why---"absolutely do not put fiberglass insulation on the crawl space floor."

also why would you put rigid foam insulation on the floor since the temperature of ground is radiating heat at a constant 55f...

MLB Construction
Re: insulating crawl space

the reason you don't use fiberglass insulation on a cement floor is that it will get wet and make a nice home for rodents.

the reason "to" insulate a concrete floor with rigid insulation is if the crawl space is accessible from a basement it will keep some of that 55 degree cold away from the basement.

the reason "not to" insulate a concrete floor with rigid insulation is if the crawl space is accessible only from outside and in no way connected to any heated space, therefore utilizing the heat from the ground to keep the space above freezing in the winter.

Ed2
Re: insulating crawl space

I may have made a mistake in how I insulated my crawl space this Fall, but I'm stuck with it and Winter is coming on. I wonder what I might do now to improve matters.

My 1000+ square ft. house is supported by sauna tubes on a grade, the floor joists are 2 1/2 to 4 ft. above bare ground with sandy gravel added. The ground is moist though not very wet, and a lot of stuff is stored there covering much of the ground. There are no openings or vents into the house from the crawl space, and no pipes in the floor. The perimeter is roughly sheathed with plywood and there are gaps open to the outside.

Right or wrong, I have already insulated the floor with craft backed 9 1/2 in. fiberglass batts, installed in 4 ft. lengths, paper side down. I'd like to seal the perimeter from the outside, as New England winters are very cold, but wonder whether this would trap too much moisture and the fiberglass would become moist. Would it be a good idea to install plastic sheeting under the insulation if I make sure it is tight?

ed21
Re: insulating crawl space
Ed2 wrote:

I may have made a mistake in how I insulated my crawl space this Fall, but I'm stuck with it and Winter is coming on. I wonder what I might do now to improve matters.

My 1000+ square ft. house is supported by sauna tubes on a grade, the floor joists are 2 1/2 to 4 ft. above bare ground with sandy gravel added. The ground is moist though not very wet, and a lot of stuff is stored there covering much of the ground. There are no openings or vents into the house from the crawl space, and no pipes in the floor. The perimeter is roughly sheathed with plywood and there are gaps open to the outside.

Right or wrong, I have already insulated the floor with craft backed 9 1/2 in. fiberglass batts, installed in 4 ft. lengths, paper side down. I'd like to seal the perimeter from the outside, as New England winters are very cold, but wonder whether this would trap too much moisture and the fiberglass would become moist. Would it be a good idea to install plastic sheeting under the insulation if I make sure it is tight?

Yes you wrongly insulated the crawl space. The insul should go between the floor joists and plastic over the earth. You didn't mention insulating the walls of the crawl space. Yes moisture will be trapped. The crawlspace should be ventilated unless it becomes part of the conditioned space, which in your case I don't see how it could.
ps. sona tubes(a brand name), not sauna tube.

Ed2
Re: insulating crawl space

Thank you for your reply. I want to be sure on a couple of points.

I did install the insulation between the joists, paper stapled to them, facing down. Do I understand correctly that the paper should have been toward the floor? If so, why? I don't understand why the fiberglass should be exposed to the outside rather than covered and held in by the paper.

I'm looking for the best solution given what I have done. What would be the harm if I now sealed the insulation by covering it tightly with plastic sheeting? Wouldn't this protect it from moisture?

Since I will not be insulating or tightly closing off the perimeter walls, is there much benefit to covering the ground with plastic? The gravel covered ground is not wet, but moist. I can kneel on it and not get wet.

With the crawl space ventilated as it is, is there much risk if I leave it as is over this winter?

keith3267
Re: insulating crawl space

The paper side should have been up, but I don't think that is going to be a big deal, at least not for one winter. Unless you did some kind of super job sealing the paper to the studs, there is probably plenty of ventilation within the fiberglass.

You don't need very much insulation in between the joists. In fact dead air is just as effective as fiberglass insulation in floors. Unlike the walls and the ceiling, the air trapped between the joists stratifies with the warm air to the floor boards and the cooler air at the bottom. If there is no circulation, this adds up to about an R-3.5 per inch, the same for fiberglass batts.

Of course, there will be some circulation if the bottoms of the joists are left open. And the big looser comes around the rim joists. You can effectively insulate the floor by putting insulation up against the rim joist all the way around the perimeter of the floor and then just covering the bottoms of the joists to minimize air circulation.

Since you have already spent the money on the insulation, just make sure that the insulation goes all the way up to the floor and down to the bottom of the rim joists to that is well insulated to the outside. You can punch holes in the paper to allow some ventilation of the rest of the fiberglass, or you can take it out and insert it paper side up, all the way to the floor boards and secure it in place with those rods that are made for floor insulation or chicken wire and any number of other things.

ed21
Re: insulating crawl space

I mis-read your original post about the insulation location. The kraft face is not allowed to be left exposed because it is flammable, but I don't think moisture will be a problem if it's left that way. I would cover the gravel with plastic sheeting & tape to minimize moisture in the crawlspace. If you cover the insulation with plastic, you could get moisture buildup in the insulation. A vapor barrier is supposed to be on the warm side. Current code (IBC) is R-30 in floors in mid atlantic area

Ed2
Re: insulating crawl space

Thanks for the replies. I have heard opposing views on this subject, as most people around here including consultants at the building supply think I did just right. It's the way about everybody does it that I've seen.

My jury is open, except I don't care to take out anything. I can hardly imagine putting the insulation in with the kraft up and the fluffy and imperfect fiberglass exposed to the cold air. It seems to me that the kraft is at least some barrier to the cold air (and moisture) just flowing into the bare fibers. I understand that any moisture that does get past the kraft would be held in more, but then why wouldn't it be better to have tightly sealed plastic sheeting over it all, which would allow no moisture in? More critically, I have a feeling the kraft may not hold its integrity in the long or not so long haul.

From Ed21, I totally take the point that I should cover the ground with plastic. At present there are many things stored under the house which have no other place to go (for which I don't hold you or anyone else responsible with regard to advice offered), such as a few tarps, a lot of plastic left on the ground from the opened packages of insulation (used to kneel on, although without it I would only have been cold, not wet, only a slight dampness on the knee) which might as well stay, and some old foam sheeting that still covers the areas around all the many sona posts, and also some piles of firewood and various building materials including numerous windows, fencing material, and tools and garden materials, and a lot of junk that needs to be sorted out and sent to a better use far away from me. As long as they are there, they cover a lot of the ground, and I could throw more plastic down here and there, but there isn't much hope of a seal. There's also the question of what would a flood do. (Thanks, I'd never seen sona written.)

From keith3267, I am amazed at the suggestion that a dead air space would be as good as fiberglass. Am I correct that to have a dead air space, you must seal it from air flow? With what? I'd better have done that than what I did. I don't much care for fiberglass, the little fibers that get into the air and all over, and lack of dependability to fluff out or hold form or stay together, especially the many batts that came damaged or pressed out of shape or partly detached. This isn't Mid-Atlantic, it's North Atlantic. R-30 was all I could fit.

Inside I used rock wool batts, and I might better have used it under the house in spite of extra cost. I could have fit R-38 of that. It would have gone in much more quickly, too. Wouldn't I have avoided the present concerns? Can this be combined with a dead air space? It seems to me that even by itself, if fit snugly, it could make a space where the air flow is mighty slow. I would qualify that I found the 3 1/2 inch flimsy, but the 5 1/2 inch holds together better. I did fill spaces over all the the 8x8s with blocks of rock wool. That may accomplished some of what you suggested.

I've got some bubble foil, haven't figured where I'll use it.

There is no potential source of fire under the house, so whatever code there may be doesn't appropriately apply to my circumstances.

keith3267
Re: insulating crawl space

The reason that dead air space is as effective as fiberglass is that the only insulation in fiberglass comes from the dead air it traps. In a bare wall cavity, air circulates up the inside sheathing (the sheetrock) and down the outside sheathing. Filling the wall cavity stops this convection of air inside the wall.

In the attic, any heat that is conducted through the ceiling warms up the attic air and it rises out through the attic vents. You need a lot of fiberglass to slow down that convection loop. But in the floor, heat that radiates into the air below the floor, warms up that air and it rises, or is trapped against the floor, unless a breeze comes along and displaces the warm air.

If you stop the air from being displaced by outside breezes, then the floor stays warm and heat loss through the floor is minimum. Unfortunately many codes are being heavily influenced by "environmentalists" who do not understand basic engineering or thermal dynamics. To them, the only answer is "more insulation" and most often, more insulation is not the answer.

I tend to agree with the people you have consulted with, even with the vapor barrier down, I don't think it is going to be much of an issue and since you have already put the insulation in, I would not go take it out. Its just if you ever do this again on another house, you can use much less insulation and get the same result.

Pages

Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.