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  1. #11
    Join Date
    Dec 2007
    Location
    Tennessee
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    1,418

    Default 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.
    Last edited by keith3267; 12-02-2012 at 11:57 PM.

  2. #12
    Join Date
    Nov 2012
    Posts
    7

    Default Re: insulating crawl space

    Thanks. T too think what I've done is okay for now. The question remaining is what materials would you use to create a dead air space? Maybe I could incorporate that into part of the roof that still need to be insulated directly. For some of the roof I used 3 1/2 inch rock wool batts with 1 inch air space, all I could get in the 5 1/2 inch deep bays because the rock wool kind of puffs out, so there is no inch left to apply bubble foil with an air space. This gives me R-15, less than I'd like. The part I haven't done is where I want to modify the roof for more head space, so I want it to be something that can be removed well, not like the 3 1/2 inch rock wool which would come out in pieces at great expense. (The R-23 5 1/2 batts that I used in the walls were far more solid.) I have some R-13 board that I will use for the upper ceiling, but the roof part would cost thousands of dollars using that. Any ideas on this?

    I'm wondering, who are the environmentalists who object to dead air space? Do they offer a reason? If there's any source for that information, I'd be interested.

  3. #13
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,418

    Default Re: insulating crawl space

    Its not that environmentalists are opposed to dead air space, its just that they are not very technically oriented. Unless it has change recently, environmental studies have been more liberal arts than math/science oriented. To them, the short answer is the R value of a physical insulation.

    Stuffing a lot of insulation into a cavity is the easy solution, not a lot of thinking involved and hard to do wrong. Creating a dead air space and making it work is a little trickier.

    I am trying not to be too hard on environmentalist. They do bring awareness and that is important. They even provide important data and observations. What they don't seem to be good at is finding good solutions. The solutions should be left to the scientific and engineering community.

    I should also point out that not all environmentalists are technically ignorant, it's just that technical competence is not a requirement to call yourself an environmentalist. Many environmentalists, especially some of the early pioneers in environmentalism came from the scientific community.

    My college roommates father was a professor of chemistry at a major university who was fired because he was telling everyone (this is the best my roommate could explain at the time) that pollution was getting into the upper atmosphere and migrating to the north and south poles and ruining the atmosphere there. I had met his dad and he did not strike me as a crackpot but in 1969, this sure sounded like a crackpot idea. I realize now that he was talking about freon and the ozone.

    BTW, he ended up teaching at a junior college somewhere in Virginia, last I heard.

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