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Thread: crawl space vapor barrier - floor or joists?

  1. #1

    Default crawl space vapor barrier - floor or joists?

    I have a crawl space that is about 3 feet high, dirt floor, nothing in the space but ductwork and wires and some plumbing (no air handlers or hot water heaters). It has vents in the masonry along the edges.

    My question is this - right now, there is insulation between the floor joists above with fiberglass insulation and a paper vapor barrier to the warm side of the floor (interior). For whatever reason, about 1/4 of the crawlspace does not have insulation over it. I know most people recommend a plastic sheet over the dirt floor as a vapor barrier, but with my situation, wouldn't that create a double vapor barrier? Should I insulate the remaining crawlspace the way it has been done so far? I don't know that I want to rip out the other insulation just to put plastic down, as the crawlspace is about 1000 sq ft. Or should I leave the vents open and put down the plastic as well?



  2. #2
    Join Date
    Nov 2004
    Boston, MA


    My last house had the plastic on the dirt as well as the paper backed insulation. Due to some renovations some of the plastic on the dirt was removed
    The reason for the plastic became very apparent
    After it was removed from that area, the entire basement has ahigher humidty level. As a result the fiberglass insulation was constantly wet & dripping water. Also a lot more spiders in the crawl space
    It was a lot drier with the plastic on the dirt

    The crawl space/foundation has since been renovated by a new owner
    They insulated the outer walls of the crawl space & took down the insulation on the floor. It was never finished & the plastic was not replaced on the dirt floor
    I think insulating the outside walls was a good choice
    I thik removing the insulation from the floor was a bad choice given the cost to heat a house these days. The crawl space always stayed fairly warm in the winter

  3. #3


    Scuba Dave has some good advice. I also prefer the crawl permeter insulated with a vapor barrier on the floor of the crawl.

    However the IRC does allow the floor to be insulated. Whoever installed this was correct putting the vapor barrier [aka paper back of fiberglass] on the warm side against the home floor. This installation will prevent the insulation holding water. Paper on the crawl side will cause a soaked insulation that will fall out and greatly increase moisture build up. Do not double vapor barrier.

    Either finish installing the fiberglass insulation on the floor with the paper up against the floor and do not install a second vapor barrier or rip all the fiberglass out of the floor joists and install styrofoam insulation around the perimeter and install a vapor barrier on the crawl floor.

    Also a consideration is if your heat ducts are insulated. If you heat ducts in that crawl is not insulated inside or outside the ducts and you keep the floor insulation you have a major heat loss 20% or more with those uninsulated heat ducts.

    I would do as scub dave suggested and also check the insulation of your heat ducts. Close your vents in the winter and open them in the summer.

    Good Luck


  4. #4


    The ductwork is insulated in the crawl space, although I'd like to replace it because the ductwork is wrapped in fiberglass without a vapor barrier. During the summer, it gets wet and drips. Anywhere to get good duct wrap? - the home stores around here don't seem to carry it.

    So if I'm understanding correctly, I should just leave the dirt and finish installing the insulation with the vapor barrier to the floor, since it is only about 200 sq feet or so and I don't want to replace all of the other stuff if I don't have to. I shouldn't put down plastic on the dirt, correct?


  5. #5
    Join Date
    Oct 2006
    Panama City, FL


    My opinion is that you should put the plastic vapor barrier on the dirt. It's better to stop the moisture at the barrier instead of coming up from the ground even if your vents are open...

  6. #6
    Join Date
    Nov 2004
    Boston, MA


    I would also put the plastic on the dirt
    When my last house had the dirt covered in plastic the crawl space was always dry. Without it the insulation was always dripping water

    My heating ducts were already insulated with the ductwork insulation
    I have seen it at local DIY stores - Home Depot & Lowes

    My crawl space covered 900 square feet & was about 3' deep

  7. #7


    The following copied sections of the IRC 2003 code may be of some help to understand principles.

    N1103.3 Duct insulation. All portions of the air distribution
    system shall be installed in accordance with Section M1601
    and be insulated to an installed R-5 when system components
    are located within the building but outside of conditioned
    space, andR-8when located outside of the building.When lo-
    cated within a building envelope assembly, at least R-8 shall
    be applied between the duct and that portion of the assembly
    furthest from conditioned space.

    N1102.1.7 Crawl space walls. Where the floor above the
    crawl space is uninsulated, insulation shall be installed on
    crawl spacewallswhen the crawl space is not vented to out-
    side air. The required R-value in Table N1102.1 shall be ap-
    plied inside of the crawl spacewall, downward from the sill
    plate to the exterior finished grade level and then vertically
    and/or horizontally for at least an additional 24 inches (610
    mm). The exposed earth in all crawl space foundations shall
    be coveredwith a continuous vapor retarder having amaxi-
    mum permeance rating of 1.0 perm [(57 mg)/(s m2 Pa)],
    when tested in accordance with ASTM E 96.

    Hope this helps


  8. #8


    This cross section in a crawl space is not a double vapor barrier: dirt - plastic sheet - large air space vented to outside - fiberglass insulation between joists - Kraft paper vapor barrier - subfloor - flooring.

    Because you have more moisture entering the crawl space (from the ground) without the plastic sheet compared with using the plastic sheet, there is nothing to lose by having the plastic sheet even with the floor above insulated. You may want to add ventilation if you did not already have that, to get rid of moisture coming down through the floor because the vapor barrier there did not form a perfect, hermetic, seal.
    Last edited by AllanJ; November 14th, 2006 at 09:16 AM.

  9. #9


    It is true with open vents you would not have trapped moisture between two vapor barriers. However in the winter when you close the vents to the crawl to avoid freezing you also create the trapped moisture double vapor barrier concern.

    Just my opinion


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