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A new video: Joe Lstiburek talks about crawl spaces

Martin Holladay | Posted in General Questions on

Why is there rain dripping down from the fiberglass insulation in my crawl space ceiling? Joe Lstiburek explains why in this video.

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  1. John Brooks | | #1

    Thanks Martin
    that video was "new to me"
    I added it to my Pinterest Board
    If anyone else has links to Lstiburek Videos that are not on my "board"
    please post a link and I will add the video

  2. Ron Keagle | | #2

    I thought we have been told that moisture does on condense on fiberglass, but rather, passes through it and condenses on the surface beyond.

  3. GBA Editor
    Martin Holladay | | #3

    The best person to answer your question would, of course, be Joe Lstiburek. I don't know anything about the crawl space shown in the photo that Joe used during his presentation.

    Like you, I'm somewhat confused by his statement, "If I were to slip my hand up in there [presumably, between the damp fiberglass batt and the floor joist], it would actually be dryer ... because those surfaces are progressively warmer."

    Of course, the temperature gradient through the fiberglass batt depends on the season. During the summer, when the house is air conditioned, the top of the floor assembly will probably be cooler than the bottom of the floor assembly. I would assume that this type of floor assembly would be wetter if the house were air conditioned -- especially if the room above has sheet vinyl flooring.

    But, as I said, only Joe can answer your question, because I don't know anything about the circumstances under which that photo was taken.

  4. John Brooks | | #4

    Ron & Martin,
    I think Joe gives a good explanation of the temperature gradients in this article

  5. John Brooks | | #5

    Ron, you may also enjoy this "insight" article ...
    Depending on the context it is possible to see condensation and frost on the surface of fiberglass batts

  6. GBA Editor
    Martin Holladay | | #6

    Thanks very much for the link, which is very relevant. For me, this was the key sentence: "The underside of the insulated floor assembly is within one or two degrees of the ground surface temperature because the two surfaces are radiation coupled."

  7. Ron Keagle | | #7

    John and Martin,

    Thanks for those references.

    At first, I thought that reaching up beyond the bottoms of the joists would be moving toward colder regions, but the coldest part of the space is the ground, so the second coldest part is the bottoms of the joists due to their thermal coupling with the ground. The warmest part is the bottom of the living space floor even though it is cooled by the air conditioning above.

    So even though the AC is not cooling to below the dew point of the outside air, it is reducing the heat loss that would otherwise keep the insulation bottom, and joist bottoms above the dew point. So when the outside air enters the crawl space, it condenses on the bottom of the fiberglass, the bottoms of the joists, and the top of the poly on the ground. I assume that the highest regions alongside of the joists would be above the dew point of the outside air, even though being chilled by the AC in the living space above.

    What I would like to observe in this crawlspace is exactly how far the wetness extends up into the fiberglass, and to compare that to how far the wetting extends up the sides of the joists.

    I would expect that the wetting extends somewhat up the sides of the joists; perhaps a few inches because the below-dew point temperature is likely to extend up into the fiberglass some distance.

    And if the temperature is below the dew point for say three inches into the fiberglass, I wonder if condensation is occurring within that three inches of fiberglass. And if it is, I wonder if it will wet the fibers within that three-inch zone. And if it wets the fibers in the three-inch zone, does the moisture hold there by capillary action, or does it fall to the bottom surface of the fiberglass?

  8. John Brooks | | #8

    Ron wrote:"I wonder if condensation is occurring within that three inches of fiberglass. And if it is, I wonder if it will wet the fibers within that three-inch zone. And if it wets the fibers in the three-inch zone, does the moisture hold there by capillary action, or does it fall to the bottom surface of the fiberglass?"

    Ron, look at page 2 (Insight-049)

    My read is that condensation will actually occur at "the dewpoint location" in the fiberglass...
    only, it does not "stay" there for long.... "moves on" and accumulates at the cold "spot of interest"...

  9. Trevor Trainor | | #9

    Since the bottom surface of the fiberglass (and joists) is the below the dew point, condensation will occur on those surfaces. Air passing through the fiberglass will be dehumidified by the bottom surface and therefore condensation will not occur inside the fiberglass batt. If enough moisture accumulates on the bottom surface, some may rise by capillary action - especially in the lumber, but the condensation will occur on the bottom surface only.

  10. Ron Keagle | | #10


    I did read that part and understand your point about it. But I see that happening as moisture moves outward through an insulation cavity where it first encounters the dew point within the permeable insulation, condenses there, and then moves on to a more interesting place to live. So it accumulates wetness at the end of the insulation zone on the sheathing.

    This would be different than the crawl space example because there, the moisture first encounters the fiberglass at its coldest point within the layer, whereas in the case of the outward moving vapor, it first encounters the fiberglass at its warmest point. So I can understand Trevor’s point that the first encounter with the dew point at the threshold of the insulation layer (in the crawlspace) would dehumidify the air right there.

    Maybe they need a sheet of polyethylene on the bottoms of the floor joists so the rising, warm, moist, outside air will hit the poly and dehumidify on that surface, and thus keep the joists and fiberglass dry.

    But, in any case, I would like to actually observe what is happening with the occurrence of that condensation, and its distribution. I would want to make sure that the whole picture fits the theory.

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