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Community and Q&A

Blown In Insulation for floor above crawlspace?

AlexD2022 | Posted in General Questions on

My house is built with a perimeter stemwall and a 2×6 subfloor resting on girders 5′ OC which has resulted in the existing fiberglass insulation to far pretty poorly and be constantly falling down. I’m wondering if putting up something like intello with strapping going across and then doing blown in insulation would make sense in a situation like this?
Location is Willamette Valley in Oregon, crawlspace is very damp – I’ve just put in a sump pump and am in the process of laying down a vapor barrier.

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  1. Expert Member
    DCcontrarian | | #1

    I recommend this article for a really good discussion of the issues in crawl spaces:

    You really have to worry about moisture drive with crawl spaces. Read the article, but I think what might be best for you is a layer of foil-faced foam to act as an air and vapor barrier and then fill the bays with fluffy insulation. Another alternative would be just spraying the whole thing with closed cell foam. Either way you have to have both air sealing and moisture sealing for the entire floor.

    1. AlexD2022 | | #3

      Thanks for the article DC, both spray foam and encapsulation are fairly expensive choices so it's unfortunate if those are the two ideal options. I have the added complication that my access is through a window well and getting sheet goods of any decent size through it is not really feasible which makes putting rigid insulation on the stem wall for encapsulation tricky.

      1. Expert Member
        DCcontrarian | | #4

        Encapsulation is also an option, but I'm talking about a layer of foam below the joists parallel with the floor.

        Access is always an issue with crawl spaces. Encapsulation is often the better option because the walls of a crawl space tend to be less area than the floor. With foil-faced foam you can slit it lengthwise and fold it without breaking the foil on the other side, which allows you to slide it through a small opening and then unfold it and have a sheet that is still air-tight.

        Do you have a way of keeping rodents out of the crawlspace? That's usually an issue with insulation.

        1. AlexD2022 | | #6

          Thanks for the tip on folding, hadn't thought of that! I'm still worried that getting sheets big enough to span girder to girder (I don't have joists, just a girder/beam that has 2x6 on the flat ontop as the subfloor).

          Funny you mention rodents :) rats burrowed around my access hole window well to get into the crawl so I'm battling that right now and will probably completely dig out the access to replace the rotting wood holding the well and get a more permanent solution done.

  2. Expert Member


    Depending on how much plumbing and wiring is in the way, you might consider a layer of foam board on the underside of the joists instead. This will give you a thermal break, as well as more R-value then just changing the type of insulation in the cavities would.

    Check you code first though. In some situations foam can not be left exposed without a thermal or ignition barrier.

    1. andyfrog | | #9

      If the concern is about moisture drive, would it be worth going with commercial Tyvek or other lower permeance membrane?

    2. Expert Member
      DCcontrarian | | #10

      I'm not sure Tyvek is right here.

      In a standard wall assembly, you've got a warm, humid side and a cold, dry side. You want the wall to dry to the cold, dry side so you put the vapor barrier on the warm, humid side and keep the cold, dry side breathable. So in cold climates, Tyvek on the outside.

      The problem with crawl spaces is that they don't fit. The underside is going to be both cooler and moister than the interior of the house all of the time. It's really more analogous to an assembly with a wrong-side vapor barrier -- like a wall with a vapor barrier on the outside, or an unvented roof. In that situation what you have to do is put an air-tight, vapor-tight, insulating layer on the cold side. The insulation has to be thick enough so that condensation won't happen on the inner face. The assembly won't dry much, but to the extent it will dry it will dry to the interior.

      This is why Lstiburek recommends either closed cell spray foam or a taped and sealed layer of foam board as the bottom layer of crawl space insulation. It's basically what you would do for an unvented roof, except upside-down.

      I worry that Tyvek would do nothing to keep vapor from entering the insulation from below. Crawl spaces are often very close to the dew point, and small swings in temperature would cause condensation to form above the Tyvek. The Tyvek would then act as a liquid water barrier and trap that moisture against the floor.

  3. AlexD2022 | | #5

    Thanks Malcolm - plumbing and electrical under the house isn't too bad, but unfortunately I don't think I can get 5 foot long sheet goods of a decent width through my access... If I had joists with normal spacing I would be much more inclined to have just done fiberglass along with making sure the ground vapor barrier was well detailed but I can't think of a good way to put batt or roll insulation up there without having to go crazy with strapping to help keep it all in place.

    That being said, it might be worth exploring how big of a sheet I can get under the house to see if it's a doable project (if I'm reading Oregon building code R302.9 correctly it can be left exposed in the crawl space with a flamespread of 75 and smoke spread of 450 - which polyiso meets). The house needs to be repiped and it seems like it would be super valuable to have all the pipes inside the rigid insulation to completely remove the worry of freezing pipes

    1. Expert Member
      MALCOLM TAYLOR | | #7


      I spent much of today re-plumbing waterlines that froze under our community hall last week. Not having to worry about anything when the weather gets cold is a real comfort.

    2. Expert Member
      BILL WICHERS | | #8

      Polyiso tends to have a relatively beefy facer. You can usually, if you're careful, cut one side's facer, score the polyiso, but leave the other facer in tact. After doing that, you can fold a panel in half, making a 4x8 foot sheet act like a 2x8 foot sheet twice as thick. If you have the clearance in the crawlspace, you can "unfold" that polyiso back into a full sheet again, using the intact facer towards you as you apply the sheet, so that the fasteners will tend to hold things flat. This might help you get things into place. If that doesn't work, it's not terribly difficult to cut a sheet in half, then either tape the seam or seal between panels with canned foam at install time.


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