GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Will foam injected between doubled I-Joists be damaged? Alternatives?

user-1119983478 | Posted in General Questions on

My 14′ x 20′ addition has just been framed and sheathed.  I noticed that they doubled up two I-joists for the floor span, with the pairs of joists separated by 12″.  This concerns me, since there are all these unreachable 11″ spaces between the joist pairs. 

My architect says we can simply drill holes in the OSB of the joists and inject closed cell foam.  My thinking is that this would damage the joists as the foam expands.  Or possibly generate too much heat, as well.

We are in Denver, climate zone 5B and the joists are over a crawl space.

I am thinking that the solution is 2″ insulation board under the joists, but that is a lot of space between the floor and the insulation board.

Suggestions please!


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    DCcontrarian | | #1

    I'm trying to figure out your description. Is this correct: You've got two I-joists touching each other, no daylight between them. Then 12" over, another pair. Then another 12" and another pair?

    When you say "there are all these unreachable 11″ spaces between the joist pairs" the 11" is referring to the vertical size of the space?

    First, I recommend reading what Joe Lstiburek has to say about crawl spaces. Start with this:

    The obvious solution would be to seal the crawl space and insulate the walls instead of the floor.

    If that's not possible, you have to treat an insulated floor over a crawl space like a wrong-side vapor barrier assembly. It's basically like an unvented roof, although not quite as critical. Either you need to have all of the insulation be impermeable, or on the cold side you have to have enough impermeable insulation so that condensation won't happen, and the assembly has to be able to dry to the interior. The impermeable insulation needs to be air sealed.

    One way you could achieve that would be with foam insulation across the bottom of the joists and blow-in insulation between them. For blow-in all you need is a 4" round hole, which an I-joist should be able to handle. Between the paired joists you'd have to drill a hole in the webbing and fill them from below. The larger gaps could either be done before the foam board using a netting to hold the insulation, or after the foam board from above through holes in the subfloor.

    1. Expert Member
      DCcontrarian | | #2

      Oh, and you have to tape the seams in the foam insulation.

      And this only works if no one is planning on running ducts or pipes in the crawl space.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |