GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Insulation in joist cavity with limited access

l_bez | Posted in Energy Efficiency and Durability on

I just moved into a house built in 2003. I’m in the process of removing the fiberglass insulation placed in the basement by the builder, to be replaced by foam (tbd sheets vs closed cell spray). This is in anticipation of finishing the basement. I’m in a northern Canadian metropolitan area, Zone 7A. 

My question is regarding one area of the basement, where an engineered joist placed lengthwise along the interior edge of the concrete wall is blocking access to the rim joist cavity — Cross sectional attached. From a few holes through this joist for utilities (gas line, AC, plumbing and drain into exterior wall above), it appears that the cavity was pre-filled with fiberglass insulation during construction.

I’m not sure how to approach this with regards to sealing it off properly to prevent condensation and mold growth.

I’m waiting to hear back from Weyerhauser regarding spacing of holes; maybe I punch some holes and remove the fiberglass to provide access for spray foam, but I don’t know if they can be spaced close enough.

Would appreciate all thoughts and advice.

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. PAUL KUENN | | #1

    Louis, what is on the exterior of that rim joist? I would just air seal and insulate (hardboard) the complete length of the engineered joist. If air can't escape to the other side where the fiberglass is, it should dry to the exterior and all be safe and the fiberglass will do the job it is intended to as long as the exterior is air sealed as well (but allowed to dry outward).

  2. l_bez | | #2

    Hi Paul, thanks for your reply. Apologies for the delay!

    I was not confident about the quality of the air sealing on the other side based on the work in the rest of the basement, and the draft coming from the perforations in the engineered joist.

    There are a few water lines, drain lines, a natural gas line, and the plumbing and wiring to the exterior AC unit that traverse the webbing in that engineered joist and then either go upwards into the exterior wall above, or to the outside. On the outside of the rim joist is siding, the gas utility line, and the AC unit.

    For this reason, I was not confident in the ability to perfectly seal off that cavity from the interior air.

    Weyerhauser recommended having a local contractor have a look at it.

    I ended up asking a contractor (family friend) to have a look. He agreed that it was safe to remove the webbing and replace with spaced out crush blocks, since the joist was not cantilevered, and the bottom was fully supported for the span by the foundation wall.

    Behind, I found a lot of areas for air leakage, and ice and mold behind the fiberglass. So I'm glad I took it down.

    Thank again for taking the time to reply. Apologies for my late kudos!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |