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

Insulating basement rim joist tight access

MossmanVT | Posted in Energy Efficiency and Durability on

Hi everyone,

   I just joined after reading quite a few posts on here.  I have a question that I am hoping someone can help me with.  I had a house built a year ago.  I built it fairly tight, but not super tight.  Zip system and flash and batt.  I’ve noticed the two sides of the house seem to lose more heat and I think I know why.

   Rim joists need to be insulated to be within code (Vermont).  When the builder called out the insulators, thee 2 X 4 stud walls were already in the basement.  So there was no access on the ends for spray foam.  The builder had them dense pack with cellulose.  Since then, I’ve moved the batts (terrible idea) and had at the stud walls spray foamed which helped quite a lot.  I’ve looked above the stud wall on one side where I thought it was dense pack, but it’s not so dense.  In fact the bump out for my bedroom just had some batts thrown in there.  The dense pack in other areas is loose at best.  I can easily jam a paint stick in there and wiggle it around.  I thought dense pack meant very tight???

  In any event, with no access at the moment, how can I add insulation in this cavity?  I do have a few inches of access on one end and less than an inch on another.  I was thinking of getting a spray foam can gun with a 40 inch wand to fill it with spray foam but I can’t imagine how many cans I would go through.  Can I blindly add spray foam within the cellulose and hope for the best?  Is there a better method?  The two part liquid spray foam is better since it goes on wet but I can’t get in there as far as I can tell  Maybe I should call back an insulator to dense pack it better?  I would appreciate any suggestions to deal with this.  Thanks guys.  

I’ve attached some pictures to help explain what I’m dealing with.

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
    NICK KEENAN | | #1

    Unfortunately a lot of people don't take insulation seriously and in spots where it's hidden it often gets short shrift.

    I don't think you want to foam over batts.

    I would consider taking off a section of the top plate of that wall over the window. It doesn't support anything, it's just there to give something for the drywall to be nailed to. You could cut it off with a sawzall. That would let you get up along the joist and at least pull out whatever's in there. You might even consider taking the entire top plate off, if you run a sawzall with a metal cutting blade along the whole length and cut through all the nails it will pop off. The foam will hold the studs in place.

    I'm not a fan of fluffy insulation along rim joists, unless they are perfectly air sealed they tend to invite condensation and get waterlogged. Spray foam is good along rim joists, as is foam board insulation if you can seal the edges. Foam doesn't work well in blind cavities because it's easy to leave gaps.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |