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Community and Q&A

Insulating rim joists

rhl_ | Posted in General Questions on

We need to air seal and insulate our Rim Joists. I am in Zone 4A.

House is 100 yrs old, foundation wall is stone, and approx 12” deep. Certain joist bays are not accessible without drilling through other joists at the end of the foundation wall, to spray into the cavity.

I have been removing old brick fireblock, to access the cavities.

Reading GBA,, 475 etc, has made me wary of spray foam like reading webmd makes me think I have cancer.

I am looking to get comfortable with this process.

I have a quote for approximately $1000 for approximately 5 inches of closed cell HFO foam, from ab insulation contractor. He is licensed in my county and carries a 1 million dollar insurance policy. His crew has been spraying for about 8 years. He claims he has never had a legitimate issue with the curing process or off gassing for year, except in one instance where he did a roof, and the  client refused to remove the fiberglass on the floor. After a core sample from both the manufacturer and a 3rd party they agreed the samples cured properly. He said the client ended up having the foam removed instead of removing the fiberglass.

I have talked to many of these foam contractors, this guy is the most responsive and reasonable as was recommended to me from a passive house builder I know. (Since he also dense packs etc).

Contractor said most of the issues in spraying (aside from not letting the foam cure) arise when barrels of the A and B products are replaced, and our joists are so small an area that we won’t even use close to a full barrel. 

I guess you see so many failures on the internet it’s hard to belief there aren’t more failures and these bad examples are examples where, really it wasn’t the contractor fault. 

Any advice to get over my angst here?  

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ryan,

    I can see how reading the Q&A forum on GBA could make you nervous about using spray foam. People share their bad experiences, they don't share when installations go perfectly fine. If they did, you wouldn't be able to find the few failures among the thousands of successful installs.

    Unfortunately, spray foam installs gone wrong are high stakes as they sometimes produce odors and are difficult to remedy. (imagine if all the homes with poorly installed fiberglass batts has an odor.)

    For environmental reasons, I'd ask you to consider options if you had a more accessible situation at your rim joist, but there are some times when spray foam is a problem-solver and an inaccessible rim joist area is one of them, as well as an important area to insulate and air seal.

    All this to say, if you have vetted the contractor and they run a professional outfit and you let them do the job the way they know how to do it, the odds are in your favor that everything goes just fine.

  2. Expert Member
    BILL WICHERS | | #2

    The fact that your contractor explained to you a job he did that did NOT go smoothly says a lot. Many contractors will pretend they’ve NEVER had a problem, and everyone has occasional problems. It sounds to me like your contractor knows what he’s doing and that’s the most important thing to avoid problems.

    Brian already said what I always say to people who are nervous about spray foam: you only here about the few problems, you don’t hear about the many, many successful installations. It’s like you said: most people don’t have cancer, but if you read a lot of medical articles you might start thinking the opposite.

    As long as you have a competent and experienced contractor doing the installation you should be ok. Rim joist insulation is one of the places where spray foam usually is the best option.


  3. frasca | | #3

    Can you do 1-2” of ccsf for the air sealing, then 3-4” of ocsf for the remaining insulation?

    Should have a lower environmental impact and be almost as good, right?

    1. Expert Member
      BILL WICHERS | | #5

      Both open cell and closed cell spray foam work for air sealing. The difference is that open cell spray foam is far more vapor permeable than closed cell is.

      My guess is for a small project like a rim joist job, the extra work involved in switching foams and making a second pass would probably outweigh any cost savings. You’re using a pretty small amount of material on a job like this too, so any environment advantage will be small as well. If it was a roof insulating project things would be very different.


  4. MAinspector | | #4

    "I have been removing old brick fireblock, to access the cavities."

    Remember to replace the fire blocking where needed. I don't know of any spray foam that is an approved fire block.

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