Hey all,

We recently hired a new tech that has previously done a bunch of Home Performance work on the East coast and he has turned me on to a new idea on how to insulate a basement rim joist that is enclosed with a ceiling. I wanted to see what everyone thinks about dense pack cellulose in an enclosed rim joist.

It makes sense that when you have a joist running parallel to the foundation wall that you can drill a hole in the ceiling and fill the joist cavity that is up against the rim. But for the side of the basement that has a wall perpendicular to the joists, can this work?

I have not heard about any contractors here in Portland OR using this technique, but in my mind it seems possible. I just wanted to make sure before we do the install, that it will pass program QC. Before I present it to the program, I wanted to hear some opinions from people that have experience  using this method.

So here's how I see it. If you drill the hole about 2 -3 feet away from the rim joist (towards the inside of the basement) in the basement ceiling, and blow material into the cavity, you should be able to make sure that the rim is fully insulated. I can imagine that the material would taper towards the hole, but as long as you pack it in, it would be fine.


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Other approved materials are allowed only in section 4 of R302.11 (above), for around mechanical penetrations, not for draftstopping or firestopping in concealed frame cavities.

I was referring to the concealed cavities where codes have required nominal 2" lumber, and where dense-pack cellulose can meet or exceed the same standard.


R302.11.1 Fireblocking materials.

Except as provided in Section R302.11, Item 4, fireblocking shall consist of the following materials.

    1. Two-inch (51 mm) nominal lumber.

    2. Two thicknesses of 1-inch (25.4 mm) nominal lumber with broken lap joints.

    3. One thickness of 23/32-inch (18.3 mm) wood structural panels with joints backed by 23/32-inch (18.3 mm) wood structural panels.

    4. One thickness of 3/4-inch (19.1 mm) particleboard with joints backed by 3/4-inch (19.1 mm) particleboard.

    5. One-half-inch (12.7 mm) gypsum board.

    6. One-quarter-inch (6.4 mm) cement-based millboard.

    7. Batts or blankets of mineral wool or glass fiber or other approved materials installed in such a manner as to be securely retained in place.

    8. Cellulose insulation installed as tested for the specific application.

It would be great if someone did a video and u-tubed it.


Practically speaking, we seal all holes less than about 2' with foam.  1/2" OSB does the rest.  We foam gypsum common wall gaps at framing, gaps between double walls, and most items.  Average house will have 3-4 places where a piece of wood is used.  

Then you're not complying with code.

In what way are we not complying with the code?
In what way are we not complying with the code?
In what way are we not complying with the code?

See R302.11.1 above.

can you be specific - I am not seeing it

"we seal all holes less than about 2' with foam.  1/2" OSB does the rest"

How is that consistent with the very specific list of allowed fireblocking materials?

And, did you mean holes less than 2 feet? Even if you meant 2 inches, that still exceeds what you reported as the maximum span for foam: not span more than 1-5/16"

Something is screwy with the site tonight

We use OSB to cover the edges of things like soffits then seal with foam, and use extruded styrofoam to seal the tops for energy,  We use the foam to cover the backs of kneewalls, and use it to fabricate top and bottom plates as necessary,  No one says anything at 2" even though the es says 1-5/16 - as they realize what the material is.  We will do this as we seal over 2500 new houses in 2012 in, as I said, 500 jurisdictions (New Jersey has a jurisdiction every couple of miles)

We have excellent working relationships with most of the inspectors that we deal with.  Based on this experience, I have no qualms about using this same system in existing houses, even though the code does not apply for the level of work we are usually doing in retrofits.

Wow thanks for all the advice! I think this is enough info to prove that this is a valid strategy.


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