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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Assuming you've already dealt with any exterior and interior moisture problems at the rim joist/basement area and there's little possibility of either leakage or condensation, dense-pack can work. But there's no way to guarantee dense-packing the perpendicular joist bays unless the interior side is blocked - otherwise you end up with little better than loose fill or use enough material to fill well into the interior so that the outer 3' can densify (do they want to insulate the whole ceiling?). And you have to make sure that there is sufficient space between the rim joist and the first interior parallel joist - sometimes there is a joist right over the inner edge of the basement wall and the space is inaccessible.

This method can work and I have done it. I can offer one suggestion to save time and materila. After you drill your hole stuff the equivalent of a 10# onion gad and dense pack the bag toward the rim joist. For the side of the basement that the joist run the lenght you pretty much will have to dense pack the entire cavity. Which would be no real big deal.

We do this technique with success.  We buy burlap bags for about 80 cents each - 22" wide by 22" deep, just folded and stitched along the sides.  If there will be re-finishing to a higher standard in the ceiling of the basement, we remove the first piece of siding outside along the band, and go in from the outside.  Either location, stuff in the closed end of the bag leaving just a collar sticking out, then fill.  Our Krendl 500's takes less than 30 seconds to fill one to densepack standards, where we were finding closer to 2 minutes without the bag - saved time and saved material.  Along the parallel joist, we drill a hole and look inside to see how big a cavity we need to fill, then proceed.  If the joists are web joists, use the bag on the parallel joists too.  Please note that we do this on the band between first and second floor too.  We are in Weather Zone 4.

How about this idea that I saw used on mid-floor balloon framed floors;  in the basement ceiling, drill a 2-1/2 to 3" hole in each bay adjacent to the foundation, insert a poly bag into the hole leaving the open end of bag down in the room, insert the insulation hose into the bag, and up into the joist cavity, fill the bag, stuff the bag ends into the hole, patch the hole.

Haven't tried it, but it feels plausible.

You need a bag that allows air to pass through, such as the burlap bags described above, otherwise the air and cellulose blows back in your face and you can't dense-pack.

I agree with Robert, burlap is the way to go. You have to control the back pressure of the application by letting the air escape or you will not achieve the 3.5-4.0 for dense pack. Also a great app. for ballon framed 2 story homes. Unbelievable reductions.

I design and build new balloon-framed two-story homes, densepack the 12" thick double walls, use the air-tight drywall system and get them down to about 2 ACH50. I've had three building inspectors let me use the cellulose as a firestop in lieu of solid wood.


There is a detail for a firewall in multifamily housing that allows cellulose.  Your inspectors are correct.

A fire separation wall is a different beast than fire blocking in vertical envelope channels. But third-party testing of National Fiber Cel-Pak has allowed it to receive fire-stop approval.

Cellulose Fire Resistance and Fire Blocking Capabilities:

"National Fiber’s cellulose insulation has been approved as a fire blocking material under Section 708.2.1, Item 1, of the UBC, Section 716.2.1 of the IBC, and is permitted as an alternate to the fire blocking in Section R602.8, Item 1, of the IRC by Omega Point Laboratories, Report for Project No. 16094-11638. When installed in a dry or spray application to a depth of 14.5 inches, cellulose outperforms conventional wood fire blocking in fire blocking tests."

Fireblocking is much simpler using any of the single part spray foams - even Great Stuff from the big box stores is an approved fireblock material.  And my understanding is that for fireblocking, there is no required depth - but there is for firestopping.  And all cellulose materials could get approvals for firestopping a party wall within a certain assembly ( specified studding, plates, offsets, drywall thickness and type, etc, etc) but they don't because it is expensive and the market is limited

You're confusing sealing mechanical penetrations with a product like Great Stuff Fireblock (orange) foam with code-required nominal 1½" solid wood fireblocking.

R302.11 Fireblocking. In combustible construction, fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space.

Fireblocking shall be provided in wood-frame construction in the following locations:

1. In concealed spaces of stud walls and partitions, including furred spaces and parallel rows of studs or staggered studs, as follows:
1.1. Vertically at the ceiling and floor levels.
1.2. Horizontally at intervals not exceeding 10 feet (3048 mm).
2. At all interconnections between concealed vertical and horizontal spaces such as occur at soffits, drop ceilings and cove ceilings.
3. In concealed spaces between stair stringers at the top and bottom of the run. Enclosed spaces under stairs shall comply with Section R302.7.
4. At openings around vents, pipes, ducts, cables and wires at ceiling and floor level, with an approved material to resist the free passage of flame and products of combustion. The material filling this annular space shall not be required to meet the ASTM E 136 requirements.
5. For the fireblocking of chimneys and fireplaces, see Section R1003.19.
6. Fireblocking of cornices of a two-family dwelling is required at the line of dwelling unit separation.


This is an interesting and confusing area.  Acceptable fIreblock materials are specifically spelled out by the code and then it adds "other approved materials".  Dow came out with orange foam in 2004 as the first of these "other" materials with an icc-es.  They copywrighted (?) the color and this material started the path to "orange" foam and we still have inspectors who want orange.  Shortly after Dow, all of the US manufacturers had the same approvals, but even the original Dow es had ALL of their foams listed, regardless of color.  Even the Great Stuff was listed in 2004 and it is still listed after several re-applications which they are required to do every 2 years.  Note in "4" above, it says that these material are not required to meet E 136 which is a combustibility standard.  A fireblock material just has to burn slower than the wood in the framing  lumber around it.  Chimneys and fireplaces must be controlled with a "non-combustibe fireblock" which turns out to be 136.

An easy way to think of it is that a firestop has a set of instructions that, when followed, offers a barrier to fire for a certain time.  Fireblocking does not have any instructions such as depth or amount of material, nor is it required to stop flames, just control the spread of smoke and flame.  The only meaningful limitation on the foam is that it not span more than 1-5/16", and that is only because that is the span listed in the first level of testing. The next is 4" and is a level of investment that none of the manufacturers have made as of yet, but internal testing tells them that it would pass easily.

Look here to see all of the Dow products that are listed as fireblocks:


All of the Great Stuff variations are listed, regardless of color - even the window foam is a fireblock.  Each of the other manufactures (Fomo and Convenience Products) have the same approval for all of their products.

We sea; new home in over 500 inspection jurisdictions in 4 states and only 2 of them won't lets us use single part foam as a fireblock.


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