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Old 07-21-2016, 01:07 AM   #1
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Sealing and insulating rim joists with fire blocking


I'm in a 1950s house that used "fire blocking" or "fire stop" in the rim joists.

This was accomplished by filling the rim joist area with rocks and sand, and then using an outer-board (on the cellar side) to hold all that material against the rim joist.

From the cellar, it looks like this:



That's not the rim joist, that's the blocking board held in place by two nails at the bottom. As you can see, an attempt was made to do some sealing (with expanding foam), but some of the outer boards are cracking/splitting.

My plan is to:

1. Picture frame the blocking boards with Silicone sealant.

2. Optionally add 2" XPS foam over that.

3. Seal the baseboards in the rooms above.

The main issue is that the real rim joists can't be accessed, so they will still leak air which will move upward through the wall cavity to the first floor. Sealing the first floor baseboards are about all I can do.

Is there anything else one might recommend in this situation?
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Old 07-21-2016, 06:07 AM   #2
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Re: Sealing and insulating rim joists with fire blocking


Where is the home located?

Most of the applications where homes have condensation and moisture issues are a result of warm, moisture laden air, moving to an exterior cold rim and condensing.

That being said, if you establish a tight barrier and insulate it from the inside, you won't have that issue to contend with. You are correct that air will still move up, but that isn't the most problematic issue with many homes. Again, just playing the averages here.

Any pictures of the outside? Perhaps you can do some air sealing on the exterior?
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Old 07-21-2016, 01:33 PM   #3
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Re: Sealing and insulating rim joists with fire blocking


Home is near Boston, MA. So humid summer and cold winters.

Here's a view of the outside of the same location. I have vinyl siding, with wooden shingles underneath.

I tried to (quickly) peek under the vinyl siding, but couldn't tell if the sill plate is exposed, or if the wooden shingles cover it up...

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Old 07-21-2016, 01:35 PM   #4
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Re: Sealing and insulating rim joists with fire blocking


Judging by the water marks on the block/joists, I'd fix the problem rather than just cover the block with foam. You'd still get wet joists, and still get excess moist/air rising up in the wall cavity, especially if balloon framed. IMO, remove the blocks, remove the dirt, canned foam the holes you can't cover with some foam board to the floor sheathing. If an outer wood rim (similar to local framing of that era), foamboard that with foil-faced, then Roxul in front and drywall if local AHJ requires it.

Gary
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Old 07-21-2016, 05:21 PM   #5
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Re: Sealing and insulating rim joists with fire blocking


I'm not sure where/when the water came from, it might have been a leaking window or ice dam drippings from above. But, there are currently no water issues.

Removing the wooden blocks and emptying all the dirt and rocks from every joist bay is a big and messy project I'd rather not undertake if at all possible, especially since the fire-blocking is there for a reason.

Is there a way to easily tell if the house is balloon-framed? I suppose I'd see studs directly behind the blocking, no?

Last edited by Alketi; 07-21-2016 at 05:23 PM.
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Old 07-21-2016, 06:06 PM   #6
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Re: Sealing and insulating rim joists with fire blocking


Check in the attic at the gable end wall for studs extending from below to end rafter, side-nailed/notched, usually with no top plate; google- "balloon frame" for many pics/diagrams. Here is an interesting one from my library; http://ncpe.us/wp-content/uploads/2012/07/Obrien.pdf

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PS; http://www.wag-aic.org/1999/WAG_99_baker.pdf
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Last edited by Gary in WA; 07-21-2016 at 06:08 PM. Reason: added link
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Old 07-21-2016, 06:29 PM   #7
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Re: Sealing and insulating rim joists with fire blocking


Quote:
Originally Posted by Alketi View Post
Home is near Boston, MA. So humid summer and cold winters.

Here's a view of the outside of the same location. I have vinyl siding, with wooden shingles underneath.

I tried to (quickly) peek under the vinyl siding, but couldn't tell if the sill plate is exposed, or if the wooden shingles cover it up...
Anything between the wood shingles and vinyl?

Balloon framing is a concern from the fire-stop standpoint. Its less and less common after WW2.
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Old 07-21-2016, 06:41 PM   #8
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Re: Sealing and insulating rim joists with fire blocking


No reason to remove the dirt, as I said; IMO, lol. That's a problem after retiring, body says "ok", mind keeps going...

The " baker"(PS) link I gave shows the fire-stop required in attic and between floors, as you have it already on the basement/ground floor.

The shingles are your old siding, I hope only a vinyl foam underlayment or MAYBE a builders paper between it and shingles... what else would be there... I removed the asbestos siding from my 1920 craftsman to expose wood shingle/lap siding below. You may see a band joist trim board or the wood shingle go right to the bottom of the rim joist.

Gary
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Last edited by Gary in WA; 07-21-2016 at 06:44 PM.
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Old 07-21-2016, 07:45 PM   #9
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Re: Sealing and insulating rim joists with fire blocking


Balloon framing it is!

There are some areas where the fireblocking needed to be removed in order to get piping out the rim joist, and low and behold, there are studs in the cavity.




BTW, my attic has blocking boards between the studs, which I presume are nailed in place.

Gary and Windows, there's nothing between my vinyl siding and old wood shingles. They just nailed the rows of vinyl directly on top.

I'm imagining if there's any room between the bottom vinyl starter strip and the old shingles, that I could take a skill saw, set the depth, and cut back the shingles to (hopefully) reveal the sill plate. But, I'm not counting on being that lucky. )

So, given balloon framing, rocks & dirt fire blocking, and a Massachusetts home, would you still recommend sealing the blocking plates and then XPS or foil-faced insulation over the top?
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Old 07-24-2016, 11:18 AM   #10
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Re: Sealing and insulating rim joists with fire blocking


Just a quick follow-up -- some additional searching uncovered a post by @bubbler , who has a virtually identical setup for the same Cape-style house, built in the same era, from a nearby (or same) town. Looks like it was standard fire-blocking practice in the 50s for balloon-framed houses in our area.

http://www.diychatroom.com/f103/insu...uction-117584/

I've reached out to @bubbler to see what he ended up doing in his situation, but haven't heard back from him yet.
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Old 07-29-2016, 03:07 PM   #11
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Re: Sealing and insulating rim joists with fire blocking


Since the wood expands/contracts with the seasonal/room RH creating air leaks and the dirt/rocks are lousy thermal barriers; yes, insulate/air seal.

Gary
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Old 01-31-2019, 11:16 AM   #12
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Re: Sealing and insulating rim joists with fire blocking


Quote:
Originally Posted by Alketi View Post
Just a quick follow-up -- some additional searching uncovered a post by @bubbler , who has a virtually identical setup for the same Cape-style house, built in the same era, from a nearby (or same) town. Looks like it was standard fire-blocking practice in the 50s for balloon-framed houses in our area.

https://www.diychatroom.com/f103/ins...uction-117584/

I've reached out to @bubbler to see what he ended up doing in his situation, but haven't heard back from him yet.
@Alketi - this thread is quite old but I'm facing the same issue (you might as well have been taking pictures in my house!) and since your last activity date is pretty recent, I'm hoping you could post an update on whether you ever got a response from @bubbler , if you ended up doing anything to seal the area, and how it fared in terms of warmth/moisture issues. I'm also wondering if you have any insulation in those walls and if so, any moisture issues with it.

Everything I read converges on stuffing Roxul up there for fireblock/insulation, then doing the standard rim-joist job of 2in of press-fit rigid foam w/ canned spray foam and more Roxul (or just spray foam kit), but I haven't seen reports on how this fares long-term in 'older' houses like this (I guess I can't call them 'old' because we do have late 1800s houses around here...)
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Old 02-11-2019, 11:44 AM   #13
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Re: Sealing and insulating rim joists with fire blocking


@whiskers , sorry I meant to reply but got sidetracked. No I never heard back from @bubbler , and I've been busy upgrading other parts of the house so haven't had a chance to do any work in this area.

You're saying 2" rigid foam first against the pre-existing fireblocking, and then Roxul on top of that, right? That would make sense as some people had claimed that exposed foam is a no-no in the event of a fire.

I've got panel walls and those old nailed together fiber-board ceiling tiles that I first need to remove to even get access to these areas. I still want to do it as I've got lots of bugs that seem to be getting in and I'm sure its from gaps in the rim joist.

BTW, I recently noticed that to run an outdoor water spigot way-back-when they avoided the rim joist cavity all together and drilled through the 4x6 sill plate instead. Yikes.

Let us know what you do!
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Old 02-14-2019, 11:54 AM   #14
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Re: Sealing and insulating rim joists with fire blocking


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Originally Posted by Alketi View Post
@whiskers , sorry I meant to reply but got sidetracked. No I never heard back from @bubbler , and I've been busy upgrading other parts of the house so haven't had a chance to do any work in this area.

You're saying 2" rigid foam first against the pre-existing fireblocking, and then Roxul on top of that, right? That would make sense as some people had claimed that exposed foam is a no-no in the event of a fire.

I've got panel walls and those old nailed together fiber-board ceiling tiles that I first need to remove to even get access to these areas. I still want to do it as I've got lots of bugs that seem to be getting in and I'm sure its from gaps in the rim joist.

BTW, I recently noticed that to run an outdoor water spigot way-back-when they avoided the rim joist cavity all together and drilled through the 4x6 sill plate instead. Yikes.

Let us know what you do!
Ha, if you start reading JLC and GreenBuildingAdvisor, you will start losing your hair seeing how houses were constructed back then in our area (I'm within the 95 belt in Boston). But they didn't know any better, I guess. Though I'm pissed but not surprised that the contractors doing the renovation on our place (waaay before we bought it) didn't care about insulating here.

I think I'm going to reach out to the town building inspector as well to get a pro opinion on what's right/up to code.
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Old 02-14-2019, 02:07 PM   #15
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Re: Sealing and insulating rim joists with fire blocking


Quote:
I think I'm going to reach out to the town building inspector as well to get a pro opinion on what's right/up to code.
Great. Please post back if you get a response.
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