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Proper blocking to prep balloon walls for cellulose?


e_doe19's Avatar
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02-26-10, 06:50 PM   #1  
Proper blocking to prep balloon walls for cellulose?

Hello, I have a 1908 construction, balloon walls, drafts, and DUST. I added cellulose to the attic and am planning to do dense-pack cellulose into the walls in the next 12 months. I haven't done a blower test yet because it's pretty obvious that I'm leaking air, everywhere. I figured I'd air seal and block around the rim joist in the basement, air seal in the attic, then see how I'm doing on my air flow before progressing. I would have air sealed before the attic cellulose, but I got led astray by the "let it breathe" mantra. So I'll be moving a lot of insulation around when I get up to the attic to work. Sigh.

I have done some research and it seems that I should use rigid foam where the walls open into the basement, to close the cavity for air seal and for later installation of cellulose. However, if blocking the balloon walls with rigid foam and sealing the seams with expanding can foam, will that be strong enough to hold the pressure of dense-packing the cellulose later? I'm a little worried that when I get around to the cellulose, it is going to blow those little blocks of rigid right off. Is there an alternative way to block those cavities at the rim joist level that will provide enough resistance against the force of the dense-pack? Is the foam way stronger than I'm giving it credit for? Also, should I be using the fire-rated expanding foam here? I just saw that kind of can foam and now I'm not sure if that's what I should use instead of the basic kind.

Thanks for any input. ~Erin

 
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02-27-10, 04:53 AM   #2  
Hi Erin, we have some code people here and hopefully they will jump in, but to add to your question, would modifying the balloon construction require real fire breaks. That's one of the reasons modern construction went away from the balloon style. I know any air sealing we do must use the fire rated for vertical paths and that is a retrofit, so it seems logical that the upgrade to the outside walls would require the same. If you don't get an answer here, check your local code people.

As for foam blocks holding up to dense pack, I would install something substantial just to be safe.

Bud

 
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02-27-10, 03:58 PM   #3  
Thank you.

Thanks, Bud. I found this image online this morning.
Erin M. Donohoe's Photos | Facebook

It makes sense to me that the rigid could support the pressure of the dense-pack this way, and it seems do-able to me. I didn't know rock wool was considered a fire block. Now I'm thinking maybe I'll try the rock wool with the rigid and go ahead and use the fire-rated foam for sealing. But I would love it if any code people would chime in.

 
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02-27-10, 05:29 PM   #4  
Gary just posted to this thread. You could PM him to get an opinion.
http://forum.doityourself.com/baseme...plication.html

Bud

 
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02-27-10, 05:53 PM   #5  
Posted By: e_doe19 I would have air sealed before the attic cellulose, but I got led astray by the "let it breathe" mantra. So I'll be moving a lot of insulation around when I get up to the attic to work. Sigh.
Are you talking about keeping insulation from blocking the roof vents? If so I would use the special vent baffles that are made for things like providing a boarder for cellulose or vermicullite.

Is the foam way stronger than I'm giving it credit for? Also, should I be using the fire-rated expanding foam here? I just saw that kind of can foam and now I'm not sure if that's what I should use instead of the basic kind.
Yes it's very strong but I wonder if it will eat at the foam before it cures? The adhesion is even more aggresive! But even if it doesn't blow out right away, would you be comfortable with that approach?

I have a half sunk basement with balloon framing also.
I had dry wall in the basement so when I got the insulation blown in, the basement walls got insulated as well. (water pipes) With only one hole required for each void!
So no fire stops but ...drywall.

Or yeah you could toenail some blocking. Instant fire blocks as well. Whenver I have to fasten something like this that requires an awkward approach angle, or limited swing, I always drill an angled pilot hole through each block and start the nails peeking out. Then it can be tacked in position with the first swing. Helps accomplish a neat secure job too.

 
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02-27-10, 06:06 PM   #6  
Posted By: mickblock Yes it's very strong but I wonder if it will eat at the foam before it cures? The adhesion is even more aggresive! But even if it doesn't blow out right away, would you be comfortable with that approach?
Just an FYI - Spray (expanding) foam has no effect on rigid foam.


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03-02-10, 04:00 PM   #7  
thanks again

Thanks to all of you for responding. Yeah, I considered toenailing some 2x4s in there but honestly, toenailing tries my patience even when conditions are optimal, and trying to do it above my head on a ladder into the little dark crevices, half of which are blocked by cross bracing would probably make me a bit like this little guy And I'll go ahead and admit that I'm still too intimidated by the pneumatic framing nailer to go that route.

I'll check with Gary and see if he agrees that the rock wool approach is acceptable.

Oh and, yes, I am going to install those baffles in the attic eaves. It just stinks that I learned after the blown cellulose went in that I really should air seal and properly ventilate. I had been advised to just leave the attic unsealed and let it breathe on its own but everything I've read from the insulation and weatherization books and things online indicate that I really should seal it up and then ventilate properly. Thanks again, guys. I love this forum!

 
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