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Old 02-02-2013, 10:11 PM   #1
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fiberglass, okay in walls but not rim joist?


Everyone says to spray foam or use XPS+great stuff on rim joists. But if you use great stuff around the edges, then install fiberglass, why isn't that good enough?

If fiberglass is acceptable (maybe not preferred) in walls with drywall (finished living areas), why not in a rim joist?

My only guess would be airflow...drywall would help stop airflow, but if you used great stuff in the rim joist edges, doesn't that stop airflow too?

BTW...I have fiberglass only right now in rim joists (no air sealing)...

Thanks.

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Old 02-03-2013, 12:25 AM   #2
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fiberglass, okay in walls but not rim joist?


Fiberglass is easy install and now comes pre bagged and works great in attic roof beams. Spray spam works great behind drywall because it creates a tight seal to block out airspace thus helping minimize airflow exiting up the wall and out of the roof. It's easy to do and I believe you can rent a machine that has a nozzle and spray on between your studs in the frame. The rim joist I think will be fine with spray foam and fiberglass in between but you may increase your overall cost and not sure how much more efficient your insulation overall would be.

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Old 02-03-2013, 02:12 AM   #3
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fiberglass, okay in walls but not rim joist?


Batts are quick and fast to install, especially if you do not want to make the envelope totally air tight. Depending on where you live, the minimum is R-13 in the bays between the joists & along the Sill. If you have the time, either use Tiger Foam, or use XPS cut to fit, and seal with either Window & Door foam in the can, or caulk the seams.

Also remember that windows are a big culprit of allowing convection cooling & heating through, and along with if they are older casements, they may not seal 100%, so they can end up allowing free air around them when closed.
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Old 02-03-2013, 02:47 PM   #4
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fiberglass, okay in walls but not rim joist?


The air sealing is the important part. After that, you can insulate with whatever you want... FG is fine. Oftentimes we'll foam the sill boxes and then reinstall the old FG if it is still in decent shape.
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Old 02-03-2013, 09:28 PM   #5
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fiberglass, okay in walls but not rim joist?


http://www.finehomebuilding.com/PDF/Free/021189072.pdf

http://www.buildingscience.com/docum...l_seal_rev.pdf

http://www.buildingfoundation.umn.ed...oistphenom.htm

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Old 02-04-2013, 12:02 AM   #6
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fiberglass, okay in walls but not rim joist?


thanks for the feedback.

Any issues using 1" XPS, seal with great stuff, then put fiberglass (R19) back on top? Main question being 1" XPS v/s 2" XPS...obviously the R value is different, but so too is the price. Any vapor issues using 1"?

I'm in east Tn.
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Old 02-04-2013, 09:37 AM   #7
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fiberglass, okay in walls but not rim joist?


Not really. The 1" will act as a Vapor barrier, just like the 2".
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Old 02-04-2013, 11:42 AM   #8
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fiberglass, okay in walls but not rim joist?


What about putting fiberglass in the rim joists and then sealing it with XPS that goes up to the subfloor? Sealed all the way around each joist and the subfloor above.
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Old 02-04-2013, 12:11 PM   #9
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fiberglass, okay in walls but not rim joist?


1" XPS (1.1 perm.) will stop air-flow but not moisture vapor from passing through. 2" XPS (0.55 perm.) will allow vapor 1/2 as fast. Biggest difference is the temperature (R-value) of the foamboard on the cavity side (being in direct contact with cold rim) insulating to prevent/raise the dew-point there. "First condensing surface"; http://www.buildingscience.com/docum...bout-diffusion

In your location, using J., F., and Dec.average low temps of 30*F, at 70* room temp; R-5 XPS = 41* (inside face) with 35% RH. Use R-10 = 47* or 44% Relative Humidity (dew-point).

IL is colder; eg. Lincoln; 3 months= 18*F at R-5 = 25% RH. R-10 = 33% RH. If you add in the extra cavity batt insulation (6"/R-19) because the rim is 6" away from the room due to the inside wall from the concrete wall; 22% RH with same R-5 foamboard.
Sealing the rim; Post #8; Insulating Rim Joist - caulk or spray foam around edges?

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Old 02-04-2013, 12:34 PM   #10
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fiberglass, okay in walls but not rim joist?


Aside from the technical aspects, I am not a fan of "sealing in" fiberglass like that as rodents, etc should be a consideration as well. That would create a perfect little nesting spot for them.
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Old 02-04-2013, 02:48 PM   #11
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fiberglass, okay in walls but not rim joist?


Thermal insulation, whether fiber glass or cellulose, when installed in side walls has very little to do with air infiltration. The U.S. Department of Energy estimates that up to 40% of a home's heat loss can be from air infiltration. But only 4% of the total loss from air infiltration occurs through side wall cavities, and most of that is around electrical outlets, a problem easily solved with inexpensive, easy-to-install electrical insulated outlet gaskets.

Air infiltration generally occurs in the areas of a home that are not insulated, such as around windows, doors, fireplaces, HVAC ductwork and perimeter joints. It can, and should, be controlled with the use of housewrap, proper caulking, and sealing of band joists, sill plates, header plates, and insulation around doors, windows, electrical outlets and other openings.

Recently there have been claims that some insulation products are better because they reduce air infiltration. Numerous research studies have been conducted to investigate this issue, among them studies by the National Association of Home Builders Research Center, researchers at Penn State University, and by a St. Louis utility. The research consistently demonstrated that if a wall cavity has been properly constructed, using drywall, sheathing and caulking, very little air will flow through the wall cavity regardless of the type of insulation.
you can use fiberglass or what ever type of insulation you want to. Just remember how air infiltration works.
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Old 02-04-2013, 05:36 PM   #12
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fiberglass, okay in walls but not rim joist?


nailbags, the question is regarding the rim joist/sill boxes, not the wall cavities.
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Old 02-04-2013, 06:26 PM   #13
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fiberglass, okay in walls but not rim joist?


Quote:
Originally Posted by HomeSealed View Post
The air sealing is the important part. After that, you can insulate with whatever you want... FG is fine. Oftentimes we'll foam the sill boxes and then reinstall the old FG if it is still in decent shape.

@HomeSealed - When you say "foam the sill boxes", do you mean using a 2 part foam to spray the entire cavity, or 1 part foam in a can to do the edges/corners only?
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Old 02-04-2013, 07:01 PM   #14
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fiberglass, okay in walls but not rim joist?


Thge easiest solution is to use 2" XPS (roughly cut or scraps) immediately after foaming the corners around the joist/rim joist area and let the foam seal the cavity and adhere the foam in place. This put the dew point inside the XPS with no condensation. If you are fussy and want, you can cover the top of the foundation wall to eliminate the lateral transmission of the wall to make you feel better.

Then just stuff in some some batts (Roxul, if possible) as thick as you want.

Do what you want later with the surface of the foundation and create space wasting cavities.

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Old 02-05-2013, 02:27 AM   #15
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fiberglass, okay in walls but not rim joist?


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Originally Posted by 88bomber View Post
Everyone says to spray foam or use XPS+great stuff on rim joists. But if you use great stuff around the edges, then install fiberglass, why isn't that good enough?

If fiberglass is acceptable (maybe not preferred) in walls with drywall (finished living areas), why not in a rim joist?

My only guess would be airflow...drywall would help stop airflow, but if you used great stuff in the rim joist edges, doesn't that stop airflow too?

BTW...I have fiberglass only right now in rim joists (no air sealing)...

Thanks.
well you answered your own question I would just put great stuff on all four sides stuff the fiber glass back in. simple you will have people that champion foam and other products but for the DYI it is simple great stuff and fiberglass batts.

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