# fiberglass, okay in walls but not rim joist?



## 88bomber (Jun 6, 2012)

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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## AppealingSpaces (Feb 2, 2013)

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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## gregzoll (Dec 25, 2006)

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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## HomeSealed (Jan 3, 2008)

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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## Gary in WA (Mar 11, 2009)

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

http://www.buildingscience.com/docu...joist/files/bscinfo_408_critical_seal_rev.pdf

http://www.buildingfoundation.umn.edu/RimJoist/rimjoistphenom.htm

Gary


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## 88bomber (Jun 6, 2012)

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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## gregzoll (Dec 25, 2006)

Not really. The 1" will act as a Vapor barrier, just like the 2".


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## mikegp (Jul 17, 2011)

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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## Gary in WA (Mar 11, 2009)

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/documents/insights/bsi-049-confusion-about-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; http://www.diychatroom.com/f103/insulating-rim-joist-caulk-spray-foam-around-edges-168849/

Gary


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## HomeSealed (Jan 3, 2008)

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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## Nailbags (Feb 1, 2012)

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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## HomeSealed (Jan 3, 2008)

nailbags, the question is regarding the rim joist/sill boxes, not the wall cavities.


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## 88bomber (Jun 6, 2012)

HomeSealed said:


> 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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## concretemasonry (Oct 10, 2006)

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.

Dick


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## Nailbags (Feb 1, 2012)

88bomber said:


> 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?
> 
> ...


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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## Gary in WA (Mar 11, 2009)

Can't go wrong there.... MN tested f.g. with paper-facing vapor retarder on rims: http://www.buildingfoundation.umn.edu/RimJoist/default.htm#@D

Their conclusion: 
"The data thus clearly demonstrates that it is not prudent from a condensation management perspective to fill the rim joists with batt insulation, regardless of whether they are covered with a vapor retarder or not. Free circulation of interior drying air in the rim joist cavity, particularly during the summer, would inhibit condensation as well as prevent moisture accumulation. Exterior insulation is effective in controlling winter rim joist condensation but tends to exacerbate summer condensation conditions, particularly in the presence of an interior vapor retarder. With the absence of interior batt insulation and an interior vapor retarder, the negative consequences of exterior rim joist insulation are mitigated. However, especially in retrofit situations, the amount of exterior insulation that can be installed (if any) may not be sufficient to provide adequate thermal insulation (at least R-10). Thus in these cases, the alternative is to install rigid insulation (such as foil-faced polyisocyanurate) flush against the interior side of the sheathing. This still allows air drying of the rim joist cavity but reduces the potential for interior summer condensation by decreasing the condensation surface to the bare minimum of the interstices of the insulation/ sheathing interface. In the winter, the insulation is protected from interior-sourced water vapor by the foil-facing. " From: http://www.buildingfoundation.umn.edu/RimJoist/rimjoistphenom.htm

A stud cavity, x4" or x6" deep is a lot easier to air-seal (drywall) than a 25' -30' x 16" or 24" joist cavity, open to interior air. Vapor diffusion through the rim joist is also an additional problem; the low R-value of wood (R-1.25 per inch) with only R--2, the dew-point on the inside face is very close to the outdoor temperature, unlike pictured; with fiberglass insulation enclosed by the drywall (air barrier) and at R- 3.7 per inch: http://www.buildingscience.com/docu...ld-weather-condensation-using-insulation/view

Gary


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## 88bomber (Jun 6, 2012)

All the responses are appreciated, but I'm a little confused with some of them. Here's what I've gathered about rim joist insulation...


no insulation = bad
fiberglass (faced or unfaced) = bad
seal cracks/corners/joints with GS + fiberglass = acceptable
1" XPS with GS + fiberglass = good
2" XPS with GS = very good
2 part spray foam = best

Where am I going wrong?


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## Nailbags (Feb 1, 2012)

here is the crucks of it all R value period bottom line you want R value. air infultration is combatted with using Good stuff cuts the air flow the zone your in requires a R-30 for crawl space 2" of XPS is not going to give you that you need the R-30 equivlant you can put r30 batts in there after the xps and GS and you got it covered. what a lot of people want to advoid is vermin making a nest in there. that is why a lot of people suggest using foam. as long as you meet the R-30 the inspector could careless what type of insulation you used. I would if it were me hire a person to spray foam in and call it good. because how many times doe you really go in to your crawl space? I would foam it and for get it.


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## Gary in WA (Mar 11, 2009)

"seal cracks/corners/joints with GS + fiberglass = acceptable"-------------------- no,

the foamboard warms the cavity side/wood rim to *prevent condensation* (and air-seal), fill with fiberglass after the foamboard. Post 16, last link. Fiberglass does nothing to warm the surface of the wood rim joist to prevent condensation of basement air there. Rigid f.b.'s quality of "insulating" raises the temperature so there is no dew-point or condensation; http://www.buildingscience.com/documents/reports/rr-0412-insulations-sheathings-and-vapor-retarders Air (infiltrating/exfiltrating) can go right through f.g. and leave its moisture on the cold rim to rot. 

Gary


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## HomeSealed (Jan 3, 2008)

as Gary said, it is not ONLY the air leakage that you want to address there, but the warming of the interior surface of the rim joist itself to prevent condensation (or sealing it from interior moisture). That is why a coating of foam or XPS board DIRECTLY sealed against the wood is best.... Think of it like your windows: Those thermal shades do a nice job of keeping your room warmer, yet every winter morning you wake up to windows covered in water or frost because you have just isolated them from the warm interior air, but not necessarily from the moisture. FG batting acts the same as the blinds in this situation. The difference is, you don't want to seal over your windows with XPS board or foam, but here you can :thumbsup:.... Is that making sense?


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## 88bomber (Jun 6, 2012)

Yes it does make sense. My only question is with the statement below in this link http://www.buildingscience.com/docu...-310-vapor-control-layer-recommendations/#Map versus the most common advice I hear.

"No interior vapor control required on the interior side of framed walls in climate zones 1, 2, 3, 4a, or 4b. In hot, humid climates, a Class I or II vapor control layer on the interior of the framing can, and often does, cause premature building enclosure failure due to inward moisture drive condensation (see RR-9302: Humidity Control in the Humid South). *BSC recommends avoiding Class I or II vapor control layer on the interior in these zones*, or any material that acts inadvertently like a Class I or II vapor control layer such as reflective foil insulations, vinyl wall coverings, glass mirrors and epoxy paints."

I am in zone 4a. So, if I follow that recommendation to avoid class 1 or 2 vapor barriers, then doesn't that mean I would be better to use 1" XPS (1.1 perms - class 3 barrier) instead of anything thicker? (2" is the most common recommendation I hear, but it is a class 2 barrier - .5 perms)

It sounds to me like 1" xps + GS + unfaced FG is the *best *option for those in zones 1, 2, 3, 4a, or 4b. Does that sound right?

Thanks for all the replies.


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## operagost (Jan 8, 2010)

The problem I have with foaming in the ENTIRE cavity is that it is now hidden from inspection. It could be rotting or infested and you wouldn't know. Foaming up the whole thing is quicker and easier than just caulking or foaming the edges to air-seal and installing foam board, but I'd be worried that something is happening under the wood and it would be much harder to check.


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## Gary in WA (Mar 11, 2009)

" *BSC recommends avoiding Class I or II vapor control layer on the interior"---------*interior, next to drywall, not toward the exterior.With 1", you will be safe at 35% RH and less as per Post #9.

Gary


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