# Insulate Rim Joists with Canned Spray Foam and Roxul



## Windows on Wash

You do need some vapor permeance control at those locations. General rule of thumb is that 2" of CC SPF will provide you the control layer you need as well as a skinned rigid foam. 

I wouldn't bother with the fire foam as it is not as user friendly and I don't think seals as well. 

Rigid foam that is cut and cobbled together can be sealed with a material compliant caulking if you like and then covered with mineral wood (Roxul) to fit between the joists (cut to fit). That would give you the added insulation and thermal barrier you require. 

Codes have evolved on this and as long as you are under 3.25", you do not require a thermal or ignition layer over it. 

http://publicecodes.cyberregs.com/st/ca/st/b400v10/st_ca_st_b400v10_3_sec017_par015.htm

I am not a fan of leaving it exposed either so Roxul is great. 

You need to address those walls before winter too.


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## sonnaps

Windows on Wash said:


> You do need some vapor permeance control at those locations. General rule of thumb is that 2" of CC SPF will provide you the control layer you need as well as a skinned rigid foam.
> 
> I wouldn't bother with the fire foam as it is not as user friendly and I don't think seals as well.
> 
> Rigid foam that is cut and cobbled together can be sealed with a material compliant caulking if you like and then covered with mineral wood (Roxul) to fit between the joists (cut to fit). That would give you the added insulation and thermal barrier you require.
> 
> Codes have evolved on this and as long as you are under 3.25", you do not require a thermal or ignition layer over it.
> 
> http://publicecodes.cyberregs.com/st/ca/st/b400v10/st_ca_st_b400v10_3_sec017_par015.htm
> 
> I am not a fan of leaving it exposed either so Roxul is great.
> 
> You need to address those walls before winter too.



Yeah, this house has sat uninsulated for 2 years now so one more winter won't matter at this point lol. I had an illness in the family so it just sat untouched for a very long time.

What type of caulk would you recommend? It's gonna be tough maneuvering the caulk gun in there at least on the sides of the house where the joists run parallel to the outside wall.

I should probably caulk all the gaps first for a good air seal and then do the pieces of rigid foam and use canned spray foam around the perimeter to seal it and hold it in, right? Just use regular Great Stuff canned foam, not the fireblock version?


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## Windows on Wash

I would put some bedding bead sealant in the box before sticking in the foam. That will give it good adhesion and seal. 

You can hard plastic extension nozzle for the caulking gun as well. 

I would use window and door specific (i.e. low expansion foam for the application of the sealing). Be sure to get a closed cell variant of it. 

Spend the money and get a pro applicators gun with the extension straws. It will pay for itself a million times over. 

Read the label on the caulking. Just make sure it is foam compliant and a good latex acrylic should be fine.


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## BarryP

sonnaps said:


> Are there any specific code requirements about vapor barrier here or that spray foam or rigid insulation are the only options? I'm in zone 4 climate, in Monmouth County in NJ.


Check with your local building official.

Typically, your approach is fine. Don't overthink this. Make sure there is no flammable vapor barrier exposed. Why not just use unfaced fiberglass batt?


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## Gary in WA

IMO, use foil-faced to stop summertime condensation at the foamboard/wood rim interface. You are in a mixed-humid, sub-tropical climate there, the foil (vapor barrier) will stop moisture from coming through to wet the Roxul/framing joists in summer. No other cavity insulation, unless meeting code; add minimum fiberglass to meet code for wood frame wall- R-4 or so. http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=New Jersey

The sealant/caulking is for the joist movement as the outside relative humidity effects the wood framing, *Post #8; *http://www.diychatroom.com/f103/insulating-rim-joist-caulk-spray-foam-around-edges-168849/

Gary


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## Olcrazy1

I would have the rim joist covered with spray foam, not the diy stuff but hire someone to come in and spray. Most air leaks come through those and sealing them makes a huge difference. I'm sure someone will correct me if I'm wrong but unless you are framing and sealing in the walls and finishing the basement leaving the walls wide open like they are you don't need a vapor barrier. Barrier is only for the closed cavity if you plan k. Finishing the basement. Spray from is a vapor barrier also if you go that route


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## sonnaps

Gary in WA said:


> IMO, use foil-faced to stop summertime condensation at the foamboard/wood rim interface. You are in a mixed-humid, sub-tropical climate there, the foil (vapor barrier) will stop moisture from coming through to wet the Roxul/framing joists in summer. No other cavity insulation, unless meeting code; add minimum fiberglass to meet code for wood frame wall- R-4 or so. http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=New Jersey
> 
> The sealant/caulking is for the joist movement as the outside relative humidity effects the wood framing, *Post #8; *http://www.diychatroom.com/f103/insulating-rim-joist-caulk-spray-foam-around-edges-168849/
> 
> Gary


I read that post of yours the other day, post #8... in there you have a link http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/ which was what I thought was my reassurance that my idea of just Roxul batt in the cavity up against the rim joist would be okay... plus the caulking though to seal any air gaps. There was a pic in there that I thought perfectly described what I wanted to do. My "basement" will be finished and is above grade. My sill gasket (if that's what you would call it) as it is now is metal flashing. I think it's more of a "termite flashing" from what I read on it but does keep the wood sills from touching the concrete block below. The diagram refers to cellulose or spray applied foam... is the cellulose comparable to roxul? It's probably a wet sprayed type, huh? Wikipedia has a description of it, first time I ever saw that aside from the blown in type used in attics.

Do things change if I plan on adding 1-2" of rigid insulation on the outside of the house when I get all the siding redone?

.


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## Windows on Wash

No.

Insulated it as it needs to be now. 

If you add rigid foam to the exterior, that should not negatively impact your insulation approach as of right now.


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## Gary in WA

_Déjà vu... _ Yes, if you will be changin_g _the exterior perm rating/dew point with the addition of foamboard, you wouldn't need foil-faced FB inside. Or any interior FB for that matter, cavity fill with unfaced batts of your choice. Or cellulose, etc.

The reason for ff FB (rather than *unfaced *FB) is to prevent moisture at FB/rim joint- when not using an interior vapor barrier or exterior FB; 
"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. "

Gary


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## sonnaps

Gary in WA said:


> _Déjà vu... _ Yes, if you will be changin_g _the exterior perm rating/dew point with the addition of foamboard, you wouldn't need foil-faced FB inside. Or any interior FB for that matter, cavity fill with unfaced batts of your choice. Or cellulose, etc.
> 
> The reason for ff FB (rather than *unfaced *FB) is to prevent moisture at FB/rim joint- when not using an interior vapor barrier or exterior FB;
> "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. "
> 
> Gary



Thanks Gary... yeah I feel like I'm running myself in circles.


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## Windows on Wash

I'll disagree with Gary here. 

You are basing a potential decision on what you may add down the road...don't do it. Plans change and so therefor do the insulation requirements. Putting up the foam board now makes sure that you don't have any condensation issues. Putting up batt insulation means you likely will have condensation issues. 

I look at the ribbon board as an assembly at that point. If you have foam on the exterior (sealed to the board), the board (obviously sealed), and sealed foam board to the interior, I view that as a monolithic piece at that point. 

There doesn't need to be any drying through it in any direction and you aren't getting any room side or exterior moisture at that point that would need to dry to either side regardless. 

I am sure the perm rating will say "X", but a skinned XPS on the exterior isn't letting any tangible amount of moisture through it to dry into the wood and then be stopped by the foil facing on the ISO board. 

If you are concerned, run an XPS board to the interior and they will have the same perm ratings on both sides. 

The perm ratings obsession is a bit of wasted energy in cases like these if you ask me. I have pulled apart homes with open cell foam (supposedly a big no-no because of the perm rating and air movement) in the band boards that are 15 years old with *ZERO* signs of moisture whatsoever. These are in unfinished basements with standard vinyl siding (i.e. cold band boards on the inside of the home) and humidifiers in some cases. 

Air is the enemy in these cases in my opinion. 

If you have a properly attached exterior cladding and no moisture infiltration into the band-board, you aren't going to have any issues.


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## Gary in WA

Getting confusing now. 

"Do things change if I plan on adding 1-2" of rigid insulation on the outside of the house when I get all the siding redone?"-

yes, things do change IF you add exterior FB, at least with my answer as I stated above. *IF* exterior FB, you don't want/need interior FB, faced or unfaced. Just add cavity insulation without a vapor retarder/barrier. That is how it changes. 
For the time between now and when you do add ext. FB, use foil-faced XPS because it stops the water vapor going from the rim (summer- warm) to the cavity (cold- AC controlled). If you used unfaced FB on the rim, moisture would then condense as the FB is not permitting vapor through, giving it room to change (condense) from a vapor to a liquid right there on the inside of the rim/FB, as the article/test showed with multiple kinds of insulation/vapor control types. IF you used ff XPS OR even a poly-wrapped unfaced XPS, it would stop the vapor from condensing completely at the rim interface. With 2" of FB exterior later, and ff XPS on the other side of rim, no problem, I was stipulating to use foil faced now rather than unfaced.

Batt alone without the exterior FB wouldn't work, it's in blue color in picture, behind the stucco.

Gary


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## sonnaps

Yes confusing....

Would it be safe to say as Windows on Wash said, better to be safe than sorry right now. Get the interior spray foamed and then problem solved no matter what happens on the outside later? If no insulation get adde outside, I'm safe. If it does like I plan, I'm still safe.... the rim band will act as a monolithic assembly and no need for drying out either way since moisture will never get to it from either side.

Although more costly, maybe spray foam is just easier install-wise than cobbling pieces of rigid insualtion and provides the best protection against air infiltration. 

It's just sooooo permanent. If something needs to be addressed in that area that stuff is never coming off. I have to think on this... all the pros and cons.


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## Gary in WA

*Use foil-faced *XPS with foil towards rim to STOP ant condensation in summer. *Do that now* and you will be safe. Adding exterior XPS won't hurt by having the foil in the middle, later.

With your humid summer, no facing (foil/poly) will give you condensation to keep the rim wet the summer, and in to winter, as the test results showed. Spray foam is easier, just be sure to add the much needed vapor barrier (poly/foil) first. 

Gary


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## MLMIB

Gary in WA said:


> Spray foam is easier, just be sure to add the much needed vapor barrier (poly/foil) first.
> 
> Gary


I thought that if you spray foam that the spray foam acts as the vapor barrier?


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## Olcrazy1

That's what I read and also what spray foam guy who did my basement said. Closed cell spray foam is also a vapor barrier, no need to add another.


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## Gary in WA

Yes, if thick enough on the inside to stop interior condensation *(heading outward)* from moisture sources- heating, cooking, bathing, pets, wood stoves, people. The ff XPS on the rim stops the summer time *(heading inward) *moisture drive that permeates the siding/wood rim to condense there as the link showed.

Gary


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