# rim-joist insulation - spray or sheet foam?



## Windows on Wash (Aug 30, 2011)

Option 1, done properly/carefully, will work just as well.

What is your time worth? That is what it mostly comes down to.


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## kuhlman (Dec 21, 2011)

I am currently doing this same thing and having trouble finding what type of foam board to use in the joist bays. EPS, EPS with film on each side, or XPS. I know XPS is recommended against basement walls. Also, the local big box stores in my area no longer seem to carry XPS.


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## titanoman (Nov 27, 2011)

Why not just stuff it with regular insulation with a barrier?

Sent from a Samsung Galaxy S2


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## kuhlman (Dec 21, 2011)

figured foam board with spray foam around the edges would give the best result (next to full spray foam). Just not sure what the best type of foam board would be.


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## Gary in WA (Mar 11, 2009)

I'd go with XPS and caulking. That foam board is not as open-cell as EPS to hold moisture leaking in from outside. Caulking would move with the rim joists (unless engineered wood) through the seasons; pp.51: http://books.google.com/books?id=iw...age&q=heel or toe bearing with rafter&f=false

Gary


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## TarheelTerp (Jan 6, 2009)

redline9k said:


> Ive got about 60 joist bays to fill in the basement of our 40yr old ranch.*
> its clearly drafty*, so id like to get this taken care of.


Before you get to the joists... what's happening at the perimeter wall?
(gaps, cracks, ungasketed doors/windows, etc)



> Option 1 is sheet foam, cut to fit the bays, caulked/spray foamed in place
> Option 2 is spray handi-foam (or similar).


fwiw... I'd hate to any spray in foam used in basement level floor joists.
well, at lest not without first doing a TON of plumbing and wiring work 
and so forth to minimize the chance I'd EVER deed to dig in later and if so to minimize the number of spots that would be likely to happen.

hth


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## Furd (Aug 26, 2007)

Definitely use the extruded foam board insulation. The foil facing can be used as a vapor retarder under certain circumstances. If this is a living space then you will need to cover the insulation with a fire retardant barrier such as 1/2 inch gypsum board.


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## HomeSealed (Jan 3, 2008)

TarheelTerp said:


> Before you get to the joists... what's happening at the perimeter wall?
> (gaps, cracks, ungasketed doors/windows, etc)
> 
> fwiw... I'd hate to any spray in foam used in basement level floor joists.
> ...


That would only apply if he was planning to foam the entire floor (every bay), which is not the case..


On the rim joist, for cost, XPS and foam or caulk is the way to go... but be WARNED, this is a total PITA. For my money ( and time), I'd buy a froth pack (two-part portable spray foam) to hit those sill boxes. By far easier, and probably more effective. 
I'd also mention that I dont believe that XPS/foam or caulk can be done for $50.


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## spring3100 (May 6, 2011)

I would (and did) use ROXUL to fill the cavities,virtually airtight,doesn't need a fireproofing cover (it is a fireblock).

And assuming cavities are at least 3 1/2 inches deep,you will get R-15 value insulation.


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## Gary in WA (Mar 11, 2009)

Either one is an air barrier (much more important than the insulating qualities), unlike fiberglass or Roxul (work fine behind the air seal). Fig. #2 here: http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation/

Check locally, it may be able to be left uncovered (though the smoke is toxic), depending on location and year of the code used; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec016_par015.htm

Some hints if you DIY: http://www.sprayfoam.com/npps/story.cfm?nppage=123

Gary


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## Beepster (Oct 19, 2009)

I am going to be a budinski for a second. Same topic different question. I am finishing of my basement in Minneapolis and plan on spraying the rim joists. 12" block, 10" joists, rim joists partially backfilled with concrete at build (1957).

My question is: do you foam spray to fill the entire rim joist cavity out to the framed basement wall (1" XPS board on wall) or do you just spray a 1-2" layer covering all surfaces?

B


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## Beepster (Oct 19, 2009)

I am going to be a budinski for a second. Same topic different question. I am finishing of my basement in Minneapolis and plan on spraying the rim joists. 12" block, 10" joists, rim joists partially backfilled with concrete at build (1957).


My question is: do you foam spray to fill the entire rim joist cavity out to the framed basement wall (1" XPS board on wall) or do you just spray a 1-2" layer covering all surfaces?


B


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## Bob Mariani (Dec 1, 2008)

titanoman said:


> Why not just stuff it with regular insulation with a barrier?
> 
> Sent from a Samsung Galaxy S2


 Because any profession with building science knowledge will know that this will only trap moisture making the fiberglass ineffective and for mold to grow. Rim joists need to be air sealed and insulated to the same R-value your local codes require for the exterior walls. R19 in my area. Here is how I do rim joists http://www.homedoctorofamerica.com/rim-joists.

Your first solution (flash an batt works fine. Easy to use Roxul batt and spray foam. Or use polyicocyanurate foam board. Spray foam is much easier and is the only method I use.


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## Bob Mariani (Dec 1, 2008)

Beepster said:


> I am going to be a budinski for a second. Same topic different question. I am finishing of my basement in Minneapolis and plan on spraying the rim joists. 12" block, 10" joists, rim joists partially backfilled with concrete at build (1957).
> 
> 
> My question is: do you foam spray to fill the entire rim joist cavity out to the framed basement wall (1" XPS board on wall) or do you just spray a 1-2" layer covering all surfaces?
> ...


In your area use closed cell foam 3" thick


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## Bob Mariani (Dec 1, 2008)

Just a note.... in a basement the air flow moves both from the exterior to the interior in the summer and from the interior to the exterior in the winter. So a vapor barrier (spray foam top and bottom edges of rim joist prior to batt installation) will trap moisture in the winter. Thus.... no fiberglass!


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## Beepster (Oct 19, 2009)

Thanks for the reply Bob.


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## DIYWorker (Jan 11, 2012)

I was looking at doing this to my basement recently- good info here.


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## SPS-1 (Oct 21, 2008)

I would not be so sure Option 2 is easier. I recently found air leaking where 4 inlet/ehaust pipes (water heater and furnace) ran through. The spray foam did not have enough adhesion and/or mechanical strength and would pull off just by gravity. (That never happens to Mike Holmes) I managed to get the job done by damming up the area to be insulated, but I would not look forward to doing a whole house like that. 
OK, I used the house brand foam of a big box store. If you bought the PU foam in two tanks, that might be better stuff. But I would tend to agree with the fellows that suggested XPS foam boards.


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## Mikenra (Jan 8, 2012)

Bob Mariani said:


> Because any profession with building science knowledge will know that this will only trap moisture making the fiberglass ineffective and for mold to grow. Rim joists need to be air sealed and insulated to the same R-value your local codes require for the exterior walls. R19 in my area. Here is how I do rim joists http://www.homedoctorofamerica.com/rim-joists.
> 
> Your first solution (flash an batt works fine. Easy to use Roxul batt and spray foam. Or use polyicocyanurate foam board. Spray foam is much easier and is the only method I use.


Following this thread. Good info. One question, what if you have electric wires running in this area?


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