# How to insulate rim joists simply



## french_guy

How should I insulate my rim joists?
But I don’t want to use the 2 bottles foam kit though
I was planning to use Roxull or rigid foam
But I’ve read so many different opinions…..that I’m lost
I’ve heard that in some areas, you cannot put the rigid foam
But it seems to be one of the most used method (using 2” foam)
I was thinking about using 2” foam + Great Stuff to seal all around, or 1” foam + GF and additional layer of Roxull
I was also planning to “fix” the foam on the rim joist with some caulk, just to keep it in place with a gap of ½” all around to seal with Great Stuff

What do you think is best?


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## bbo

best is really perspective.

I used closed cell foam. no worries about air leakage. but more $$ for sure.


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## Dave Sal

I did the 2" rigid foam with silicone caulk to seal around the edges. It was relatively easy, inasmuch as squatting around in a crawl space can be. I also closed off the two outside air vents and sealed them up with the rigid foam as well. The crawl space temps now stay an average of 63 degrees all year round. Pic attached of rigid foam in place prior to caulking the edges.


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## concretemasonry

I precut some scrap 2" XPS that was sized a little loose for the typical opening (box cutter and snap). I put a small bead of Great Stuff around to seal and then immediately jammed the XPS pieces in and left the foam to fill and adhere the XPS. I then stuffed in some fiberglass (probably not necessary) but it was handy, but Roxul would be better and you could do it at on you time table after the XPS and foam were in place.

It made a world of difference and I got rid of a lot of "waste".

Dick


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## cleveman

When I build, I sheath with 1" xps and I save all my window and door cut-outs. I rip them to the joist width on a table saw, then set my saw up in the basement and measure each space and cut them to length plus a bit.

I "friction fit" the pieces in, 2" thick. If I have some 2" stuff left over from the concrete walls, I'll use it up as well. I'll use a hole saw to cut out for the 2" pvc going to the furnace, and any sleeves I have for the line set, etc., then cut them in half at the hole and put them in.

No problems getting to fit tight with the friction fit, but if I have some extra free caulk, I'll caulk the edges. Then I put some fiberglass on top of it.

With 1" of xps on the outside of the rim joist and 2" on the inside, I figure I have r-15, then I put 13 of fiberglass on top of that and call it good.


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## Gary in WA

french_guy said:


> How should I insulate my rim joists?
> But I don’t want to use the 2 bottles foam kit though
> I was planning to use Roxull or rigid foam-------
> 
> ---------- Roxul alone will not stop the exterior air infiltrating or basement moisture from condensing on the rim. Requires foil-faced and thick enough to prevent condensation. The more fibrous insulation behind it, the thicker fb required because it will be insulated from the heat source and run colder--- exterior fb doesn't matter; last paragraph-- *http://www.buildingfoundation.umn.edu/RimJoist/rimjoistphenom.htm*
> 
> 
> But I’ve read so many different opinions…..that I’m lost
> I’ve heard that in some areas, you cannot put the rigid foam---
> 
> -- many locales require an ignition barrier or thermal barrier= additional fiberglass batt or mineral wool batt---- or drywall, check locally with AHJ. Many accept foil-faced polyiso (PIC) as it doesn't require either. Google Thermax. Caulk the rim joints to sheathing/mudsill/joists - then add the PIC/canned foam.
> 
> 
> But it seems to be one of the most used method (using 2” foam)
> I was thinking about using 2” foam + Great Stuff to seal all around, or 1” foam + GF and additional layer of Roxull
> I was also planning to “fix” the foam on the rim joist with some caulk, just to keep it in place with a gap of ½” all around to seal with Great Stuff
> 
> What do you think is best?


That should work.... Gary


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## ThatDaveGuy

Did the 2" foamboard + GreatStuf on the rim joists here, this morning is below freezing with goddawful winds (my winter standard apparently) and I can already see a marked difference. Quick + cheap + (relatively) easy for an improvement I can feel? I'll take it every time, all that swearing in the crawlspace was worth it.


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## french_guy

So how did you do it *exactly* (step by step)?


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## Gary in WA

If asking Dave, ignore the rest of this post....lol. 



Caulk all joints in the cavity on the rim joist. This is your first-line of defense against air infiltration- no point in letting it get to the inside of rim to condense. Cut foil-faced foam board (2") min. 1/2" smaller than width/length dimensions of cavity. It HAS to be foil-faced to stop condensation/moisture passage; " 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

Add foam compatible adhesive on the back around the perimeter and a center bead vertically to make two sections (in case you missed a 1/32" gap to the outside with the caulking) - this is second-line of defense that will limit air/moisture movement by 1/2 surface area. Press it against rim after adding glob of hot-melt glue or two for temp. holding power, centering it in space. When adhesive is dry (per label), add low-expanding "Window-n-door" canned foam around perimeter, not "Great stuff"(high-expanding). More on reusing foam can; http://netcrafting.com/homemx/greatstuff.html

Reason for caulking rim joist first; http://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange/

And, *page 51*; http://books.google.com/books?id=iw...um=3#v=onepage&q=cutting floor joists&f=false

Remember; the more fibrous cavity insulation you add in front of the foamed rim, the thicker the fb required to keep the dew-point down. 

Gary


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## hammerlane

spray foam


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## french_guy

Cool, thanks
Very well explained
I went to my local HD, and I didn't see the 2" foil faced foam though.....!!!!


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## Dave Sal

french_guy said:


> Cool, thanks
> Very well explained
> I went to my local HD, and I didn't see the 2" foil faced foam though.....!!!!


I bought my 2" foamboard from Menards and it also wasn't foil faced. To my knowledge, they don't carry it, but they might be able to special order it. I have no issues with moisture though. I used PL Premium as the adhesive to stick the foam board to the rim joist and then used silicone caulk to fill in the gaps around the edges. As soon as a piece of foam board was glued in place and sealed up around the edges I could feel the air movement suddenly stop when I was doing the west side, as there was a strong westerly wind that day. This job alone probably made the house more comfortable than anything else that I've done. The crawl space no longer gets ice cold during the winter. I was just down there the other day getting holiday stuff out of there and I was in a T-shirt and pants and wasn't cold at all, as it now stays around 63 degrees all year. :thumbsup:


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## Gary in WA

Tried this store for your location/zip, they carry 1", just double it up; http://www.homedepot.com/p/Unbrande...sulation-787264/100549260?N=baxx#.UpQYmicbHAU

Hammer, you really need to replace the missing places of X-bracing for the floor to act as a unit rather than sections of smaller units. Flat 2x4 under the ducting works, in-line with the x-bracing, a 1x4 flat, nailed to the joist bottoms will improve the structure immensely; http://www.ewpa.com/Archive/2004/jun/Paper_278.pdf House wrap on the fibrous insulation will stop the attic inherent convective loops from degrading the top few inches of insulation of R-value; http://digitalcommons.unl.edu/cgi/v...y rising` attic air from passive ventilation"

Gary


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## princevlad

Hammer, Do you feel a big difference with adding more insulation in your attic. Heating/cooling wise and HVAC not running as much?

I was debating to add more, but on the fence.


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## hammerlane

princevlad said:


> Hammer, Do you feel a big difference with adding more insulation in your attic. Heating/cooling wise and HVAC not running as much?
> 
> I was debating to add more, but on the fence.


The attic area I refer to in the photo is above my family room and garage. IT actually extends back into the picture another 25 feet or so.

The doubled up batt insulation did help I think in warmth in the family room. On the other side of the wall that is spray foamed in the attic is a walk in closet and the sprayf foam did help in there.

The spray foam in the basement in the rim joist cavities not only helped with air infiltration, but also I have not noticed as many dead bugs in the basement. Previous to the rim joists being spray foamed, the cats we have would hunt at night in the basement killing bugs.


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## handyman_squire

Hammer which type of spray foam did you use? I would like to do this in my crawl space as well as there is no insulation at all between the rim joints. And in my attic as well I have a wall like the one you posted that shares a living space on the other side that doesn't have insulation on it ether. Thanks


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## ccleme11

Gary in WA said:


> Cut foil-faced foam board (2") min. 1/2" smaller than width/length dimensions of cavity. It HAS to be foil-faced to stop condensation/moisture passage;


So, am i right in assuming that the foil needs to go on the inside of the basement?


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## Gary in WA

Yes, toward the heated room. Post #13; http://www.diychatroom.com/f103/frozen-water-pipe-insulation-question-193523/

You could leave a space between the fb/fibrous insulation to allow a little room heat to keep the temperature of the foamboard warmer, also preventing condensation (if the thickness is not enough to prevent condensation the whole year).

Gary


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## chronojosh

If you cant' get foil faced xps, can you buy regular xps and attach another radiant barrier insulation to that? The only reason I ask is because I don't see any at Lowes or HD.


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## Windows on Wash

At that point, just cover the exposed foam with mineral wool.


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## chronojosh

Since I'm a simpleton, would the 2" of non foil faced xps against the rim joist and then a layer of roxul against that prevent condensation? I do not have any exterior foam insulation.


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## Windows on Wash

Seal the foam. Condensation will not happen on the foam.

You need the insulation for fire/code compliance.


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## Gary in WA

Windows on Wash said:


> Seal the foam. Condensation will not happen on the foam.
> 
> You need the insulation for fire/code compliance.


I don't agree. The Roxul or any fibrous/cavity fill is to bring the wood rim joist up to the code required R-values for a wall= *R-20*; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Connecticut

The foam should not require an ignition or thermal barrier in that area of application;http://www.buildingscience.com/documents/information-sheets/critical-seal-spray-foam-at-rim-joist

Chronjosh, check locally to make sure your area is following this; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_3_par199.htm?bu2=undefined

If you add Roxul to the foam (to meet code) be careful not to add too much thickness as it will keep the room warmth air (insulating) from reaching the foam board and the lowered temp there (because it is in direct contact with the rim/outside temps) could cause condensation easier.
Eg.- with R-10 FB and R-15 (3-1/2") Roxul, the dew-point is at *43% Relative Humidity*, using 68*F room air and 3 coldest months averages for Conn.; http://www.weather.com/weather/wxclimatology/monthly/graph/USCT0094

With R-10 and 6" Roxul (R-23)= safe to *38%RH* in cavity, with 7-1/4" Roxul (wrongly thinking- more is better) only *35%RH* dew-point and below is safe.

This explains ratio of cavity fill/FB well; *Fig. 4*, notice the temp change going from R-12 to R-19; http://www.buildingscience.com/docu...ld-weather-condensation-using-insulation/view

The unfaced FB is fine, just don't add too much Roxul to defeat it...just meet code minimum, IMO.

Gary
PS. as you don't say if drywall will be applied (and reduce/stop any interior air condensation), ADA it if/when you do; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/


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## chronojosh

Thanks all. Gary, I read through some of that. I just want to make sure that if I just use non foil faced xps, and didn't put roxul in front of it, should I still worry about condensation? I know people do this all the time, but this is the first thread I have seen the condensation issue discussed. That is part of the reason why I am so curious about it.


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## Windows on Wash

Gary in WA said:


> I don't agree.
> 
> The Roxul or any fibrous/cavity fill is to bring the wood rim joist up to the code required R-values for a wall= *R-20*; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp?state=Connecticut
> 
> The foam should not require an ignition or thermal barrier in that area of application;http://www.buildingscience.com/documents/information-sheets/critical-seal-spray-foam-at-rim-joist


I don't care what code says in this application with regards to foam being acceptable to left uncovered. If his inspector or county is working off an older variant, it should be covered or treated with an itumescent. In an area where fireblocking is so critical, I would never leave a combustible like foam uncovered. 

Some counties still do not recognize what the newer code allows for so be sure to check with your local folks. 

Where in your link does it mention rigid board foam and code compliance? I know there are other links out there but your link reference SPF.

From OC on their Foamular product:

*Q: Can FOAMULAR® be left exposed in interior applications?*

_A: No. To comply with building codes, all foam plastics must be covered with a 15 minute thermal barrier. Gypsum board, ½” thick is a common covering._

http://www.foamular.com/foam/faqs/#appgen

Can you also clarify as to whether or not the OP needs to bring his insulation schedule up to current code at the rims joist locations? 

Per the first code link, the mass wall R-value is 13/17. Would they consider the ribbon board part of the mass wall or the wood framed wall?


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## mikegp

Dow makes a rigid foam that does not have to be covered or protected by fire in any way. Says it right on the board.


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## Windows on Wash

mikegp said:


> Dow makes a rigid foam that does not have to be covered or protected by fire in any way. Says it right on the board.


My Dow rep was down yesterday and told me about that. We are using it for crawl applications now.

Between you and me...we will still be covering it. The smoke formation from all the foams I have seen is not pretty.


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## mikegp

I was actually about to add to my post that I would still cover it, whether it was necessary or not. Totally agree with you.


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## Gary in WA

I agree FB requires covering on the walls. As far as the rim joists, I understood both SPF and XPS are made with fire-retarding chemicals added to get their burn/smoke rating, but as I stated before that you missed; "*Chronjosh, check locally* to make sure your area is following this; http://publicecodes.cyberregs.com/ic...?bu2=undefined" This will explain the local jurisdictional requirements on application. 

The rim is wood frame, same as the walls. "*LIGHT-FRAME CONSTRUCTION.* A type of construction whose vertical and horizontal structural elements are primarily formed by a system of repetitive wood or cold-formed steel framing members. *" *So, while adding 1-1/2" ofmineral wool will meet ignition barrier requirements, it wouldn't meet energy requirements for the wall that needs minimum of R-10 (3-1/2" batt). Any more thickness of insulation and the dew-point would really drop as the fb is only a Class 2 (vapor retarder) at this thickness. Hence; using foil-faced Thermex for the vapor barrier (Class 1) and better for fire to boot. Yes, you are safe to only 43% basement room air humidity, then may get condensation- only during those three coldest months when fb temp is at 45*F on inside.http://www.buildingscience.com/docum...nsulation/view

The walls may be mass construction: "*MASS WALL.* Masonry or concrete walls having a mass greater than or equal to 30 pounds per square foot (146 kg/m2), solid wood walls having a mass greater than or equal to 20 pounds per square foot (98 kg/m2), and any other walls having a heat capacity greater than or equal to 6 Btu/ft2 · ° [266 J/(m2 · K)]. *"

*Some local AHJ require wood fire-blocking *between fb* on the walls, some accept just a* covering over* the fb as fg insulation. But nothing to stop fire from burning through the foamboard laterally. Check locally. 

Further research, (thanks, WoW) the foams are different in fire qualities, boards require an ignition barrier- except possibly; Thermax; http://www.pureenergycoach.com/uplo...ir_sealing_insulating_using_foam_products.pdf Check locally.... IMO, use two layers of (2") Thermax on the rims, forget the Roxul.... 

Gary


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## chronojosh

So, I started cutting 2" xps to fit against my rim joists. Before placing the xps I applied "great stuff" around the perimeter of the rim joist area and then pushed the xps into place to seal it. Then I sprayed around the xps with more "great stuff" to fill in any gaps. I think that as the spray foam expanded the xps may have pulled away from the rim joist a bit....maybe 1/4". Will this a problem as far as condensation goes?


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## concretemasonry

It sounds like a good plan that I have used several times on homes if owned over the last 30 years. You will get some criticism from code reference citers and hair splitters that deal on .001's on perm ratings. It is just a small space, so do not worry about much because what you propose is estimated at a 98.7768365% improvement for a small area relatively.

Dick


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## Gary in WA

Lol. 

Cj, if you air sealed the rim joist (with a sealant) from the mud-sill plate and deck sheeting above before adding the foam board, you will be fine. The rim will expand/contract over the seasons allowing infiltrating air, sometimes moist; http://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange/

*Page 51*; http://books.google.com/books?id=iw...um=3#v=onepage&q=cutting floor joists&f=false

Gary


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