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Old 12-01-2013, 12:31 PM   #16
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How to insulate rim joists simply


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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Old 01-20-2014, 01:21 AM   #17
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How to insulate rim joists simply


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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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Old 01-22-2014, 04:51 PM   #18
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Yes, toward the heated room. Post #13; Frozen water pipe - insulation question

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).

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Old 01-23-2014, 02:00 PM   #19
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How to insulate rim joists simply


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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Old 01-23-2014, 04:50 PM   #20
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How to insulate rim joists simply


At that point, just cover the exposed foam with mineral wool.
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Old 01-23-2014, 06:36 PM   #21
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How to insulate rim joists simply


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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Old 01-23-2014, 10:19 PM   #22
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Seal the foam. Condensation will not happen on the foam.

You need the insulation for fire/code compliance.
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Old 01-24-2014, 12:10 AM   #23
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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/EnergyCode...te=Connecticut

The foam should not require an ignition or thermal barrier in that area of application;http://www.buildingscience.com/docum...m-at-rim-joist

Chronjosh, check locally to make sure your area is following this; http://publicecodes.cyberregs.com/ic...?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/wxcli...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/docum...nsulation/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/docum...wall-approach/
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Last edited by Gary in WA; 01-24-2014 at 12:13 AM.
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Old 01-24-2014, 06:54 AM   #24
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How to insulate rim joists simply


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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Old 01-24-2014, 07:02 AM   #25
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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/EnergyCode...te=Connecticut

The foam should not require an ignition or thermal barrier in that area of application;http://www.buildingscience.com/docum...m-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, ˝” 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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Old 01-24-2014, 07:27 AM   #26
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How to insulate rim joists simply


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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Old 01-24-2014, 08:02 AM   #27
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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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Old 01-24-2014, 09:00 AM   #28
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How to insulate rim joists simply


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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Old 01-24-2014, 01:23 PM   #29
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How to insulate rim joists simply


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/uploa...m_products.pdf Check locally.... IMO, use two layers of (2") Thermax on the rims, forget the Roxul....

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Old 02-02-2014, 08:02 PM   #30
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How to insulate rim joists simply


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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