Joist insulation
Last Post 08 Dec 2014 05:01 PM by Dana1. 31 Replies.
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John7User is Offline
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04 Dec 2014 03:21 AM
I brought the poly vapor barrier up from the wall below and sealed it to the bottom edge of the polyiso with that black, never hardens acoustic seal. Then drywall as usual. John
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04 Dec 2014 05:10 AM
Mitigating air infiltration is normally preferred, however, I would not ignore Dana’s warning about potentially creating a condensing surface. Your local climate does matter and should be considered.
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04 Dec 2014 06:46 AM
Sometimes DIY is impractical. Such is the case with sealing a rim joist. Though the majority of our work is retrofitting radiant floors in old houses, we do design/build and often design/specify radiant systems for additions and new homes.

It is precisely because of the regular complexity of the rim-joist assembly with the inconsistency of the quality of construction and random penetrations that I specify 2# foam, which covers infiltration, vapor, condensation and is relatively easy to visually inspect for quality and quantity.

It may be the best money you spend on insulation after the ceiling is blown.

We do you fiberglass below our radiant plates as a practical matter but trapping warm air in each joist cell is our real goal so one can make a case for rigid insulation foamed at the edges. Cost is the limiting factor here, both in time and material without appreciable gain.
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04 Dec 2014 07:01 AM
And sometimes DIY is the only practical option. There are many factors to be considered before making that decision. And one should always take the time and do the required research to become educated before making that decision.
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04 Dec 2014 04:21 PM
What about Roxul on outside (touching the rim joist), then continue the wall's vapor barrier into the joist space and seal with tape?
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04 Dec 2014 05:54 PM
Posted By Vancouver123 on 04 Dec 2014 01:42 AM
thanks for all the great advice. It sounds like for rim joists, foam-board insulation (with spray foam around the edges) is a relatively simple solution. Also sounds like the order should be rim joist, then roxul, then the foam board, then joist space. I have no idea how to seal the bottom of the foam board to the drywall below, is that necessary?

No, the order should be

Rim joist / enough foam for dew point control at the foam-fiber boundary / fiber insulation  / semi-vapor permeable air barrier

http://www.buildingscience.com/documents/information-sheets/content/basement-insulation/images/bscinfo_511_figure_02.jpg

Also, to be clear, dew point is a temperature, not a location.  The relevant dew point for this discussion is ~40F, which is the moisure level in 64-70F / 35% RH air, common indoor conditions in cool/cold climates.  And it's the average wintertime temp at the foam/fiber boundary, not the peak that is important.  As long as the temp at that boundary averages 40F or higher you won't get liquid moisture loading in the cavity, even though you'll get surface condensation (or even surface frost) on the surface of the foam during the winter low temps.

Using a foam with a permenace between 0.4 and 1.5 perms (at the installed thickness) gives the rim joist reasonable drying capacity toward the interior. Using foil faced goods or polyethelene sheeting REDUCES the moisture resilence of the assembly due to extremely low vapor permeance. That's OK if there is back-vented siding on the interior, but it increases risk otherwise.
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05 Dec 2014 03:27 AM
Dana, I appreciate all that and your depth of knowledge. You raise two points at the end and indeed our provincial building code does require both polyethylene vapor/air barriers and rainscreen siding. I must insulate and put an impermeable barrier on the conditioned side. Whether 6mil polyethylene or foil faced polyiso it makes no difference. Looking at the diagram I am curious what makes the rim joist require foam yet the stud wall above is perfectly fine with just fiber insulation. John.
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05 Dec 2014 01:26 PM
I appreciate all the input which frankly for a non-construction guy has been very confusing! After some discussions with my contractor and the city inspector we're going a slightly different route. We're going to put roxul directly on the rim joist, then continue the wall vapor barrier up into the rim joist and acoustically seal on all sides and top.

I think this will be OK because (a) Vancouver's average temperature is around 50F (year-round) and 40F (in winter) so I imagine there will be a lower risk of condensation in this mild climate despite the higher radiant-related heat in the joist cavity, (b) there are some cost savings vs. the rigid foam and (c) contractor and inspector think this is a fine solution.

Not too late to change so if this is a bad idea please let me know.
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05 Dec 2014 01:50 PM
Posted By John7 on 05 Dec 2014 03:27 AM
Dana, I appreciate all that and your depth of knowledge. You raise two points at the end and indeed our provincial building code does require both polyethylene vapor/air barriers and rainscreen siding. I must insulate and put an impermeable barrier on the conditioned side. Whether 6mil polyethylene or foil faced polyiso it makes no difference. Looking at the diagram I am curious what makes the rim joist require foam yet the stud wall above is perfectly fine with just fiber insulation. John.
I take it you're in western B.C. then?  Code does not specify polyethylene or foil.  Under than National Building Code a vapor retardency of just over 1-US perm or less qualifies as a vapor barrier.  A 1" shot of closed cell foam (or 1" XPS) is about 0.8-1.5 perms.  At 2" (or even 1.5") it would reliably meet that part of the code.

In your climate you also don't need more than 15% of the total R to achieve dew point control at the foam/fiber boundary, unless B.C. spells it out differently from the NBC.

As for the detail drawing, you'd have to ask the person who drew it. Maybe there vapor retarder in that upper wall assembly? It's probably simply because they were not detailing the upper wall, only the rim joist.

It's damned near impossible to make an interior side air/vapor barrier perfectly, especially for the long term,  and it's far more reliable & resilient to make the vapor retardent layer tight to the rim joist, and use the R-ratio for dew point control at the foam/fiber interface. It still needs an interior side air barrier, but it doesn't turn into a disaster when it leaks air in 20 years, allowing moisture to get into assembly, but impeding the drying path the way a foil facer or poly sheeting would.
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07 Dec 2014 03:05 AM
Posted By Vancouver123 on 05 Dec 2014 01:26 PM
I appreciate all the input which frankly for a non-construction guy has been very confusing! After some discussions with my contractor and the city inspector we're going a slightly different route. We're going to put roxul directly on the rim joist, then continue the wall vapor barrier up into the rim joist and acoustically seal on all sides and top.

I think this will be OK because (a) Vancouver's average temperature is around 50F (year-round) and 40F (in winter) so I imagine there will be a lower risk of condensation in this mild climate despite the higher radiant-related heat in the joist cavity, (b) there are some cost savings vs. the rigid foam and (c) contractor and inspector think this is a fine solution.

Not too late to change so if this is a bad idea please let me know.


That will work fine. Don't let the "debate" distract you. John.
John7User is Offline
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07 Dec 2014 03:42 AM
Posted By Dana1 on 05 Dec 2014 01:50 PM
Posted By John7 on 05 Dec 2014 03:27 AM
Dana, I appreciate all that and your depth of knowledge. You raise two points at the end and indeed our provincial building code does require both polyethylene vapor/air barriers and rainscreen siding. I must insulate and put an impermeable barrier on the conditioned side. Whether 6mil polyethylene or foil faced polyiso it makes no difference. Looking at the diagram I am curious what makes the rim joist require foam yet the stud wall above is perfectly fine with just fiber insulation. John.
I take it you're in western B.C. then?  Code does not specify polyethylene or foil.  Under than National Building Code a vapor retardency of just over 1-US perm or less qualifies as a vapor barrier.  A 1" shot of closed cell foam (or 1" XPS) is about 0.8-1.5 perms.  At 2" (or even 1.5") it would reliably meet that part of the code.

In your climate you also don't need more than 15% of the total R to achieve dew point control at the foam/fiber boundary, unless B.C. spells it out differently from the NBC.

As for the detail drawing, you'd have to ask the person who drew it. Maybe there vapor retarder in that upper wall assembly? It's probably simply because they were not detailing the upper wall, only the rim joist.

It's damned near impossible to make an interior side air/vapor barrier perfectly, especially for the long term,  and it's far more reliable & resilient to make the vapor retardent layer tight to the rim joist, and use the R-ratio for dew point control at the foam/fiber interface. It still needs an interior side air barrier, but it doesn't turn into a disaster when it leaks air in 20 years, allowing moisture to get into assembly, but impeding the drying path the way a foil facer or poly sheeting would.


That last part would create the all to be avoided double vapor barrier. I can't use semi permeable vapor barrier.......... My hands are tied with our building code, what products are certified vapor barriers, city hall plan reviews (permitting), and ornary inspectors........... True the code does not specify polyethylene however it must be lab certified to meet CAN/CGSB-51.33-M. AFAIK no manufacturer has be interested in spending the required money to get their product certified for use in Canada under part 9 of the building code - our market is too small to make it worthwhile............ 9.25.4.2: 5) Membrane-type vapour barriers other than polyethylene shall conform to the requirements of CAN/CGSB-51.33-M. "Vapour Barrier Sheet, excluding Polyethylene, for Use in Building Construction."............ I would loved to have been able to use semi permeable vapor barrier but was blocked every step of the way. It took 7 different wall assembly schemes before I got one I was comfortable with and they would let me build. (At one point they wanted me to drywall and vapor retardant paint the rim joists insulation!!! stop the insanity!) I consulted with industry experts, building science experts, code consultants and those that actually serve as advisors to the code. That is why I am passionate about this subject............. I am still steamed by the whole thing and am going to move on to some youtube cat videos......... John.
Dana1User is Offline
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08 Dec 2014 05:01 PM
Certainteed has had at least some third party testing by/for the CCMC and NRC .  From the spec sheet :

National Building code of
Canada 2005 & 2010
Articles 9.25.4.2
Articles 9.25.3
CCMC Evaluation Report #13278-R


How did those cats learn to play piano so well, anyway?
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