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Old 01-01-2014, 12:19 AM   #16
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Originally Posted by Msradell View Post
I'm not sure why everybody is trying to make this much more complicated than it needs to be. Obviously decoupling the stringers so they don't transmit the cold would be the ideal situation but in this case I'm not sure how valuable that approach would be. The rest of your house probably has many other installation problems that would provide a greater benefit. It was my home I would fill that cavity with spray foam as well as spraying the floor under the bedroom with at least 6 inches of closed cell foam. We did that for our over garage master bathroom a couple of years ago in it may be significant difference!
Overkill filling in that 2 1/2 for cavity with Spray foam, along with decoupling.

No one is making this more complicated than it is. Well, the OP driving and answering to forum posts on his smartphone, although the reading Gary's information, makes it harder for the OPto understand what is being stated.



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Old 01-01-2014, 08:43 PM   #17
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Please don't read while driving..... R-30 is code OR R-19 min. OR enough to fill the cavity, footnote "g"; http://energycode.pnl.gov/EnergyCode...e=Pennsylvania

If you want the space above; http://www.buildingscience.com/docum...al-performance Just add enough to meet code.

No need for faced as the exterior glue in the plywood is lower perms than the facing paper.

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So looking at the diagram above, does the rigid foam still need to be 2" thick on the bottom of the joists? Just thinking about the already low ceiling height.
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Old 01-02-2014, 06:00 PM   #18
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Per your location using the coldest 3 months average temp of 29*F, using R-10 (2"XPS) the cavity is safe to 38%RH in the room above. With R-7.5, the dew-point in cavity is 35%RH room air moisture. Depends on the temps in the garage compared to outside, if garage is always warmer due to less wind, furnace removes excess RH, room used only occasionally, etc. , you could get by with the lesser thickness. I didn't even run the numbers for less FB because of your location. http://www.google.com/url?sa=t&rct=j...58187178,d.cGU

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PS. you could always add some XPS between the joists and use PIC on the joists in only 1/2" thickness, depends on local products...
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Last edited by Gary in WA; 01-02-2014 at 06:03 PM. Reason: added thought
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Old 01-02-2014, 06:18 PM   #19
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Originally Posted by Gary in WA View Post
Per your location using the coldest 3 months average temp of 29*F, using R-10 (2"XPS) the cavity is safe to 38%RH in the room above. With R-7.5, the dew-point in cavity is 35%RH room air moisture. Depends on the temps in the garage compared to outside, if garage is always warmer due to less wind, furnace removes excess RH, room used only occasionally, etc. , you could get by with the lesser thickness. I didn't even run the numbers for less FB because of your location. http://www.google.com/url?sa=t&rct=j...58187178,d.cGU

Gary
PS. you could always add some XPS between the joists and use PIC on the joists in only 1/2" thickness, depends on local products...
Well I just bought a small 30k BTU wall mount heater for the garage. Not to keep it up to a desired room temp but to at least take the chill off. I may have to put the XPS between the joists and then just re-drywall it due to my garage door/ceiling clearance.



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Old 01-03-2014, 11:48 AM   #20
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Ouch, well- anything with FB is an improvement....

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