# What thickness of XPS/Roxul?



## Windows on Wash (Aug 30, 2011)

1. Well thought out and scripted plan.
2. I would go with 2" XPS everywhere in including bands. You will want that thermal resistance and vapor control level. The area should be covered by drywall as part of your finishing plans so no worries there and you can cover with whatever batt insulation you see fit.
3. I would go 2" personally. Pretty cold up there. 
4. Meh..? Not really sold on that stuff and if you build it right, you shouldn't have any moisture issues.

I know that IRC up in those parts requires a vapor retarder level of some significance but just make sure the exterior envelope is supremely well sealed before you start with the put back and make sure (as you mentioned you will) you take do ADA on the sheetrock (Airtight Drywall Approach).


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## mlowrie (Nov 6, 2009)

1. Thanks. At least I have one person agreeing with my method and based on what I've read and see, it looks like the optimal way to do it
2. Yes, the area will be finished with drywall. I really don't want to leave unfinished wall exposed too long. I have enough of those already!
3. Depends on your definition of cold. We don't get below freezing very often but I'll see if I can squeeze another inch on the foundation wall - no problem for joist cavities. 
4. That's what I thought. As long as wood isn't touching concrete then everything should be fine. 

So after thinking about this two things came to mind:
1. Having the vapour barrier completely on the interior wall makes it a bit difficult to seal effectively since above the foundation wall, it will be ~6" from the exterior wall. With the subpanel and all the wiring, there will be lots of penetrations. What if a "middle plate" is installed so that the vapour barrier can run up along the batt insulation in front of the foundation, then back so it is against the batt insulation of the exterior wall? ie the void would be behind the drywall, not behind the drywall and vapour barrier. (does this make sense?) It seems this would be the same finished profile you would obtain from spray foam. 

2. If this is done, then the moisture barrier and vapour barrier will create a closed cavity. I suppose then the moisture barrier shouldn't be used at all?


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## Windows on Wash (Aug 30, 2011)

Get the outside wall airtight and airtight the drywall. 

After that, you shouldn't have any moisture issues and I am not a fan of plastic, interior, vapor barriers anyway.


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## Gary in WA (Mar 11, 2009)

IMHO, 2" is overkill for your location. On the rim joists, the 1"XPS is more than enough, same as Zone 4C;http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements

On the poly, some reading for you; http://www.buildingscience.com/docu...Hygrothermal_Modeling_Basement_Insulation.pdf

Fill the 6" gap between insulation/walls with cheaper Fg to limit convective looping/air movement there; https://docs.google.com/viewer?a=v&...Foimib&sig=AHIEtbQYAZ0SWjnVJNXhfHFsk1RrK9G2HQ

Using 3 coldest months averages of high/low for your location; http://www.weather.com/weather/wxclimatology/monthly/graph/CAXX1710

is *40*F*-- temperatures of different thickness FB on cavity side (above grade/1' below grade) is; *R-10* (2")XPS 9=(unfaced) with R-14 cavity - safe to basement air below *57%RH*--- FB at 52*F.

*R-5* XPS and R-14= FB at 47*F and *48%RH*

*R-2.5* XPS and R-14= FB at 44*F and *43%RH*

Check your transition ducting to termination hood for dryer, appears you have a leak... canned foam any wiring holes through studs/plates to limit air movement in wood frame wall, cc pipe insulation on water supplies as they will be much colder once cavity-fill/new wall is added, fire-stop new wall area before install at ceiling to prevent joist cavities fire path and every 10' horizontally as in US codes; http://publicecodes.cyberregs.com/icod/irc/2006f2/icod_irc_2006f2_6_sec002_par017.htm

Gary
PS. "no burst" washer hoses are cool...


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## mlowrie (Nov 6, 2009)

Thanks for all the info Gary. That was some interesting reading and there seems to be a whole slough of stuff out there regarding this topic and it's hard to judge which is best for the area and design of the house. 

So 1" XPS and R14 batt will be my method and I'll go through the calculations again myself to verify the numbers you got and ensure that condensation won't occur. 

As for the "dead space", if it is on the warm side of the insulation and vapour barrier, are air currents still a factor? I don't really want to spend a whack more cash on filling in this void space just so the vapour barrier is in contact with the drywall. Not much different than creating a stepped wall, then deciding to eliminate the step later in the future. 

As for my plumbing and vents, they are all getting replaced. New venting, new plumbing (PEX with manifold system), updated electrical, the whole works. I only want to do this once!

I'll check into the fire stop requirements here to make sure I don't miss anything. 

Mike


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## Windows on Wash (Aug 30, 2011)

Air currents aren't an issue if the space is tight, temp stable (i.e. convection), and drywalled tight.


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## Gary in WA (Mar 11, 2009)

You're welcome! The gap fill will stop any warm wall (interior one) temperatures mixing with the cold wall (exterior frame wall) temperatures through the studs (thermal bridges) slowly drawing your heat away, every time you warm the room. Standard 16"oc wall frame loses -9% to wood that is rated at R-1.25 per inch or R-4.4 at every stud and plate. Drops your wall rating from R-14 to R-9....

Radiation/conduction could also take place depending on the temps/frequency. Horizontal airspace has R-1 but with a cold upper wall (above grade) and a warm (interior) whole wall, IMO, insulate to control any tempering of the air space- your mild winters- you might not want to "cover all the bases" but at least you were informed; pp. 32-40; http://www.google.com/url?sa=t&rct=...0IDQCw&usg=AFQjCNFRdfFAHayhaIygcKMAa3A8yDuYqg

Gary


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