John7
Basic Member
Posts:114
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| 17 Jan 2012 12:28 AM |
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Hi
For various reasons I have settled on a thick wall of dense pack cellulose.
Double stud wall:
The current design is for drywall/2x3 stud wall/1" separation/2x4 structural stud wall/ 1/2" plywood/housewrap/rainscreen strapping/fiber cement siding. I will be working with a small crew and/or doing a lot of the work myself. I would like to "get to the roof" as quickly as possible and am considering installing the interior 2x3 stud wall after the roof is on. However I think this will be very difficult with the interior partition walls in place (limiting work space) and not being able to tip up exact full height walls against the ceiling joists.
Modified Mooney wall:
An alternative would be 2x6 structural stud wall/ 1/2" plywood/housewrap/rainscreen strapping/fiber cement siding. Once the roof is on, return to the walls and on the interior apply 2" wide strips of 1" XPS, and 2" wide strips of 3/4" plywood over that. Effectively furring out the studs 1.75" (not really Mooney but similar advantages). I realize this would be a bit labor intensive and some what more materially expensive but has some advantages. Fitting XPS and plywood strips would seem to be easier than shimming 2x3 walls that have to be built short. I also avoid having to plywood box every window opening (thermal bridge) and line up those studs. Seismically I have a 2x6 wall with plywood on one side and drywall on the other (rather than two separate walls of smaller studs).
Has anyone done this or similar? Do you foresee any issues?
Thanks
John |
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DickRussell
Basic Member
Posts:182
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| 17 Jan 2012 10:21 AM |
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John, the first thing that comes to mind regarding the double frame option is the location of the air barrier layer and how you will be getting that to be continuous. Some prefer to establish it on the outside, others on the inside. Your local climate may have some bearing on what your options are, to avoid moisture issues. Given your choice of a very well insulated wall, I'm guessing you are in zone 5 or 6 (heaing dominated).
One problem with tacking on inner wall frame sections after you have closed in is that you'd have some inner partition walls poking through what otherwise would be a continuous inner wall. That could be a problem if your air barrier layer is to be on the inside. You might want to arrange your stud spacing so that you can have drywall on the exterior wall pass continuously through the partition wall, and have the partition drywall butt up against the exterior wall section, and supported by framing close to that intersection.
For new construction, getting a really high R wall, say R30 or better, with framing techniques, most will agree that the double wall is the way to go, as the insulation cavity can be whatever you need it to be. The Mooney wall technique is aimed more at retrofit situations. The total insulation cavity obtainable is basically the sum of your structural wall stud depth plus the 1.5" of horizontal strapping. Your comments give the impression that you really want to have the double wall, but are concerned about the delay in getting the project under roof cover. Will you be framing in winter, with all the problems of snow and ice taking up a lot of time?
Framing up the inner wall doesn't take all that long, and you have recognized the advantages of having both wall frames up together and tied together at their tops (something your inspector may require for firestopping purposes, despite the fact that some inspectors accept dense packed cellulose as firestopping. You may decide that the little delay in getting the roof on is worth it.
For background, I chose the double wall option for our new house. I was looking for R40, and a 12" cavity free of thermal bridging gives that. The crew of three took perhaps a day and a half or so total to frame up and tie in the inner wall frames. The house is on two levels (built on a hill; lower level is half walkout, with full frame walls). The subfloor for the upper level ties together the inner and outer wall frames of the lower level. The outer wall of the lower level bears the roof load. The inner wall of the lower level (where there is framing) bears the floor load and is 2x4. Where the lower level walls are just (insulated) concrete foundation walls, there are two sills. The floor system rests on the inner sill. The rim joist also rests on the inner sill, or on the inner wall frame where there is framing below the floor. The upper level outer wall frame rests on the subfloor and thus the outer wall frame of the lower level in those places where there is framing below the floor. In these places, the rim joist is insulated on its outside face as part of the lower wall cavity. In places where there is full height foundation wall (the uphill sections), the upper level outer frame rests on the outer sill, and the rim joist is insulated on its outside as part of the upper level wall cavity.
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Dana1
Senior Member
Posts:6991
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| 17 Jan 2012 12:12 PM |
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Double studwall construction is pretty straightforward, but unless you're using finger-jointed stock on the 2x3s you'll have a flatter finish wall by going with 2x4s on the interior. (Standard milled 2x3 studs tend to come with a lot more twist & bends.) Steel 2x3s are dead-straight, but also have greater bridging- you'd need to add about inch of depth to make up for it to achieve the same whole-wall R.
In zone 5 the rainscreen alone provides sufficient drying capacity to protect the plywood sheathing (per IRC 2009), which would allow you to use the finish latex paint (a type-III vapor retarder) as your only interior vapor retarder. In zone 6 that wouldn't meet code, but with a thick wall and exterior rainscreen the dense-packed cellulose buffering and relative air tightness is probably still protective enough (a WUFI simulation of your wall may be in order.) With a thick cellulose wall it's better to keep it relatively vapor-open to allow maximum drying, since it can otherwise buffer a lot of moisture, taking forever to dry. Worst-case you can probably use a variable-permeance vapor retarder like Certainteed MemBrain,which is fairly vapor-open when the humidity is high (summertime air, or humid inside the insulation layer) but a class-II vapor retarder when the humidity is low (winter air.) |
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greentree
Advanced Member
Posts:587
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| 19 Jan 2012 11:41 PM |
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John,
You don't need to tip up walls necessarily. You can nail your plates and tap in your studs after the roof is on. It does not take much longer than platform framing when you develop a system to do it. Some guys also tack bottom plates and tip up studs and top plate but if the roof is on you could run into trouble with the latter if you have any "issues."
You may want to tack the top plate though and remove the nails from the trusses and use the slotted clips to allow for truss uplift if your truss bearing is the exterior wall.
When doing this, after the plates are up, you go down the line and measure each stud length and make a cut list. 95% of the studs or better will be the same length, if they're not your framing is jacked up.
Have fun with those 2x3's, I'm sure they'll be dead straight. |
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John7
Basic Member
Posts:114
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| 20 Jan 2012 12:13 AM |
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I am going with option 2 consisting of 2x6 standard stud wall with 1" XPS and 3/4" plywood furring strips, and naming it "Walker Wall". |
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NFC
New Member
Posts:59
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| 20 Jan 2012 12:58 PM |
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It would make more sense to put the XPS on the outside (everywhere), and make it 2", and would probably be an easier install, and would better improve your overall R value. Your setup you only have R5 at every stud, and a bunch of extra labor. |
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John7
Basic Member
Posts:114
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| 20 Jan 2012 11:22 PM |
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I tried that but my envelope consultant advised against it. Our zone 4c climate won't tolerate having two vapour barriers. Even with VB paint, no poly. |
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Dana1
Senior Member
Posts:6991
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| 23 Jan 2012 12:24 PM |
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At 2" XPS is not a vapor barrier, and has about 2x the drying capacity of VB paint.
If you went with 2.5" of unfaced EPS (also R10) you'd have about 3-5x the drying capacity of XPS- sufficient drying capacity to be able to dry into a rainscreen cavity even in winter. (At 2- 2.5" EPS has comparable vapor retardency to standard ordinary latex paint, or about as vapor-permeable as 3/4" CDX plywood.)
In zone 4C with that much exterior foam and you shouldn't need ANY vapor barriers, and with cellulose it's better to keep it relatively vapor open on both sides. With some amount of EPS on the exterior and a rainscreen between the EPS & siding it can work well. (You can run a WUFI simulation on the stackup if you need more convincing.) |
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John7
Basic Member
Posts:114
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| 31 Jan 2012 12:59 AM |
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Dana
Thank you for your information, however my BI is totally inflexible and sticking to the letter of the (local) building code. To make matters worse our building code does not allow you to delete the vapour barrier - only make substitutions like ADA or (certified) VB paint.
My envelope consultant has strongly advised me to not use 2" of exterior XPS due to its lack of vapour permeability (less than 1 perm). (and anything less thick isn't guaranteed to keep the sheathing above dew point.)
Google "vancouver leaky condo" and you will have some understanding why our local BI's are completely unwilling to vary from the protection of our building code's boiler plate.
John |
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Dana1
Senior Member
Posts:6991
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| 01 Feb 2012 06:39 PM |
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Almost everything I read under "vancouver leaky condo" has to do with improper venting of rainscreens on brick veneer and stucco clad buildings, which have nothing to do with the interior moisture drives, and everything to do with the moisture-reservoir cladding and EXTERIOR moisture drives from rain-soaked brick. And in those buildings lower perm exterior foam and a high-perm interior would protect the wood better from the exterior moisture, at very low risk from interior moisture drives.
IIRC to meet code in Canada to get away without interior vapor retarders the exterior foam needs to be thick enough to prevent the sheathing at the outside DESIGN temp, not the AVERAGE winter temp, which is a lot more stringent than using the average temps (which would be more like the IRC). But in Vancouver that's still not too tough, as compared to say, Winnipeg, but on a fatter high-R wall it's still an issue.
But EPS is not the same as XPS- you can go up to ~ R14-R16 with Type-I EPS and still be above 1 perm, whereas you cross that (arbitrary) boundary at ~ R8 with XPS. If R15 is not good enough for the dew-point calc for your thickness of cellulose, if you instead went with 4-8lb density rock wool panels the sky is the limit- it's VERY high-perm. Have you even considred panelized high density rock wool panels?
http://www.thermafiber.com/images/uploads/pdf/rainbarrier%20data%20sheet.pdf
http://www.roxul.com/residential/products/roxul+comfortboard+fs
http://www.roxul.com/files/RX-NA-EN/pdf/Residential%20Sell%20Sheets/ComfortBoard%20IS.pdf
http://www.roxul.com/files/RX-NA-EN/pdf/ComfortBoardFS.pdf
http://www.roxul.com/files/RX-NA-EN/pdf/RockBoard80.pdf
http://www.roxul.com/files/RX-NA-EN/pdf/Residential%20Sell%20Sheets/DrainBoard_Sell%20Sheet_Eng.pdf
Rock wool panels are similar in R/inch to EPS, and have an order of magnitude higher drying capacity. (I have no idea how they compare in R/$.) OSB sheathing is also on the order of 1-2 perms, so rock wool panels would never be the limiting drying factor. With rainscreened siding properly vented that would provide the MAXIMUM capacity for drying toward the exterior, and even if it didn't meet the stringent dew point standards on R-value, it could still tolarate a 2-perm interior in that climate (but only if rainscreened.)
No matter what you do for exterior insulation, if they insist on an interior vapor barrier, rather than interior poly or vapor retardent latex, use MemBrain, which goes high-perm should the cellulose ever take on moisture, but stays low-perm when dry. |
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John7
Basic Member
Posts:114
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| 02 Feb 2012 12:03 AM |
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Dana
I'd like to provide some background to my decision and response to some of your comments.
Vancouver leaky condo:
I see at least half a dozen of these under tarps on my commute - every day for the last ten years. I've observed many "inverted gut jobs" and it’s downright scary. It’s a mix of some high-rise EFIS and a lot of low-rise stucco or vinyl/wood siding. Brick is less than 3% of residential construction here so I am not sure what reports you are reading. Having said that I agree with your statement that a VENTED rainscreen is paramount. I am also avoiding stucco since its often installed old school with tar paper/metal mesh/scratch coat. Regardless of installation style it invariably it cracks and water is sucked into the wall. (I have a person preference for (fiber cement) clap board type siding.) Based on my observations I figure a lot of the problems are due to improper or omitted flashing. That's not vapour drive, I agree, but trapping an eventual leak behind XPS would seem disastrous.
Roxul panels:
I have considered Roxul panels. They look great but this is cutting edge stuff here and my BI would insist on an envelope engineer. They are also special order from Ontario. The typical assembly I've seen is siding/rainscreen strapping/Roxul panel/housewrap/ 1/2" sheathing/stud wall. I cannot imagine how you would successfully flash a window mounted to that (back to my original observation).
Climate:
I am in a temperate rainforest. Summers are warm and dry. Winters however are very wet. It usually rains (lightly) most of the day for 5-6 days a week. The sun may not shine for up to 60 days. Temperatures hover just above freezing (32-42°F). Winter ambient relative humidity is 70% and the exterior drying potential is nil.
MemBrain:
I considered this but am not convinced it would work as intended in our humidity "backwards" climate. Any thoughts?
Thanks
John
PS check some of the gems attached. |
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John7
Basic Member
Posts:114
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| 02 Feb 2012 12:07 AM |
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Ok I figured out how to attach photos...  Flashing over rainscreen strapping?  and again?  |
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Dana1
Senior Member
Posts:6991
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| 03 Feb 2012 12:13 PM |
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Posted By John7 on 02 Feb 2012 12:03 AM
Dana
I'd like to provide some background to my decision and response to some of your comments.
Vancouver leaky condo:
I see at least half a dozen of these under tarps on my commute - every day for the last ten years. I've observed many "inverted gut jobs" and it’s downright scary. It’s a mix of some high-rise EFIS and a lot of low-rise stucco or vinyl/wood siding. Brick is less than 3% of residential construction here so I am not sure what reports you are reading. Having said that I agree with your statement that a VENTED rainscreen is paramount. I am also avoiding stucco since its often installed old school with tar paper/metal mesh/scratch coat. Regardless of installation style it invariably it cracks and water is sucked into the wall. (I have a person preference for (fiber cement) clap board type siding.) Based on my observations I figure a lot of the problems are due to improper or omitted flashing. That's not vapour drive, I agree, but trapping an eventual leak behind XPS would seem disastrous.
Roxul panels:
I have considered Roxul panels. They look great but this is cutting edge stuff here and my BI would insist on an envelope engineer. They are also special order from Ontario. The typical assembly I've seen is siding/rainscreen strapping/Roxul panel/housewrap/ 1/2" sheathing/stud wall. I cannot imagine how you would successfully flash a window mounted to that (back to my original observation).
Climate:
I am in a temperate rainforest. Summers are warm and dry. Winters however are very wet. It usually rains (lightly) most of the day for 5-6 days a week. The sun may not shine for up to 60 days. Temperatures hover just above freezing (32-42°F). Winter ambient relative humidity is 70% and the exterior drying potential is nil.
MemBrain:
I considered this but am not convinced it would work as intended in our humidity "backwards" climate. Any thoughts?
Thanks
John
PS check some of the gems attached.
MemBrain was tested for nearly a year under unvented stucco and compared to other assemblies by the Washington State University folks at their Natural Exposure Test Facility in Puyallup WA (west of the Cascades, a bit south of Seattle) with a climate & rainfall very to Vancouver, and it performs quite well- I'd even go so far as to say it's performance ROCKS in marine zone 4! See the discussions regarding Wall 2 and Wall 8. Note that even an inch of exterior foam made a remarkable improvement compared to similar constructions using poly vapor retarders. And even unvented the peak moisture content of Wall 8 with the exterior foam/interior MemBrain outperformed most other stucco clad assemblies (even vented stucco w/ interior poly), never exceeding 14%. Compare the MC probe graphs for vented/unvented stackups in the appendix. It's safe to say that MemBrain would do even better with a vented non-reservoir cladding, but that configuration was not tested. In a cellulose-insulated cavity you have the further protection of the buffering capacity of the cellulose, which is well known & studied regarding it's ability to reduced the peak moisture content of the sheathing. EIFS is almost always a disaster in a rainy climate- too many details would have to be perfect on day-1, and the reliability of "perfection" over time simply isn't possible. A YouTube search on "Vancouver Leaky Condo" yielded: http://www.youtube.com/watch?v=1bGDOmfa08I |
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John7
Basic Member
Posts:114
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| 03 Feb 2012 11:28 PM |
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I forgot to mention, MemBrain and most vapour barrier paints have not been tested to Canadian Standards and therefore are non-conforming to our code.
Too bad because MemBrain does look like an excellent product.
John |
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DickRussell
Basic Member
Posts:182
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| 04 Feb 2012 11:09 AM |
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John, great shots of how to botch a window flashing job, and this is on an EIFS repair job? Flashing over the rainscreen furring strips rather than under the WRB - what a way to ensure water getting behind the flashing! I see another lawsuit in the making. |
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Dana1
Senior Member
Posts:6991
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| 06 Feb 2012 03:10 PM |
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Posted By John7 on 03 Feb 2012 11:28 PM
I forgot to mention, MemBrain and most vapour barrier paints have not been tested to Canadian Standards and therefore are non-conforming to our code.
Too bad because MemBrain does look like an excellent product.
John
Are you sure about that? http://www.certainteed.com/resource...Canada.pdfHave you looked up the alphabet soup on top of the center column, page 2?: 4. TECHNICAL DATA Applicable Standards: • Model Building Codes: – National Building code of Canada 2005 Articles 9.25.4.2(1), (2) and (3) Articles 9.25.3.1, 9.25.3.2 and 9.25.3.3 CCMC Report #13278-B – BOCA, ICBO, SBCCI and ICC See also:
http://www.nrc-cnrc.gc.ca/ccmc/...3278_e.pdfWorst- case, poly on the interior and 2-3" of unfaced EPS (R8-R12, and greater than 1 perm) over the structural sheathing , under rainscreened siding still works pretty well that climate as a drying toward the exterior only stackup, and BETTER than the same wall without the exterior foam. With the interior poly it's a bit less resiliant to exterior moisture drives though. |
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Dana1
Senior Member
Posts:6991
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| 06 Feb 2012 03:13 PM |
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Dick- I thought those pictures were supposed to be illustrating/exposing some of the sources of the problems, not the intended solution. |
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John7
Basic Member
Posts:114
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| 07 Feb 2012 12:02 AM |
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I know there are a lot of testing standards in there (MemBrain) but I don't see one that satisfies our code requirement:
9.25.4.2 Vapour Barrier Materials...
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."
Likely our market is too small to justify the costs of testing and certification.
John |
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John7
Basic Member
Posts:114
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| 07 Feb 2012 12:06 AM |
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Posted By DickRussell on 04 Feb 2012 11:09 AM
John, great shots of how to botch a window flashing job, and this is on an EIFS repair job? Flashing over the rainscreen furring strips rather than under the WRB - what a way to ensure water getting behind the flashing! I see another lawsuit in the making. The first picture is of a 20 storey leaky EIFS tower. It was scafoldded and shrink wrapped for a year! The other two are of low-rise new construction (frame with siding). There is a second head flashing under the trim but I am not confident it is installed any better (note the lack of end dams). What exactly is the upper head flashing intended to do as installed? All are unrelated sites. |
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