# First Post! Insulation question!



## joecaption (Nov 30, 2011)

You would also have to add jamb extentions to all the windows and door jambs.
Since you already have it all torn apart I guess it would be worth it.
Spend the time to air seal any holes where any wires or plumbing were run.
Air seal the attic, make sure there's enough insulation in the attic.


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## quackaddict (Jan 23, 2013)

I'm aware of the jam extension issue, the windows already have them and I'm confident I can remove those and add the thickness I need. The garage was a perfect training for sealing this room, took forever!

I had planned on adding more insulation to the current attic insulation once poly barrier and drywall have been added. It has an r40 installed in 1980.


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## AGWhitehouse (Jul 1, 2011)

Running a layer of foam behing the gwb would give you much more of an impact than extending the stud widths. Use 3/4" polyiso (R-5 +/-) and un-faced batts. You'll need extensions as joe said including your electrical boxes. you should be able to get mud rings for that additional depth.


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## quackaddict (Jan 23, 2013)

The rooms will receive the total redo, including electrical, so I'm not worried about that. I would imagine the foam would be used to stop air flow? 

I have been looking for air penetration in parts of the house I have opened up, from what I am seeing the stucco is doing a good job of preventing air penetration. I do like the idea of the foam board thou, should be close in r value to what I could get and would ensure minimal air flow.


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## HomeSealed (Jan 3, 2008)

Have you considered spray foam? Obviously it is more $$$, but you could get the extra r-value without all of the furring, etc. Dense packed cellulose would also be better per inch than the fg batts.


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## AGWhitehouse (Jul 1, 2011)

quackaddict said:


> I would imagine the foam would be used to stop air flow?


It's purpose is to reduce the thermal bridging of the studs. Stopping air flow is achieved through caulking and following the Airtight Drywall Approach (ADA to some).


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## quackaddict (Jan 23, 2013)

AGWhitehouse said:


> It's purpose is to reduce the thermal bridging of the studs. Stopping air flow is achieved through caulking and following the Airtight Drywall Approach (ADA to some).


So Im clear you would add it as a sheating on the face of the studs, then drywall over? That seems like an interesting approach. I would imagine you just use longer drywall screws and have at it. 

Tuck taping the foam board would help with any sealing issues you havent gotten already with caulking and such.


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## quackaddict (Jan 23, 2013)

HomeSealed said:


> Have you considered spray foam? Obviously it is more $$$, but you could get the extra r-value without all of the furring, etc. Dense packed cellulose would also be better per inch than the fg batts.


Havent asked anyone but with the small SQ ft required it may be a viable option. Worst I cans ay to a quote is no thanks.


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## AGWhitehouse (Jul 1, 2011)

quackaddict said:


> So Im clear you would add it as a sheating on the face of the studs, then drywall over? That seems like an interesting approach. I would imagine you just use longer drywall screws and have at it.


Yes..



quackaddict said:


> Tuck taping the foam board would help with any sealing issues you havent gotten already with caulking and such.


You will need to tape it to provide the required vapor retarder system.


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## AGWhitehouse (Jul 1, 2011)

If you choose the spray foam route, you can put 3/4" thickness furring strips horizontally across the face of the studs at 16" o.c. spacing. Then the spray foam can fill the voids to the face of the furring strips thus providing a similar thermal break along the face of the studs as the rigid foam board. I would only recommend 2.0lb closed-cell spray foam which provides R-6.75 per inch of thickness. It can only be installed in 2" lifts, so multiple applications will be needed.

I find open cell spray foam to be a waste of $$. At R-3.7 per inch it isn't much better (thermally) than most batt. products. With a little caulk and patience you can make a batt. wall perform equally well to an open-cell spray foam wall.


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## quackaddict (Jan 23, 2013)

Would it be a better option to attach the Foam board with some sort of OSB Furring strips then attach drywall to those strips? I could knock out a couple of birds with one stone if I attached the strips horizontally to allow horizontal drywall with 100% edges for screwing.


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## AGWhitehouse (Jul 1, 2011)

quackaddict said:


> Would it be a better option to attach the Foam board with some sort of OSB Furring strips then attach drywall to those strips? I could knock out a couple of birds with one stone if I attached the strips horizontally to allow horizontal drywall with 100% edges for screwing.


You'd be losing more room area by doing so, but it's not an impossibility. Using a 3/4" polyiso board would need 2-1/2" gp screws for 1/2" gwb attachment.


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## quackaddict (Jan 23, 2013)

Thanks for the ideas, I have been mulling the various aspects of this over for a while and am probably going to use a slightly modified approach that wont require outlet box extensions and will allow me to use 1.5 foam. I will have the extra thickness of the wall but since Im removing an inch of plaster product and replacing with 1/2 drywall, I should have a net width of only 1/2-1 inch less than the current width of the room. That I can deal with.


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## Gary in WA (Mar 11, 2009)

What is the exterior wall make-up? 
If you have wood board sheeting with tar paper, the inside face of the board is the first condensing surface because it is at/near outdoor temperature; Fig. 6, 7; http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

Putting foamboard on the inside will create a vapor retarder, which you want in your climate. However, the foam will also keep the sheathing (if any) colder, increasing the condensation there by having a lower dew point temperature. This is the “lowest bang for the buck” from a moisture standpoint, from Building Science, pp.6,7; https://docs.google.com/viewer?a=v&...iYVZpx&sig=AHIEtbRVXHNlbPlXiHMzGIwFLQF3YvmyFw

If you “piece and cobble” rigid foam board between the studs- canned foam to the boards and around the foam perimeter, the first condensing surface will be the inside face of the foamboard. Outside f.b. would be best-- though impossible for you, go with the “next best”. Similar to a wood rim joist- need to stop infiltrating/exfiltrating air movement (multiple areas with individual boards on exterior wall); http://www.buildingscience.com/docu...joist/files/bscinfo_408_critical_seal_rev.pdf

But, it will be warm enough not to worry about as you would if the f.b. was inside, on stud faces. Inside f.b. will also stop exterior solar driven moisture through the stucco (a moisture reservoir), if on the South or West side, from wetting the cavity insulation, degrading the fiberglass R-value by up to 60%; http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/90-controlling-heat.html

Inside f.b. would not stop any convective looping of the fibrous cavity insulation, where cavity f.b. does stop infiltration from the numerous gaps; http://www.buildingscience.com/documents/information-sheets/installation-of-cavity-insulation/

Then the cavity would remain closer to house temperatures (conditioned by HVAC) rather than cold enough to have condensation on the boards for a much longer time period during the year, able to mildew, mold or rot the wood framing; http://www2.dupont.com/Tyvek_Weathe...rior Insulation Building Science Bulletin.pdf

For your location; http://www.usclimatedata.com/climate.php?location=USMN0448
with an average low of Jan., Feb., and Dec. of 5*F --------- using cavity 2”XPS (R-10) the sheeting temp. at the stud side would be 34*F for condensation there at 27% Relative Humidity with 70* room temperature. With inside foamboard, temperature of the cavity (most of)/sheeting boards would be same as outside temp (5*F) and only 9% RH. Basically, the dew point changes from the boards face to the foamboard face = and a warmer/drier cavity and insulation. Adding more cavity foam raises to temps/dew points. Inside f.b. stops the air (if sealed, though ADA is best), slows the heat but gives a much colder sheeting temperature for more condensation from outside/inside air movement. 

You should add some f.b. strips (1-1/2” wide) to the inside studs/plates edge for a thermal break as said already.

Gary


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## quackaddict (Jan 23, 2013)

Gary, thank you for the post,

So I understand correctly, the addition of rigid foam in the cavities to seal them and more rigid foam over the face of the studs would provide the "best" possible for this installation? You would minimize air movement and create a thermal break on the interior of the wall?


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## AGWhitehouse (Jul 1, 2011)

Sounds like he wants you to cut rigid foam and fit it within each stud bay. tight to the exterior sheathing. Then spray foam all those edges. Then put batt. insulation with thickness to fill the remaining depth. Also adding 1-1/2"x1-1/2" rigid foam strips to the face of the studs.

Pretty labor intensive and alot of expensive foam waste...imo...plus it doesn't really provide you with a true moisture retarding system that is code worthy. Code requires a vapor retarder on the warm side in winter unless you have a continuous R-10 foam layer on the outside (for zone 5). With you in zone 6/7, that R-10 requirement is likely higher, if allowable at all. The method explained above does not provide a continuous layer and therefore does not satisfy that aspect of the code. You would still need to provide a vapor retarder (kraft paper facing) at the interior. BUT, with 2" XPS on the exterior side of the cavity, adding kraft paper on the inside would form a double class II retarder system, which is even worse for moisture issues than a continuous layer of rigid beneath the gypsum board.


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## Gary in WA (Mar 11, 2009)

Yep, AGW nailed it, at least in the description. You will have a warmer cavity for the f.g. and air-tight from the exterior air this way. ADA the drywall for air-tight from the interior. Vapor diffusion will be very slight compared to all the gaps between boards for sheathing. Is more work, lol. Is the wall 11' or 20' exterior wall? 16"oc, 14-1/2" between into a 48" sheet= 4-1/2" rip of waste to use in the attic where the roof slope meets the exterior wall...

Gary


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## quackaddict (Jan 23, 2013)

I would tend to agree...

Basically looking at gutting, adding r-15 Fiberglass, then furring horizontally to accommodate drywall (Something like 24 OC with 2x2 and 2x3) and some vertical only to allow for outlet attachment. Then go back and fit 1.5 foam between and use caulk to seal between joints. Then I can drywall over with a theoretical R in the 22 range. I would loose some to thermal bridging in the end but placing the drywall support on horizontal would minimize that.

I would probably also take the time to seal the slat board as best I could with silicone. That in itself will take forever but such is life I suppose. 

Does stucco have an air permeability? It seems like this house is very well sealed from drafts and such, I would assume it is from the continuous siding the stucco provides.


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## quackaddict (Jan 23, 2013)

Gary in WA said:


> Yep, AGW nailed it, at least in the description. You will have a warmer cavity for the f.g. and air-tight from the exterior air this way. ADA the drywall for air-tight from the interior. Vapor diffusion will be very slight compared to all the gaps between boards for sheathing. Is more work, lol. Is the wall 11' or 20' exterior wall? 16"oc, 14-1/2" between into a 48" sheet= 4-1/2" rip of waste to use in the attic where the roof slope meets the exterior wall...
> 
> Gary


One of each, the room has two exterior walls, which is why I want to insulate as best I can...


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## Gary in WA (Mar 11, 2009)

"When moisture storage effects are added, things get real interesting. Take a
brick veneer cladding that experiences rain-wetting followed by exposure to
solar radiation. The sun tends to drive moisture inward. It makes sense to put
a vapor barrier behind a brick veneer to stop this inward flow. But what
happens when you install a vapor barrier behind a brick veneer in a cold
climate? Do you also want to install a vapor barrier on the inside? I think not ?
that would be a vapor barrier on both sides. Maybe we want a *vapor barrier on
the outside behind the brick veneer and a **vapor retarder on the inside*. Maybe
we should use an insulating sheathing that is vapor semi-permeable on the
outside with sufficient thermal resistance to elevate the temperature of the
condensing surface during the heating season, and have a vapor permeable interior finish to permit drying to the interior? It sounds complicated and it is."

"So under dry-cup testing the kraft facing has a
perm rating of 1 perm, and under wet-cup testing the kraft facing has a perm
rating of 5 perms. In the winter months, when the inside relative humidity is
high, we have vapor retarder on the inside, that permits drying to the inside." Bold is mine, from: http://www.buildingscience.com/documents/reports/rr-0004-air-barriers-vs-vapor-barriers

Gary


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## quackaddict (Jan 23, 2013)

Dang stucco! Always causing me problems!

Im understanding what you are saying Gary, just trying to come up with a way to get everything done without spendin 15 days insulating.


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## Gary in WA (Mar 11, 2009)

Yet another link, lol. Paper facing "checks" moisture, Ginger vs. Mary Ann: " *Figure A:* The traditional “vapor barrier” on a fiberglass batt is actually “asphalt-coated kraft paper” that varies in vapor permeance as a function of relative humidity. It pretty much is a vapor barrier only in the absence of vapor. It really functions much like a valve that opens and closes depending on available moisture. In the winter in most older homes the interior relative humidity is in the 20 percent range whereas in the summer the interior relative humidity is in the 50 percent to 60 percent range. So in the winter a kraft-faced batt has a vapor resistance of approximately 1 perm—an interior vapor retarder on the correct side of the thermal control layer—reducing outward vapor flow. And in the summer the same kraft faced batt has a vapor resistance of approximately 10 perms—allowing the same assembly to dry inwards. It is interesting to note that latex painted gypsum board works pretty much the same way. And now we have new “smart materials” that are engineered to take advantage of the differences (2nd Generation Vapor Control Membrane)." From; http://www.buildingscience.com/documents/insights/bsi-024-vocabulary

Gary


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## Gary in WA (Mar 11, 2009)

LOL. Just do it the "easy" way as AGW said, and let us know how that works out for you. Remember... you came here...lol.

Gary


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## quackaddict (Jan 23, 2013)

Thank you gary, I do appreciate your comments! Your logic makes sense, Do you believe I would need 1.5 or 2 in the cavites to get the vapor retardant or could go with a thinner (.5 ) to seal and still have space for the fiberglass?


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## AGWhitehouse (Jul 1, 2011)

Gary does have alot of internet links and quotes, though just because it's on the internet doesn't make it right (my info included). Here in CT, code is code, and unless you can convince the local officials that the code is wrong and get a signed approval, you'd best stick to the code. Gary doesn't like code in some instances, and this is one of them. So, if you're going to follow Gary's advice then I'd check with your local official before you spend all that time and money. It would be horrible to get it all done and either have to rip it out or pay some kind of fines (don't know your laws).

I will say that buildings have been placing the vapor barrier on the warm side in winter for MANY years now and there aren't too many cases where that has caused extreme issues. Sure there are cases of mold/etc. but that usually stems from other problems with the constructed assembly (bad gutters, leaky roof, leaky siding, etc.). In your climate (in winter) the majority of the moisture is generated within the living space. Not allowing it to enter the wall cavity is of great importance (hence the code). Your studs are still directly touching the exterior cold and are not so thermally broken as to mitigate that cold within the same dimension as the foam and will have colder surfaces behind that foam layer. Those cold surfaces, left exposed to moist air, may condense the moisture laden air coming from the interior.

I always like to side on the code as it is a set of rules that is reviewed year after year and changes are usually well thought before being enacted. The same process goes for variances to the code. You must submit documentation and reasonings based on your specific project's parameters. The officials will review and make a determination.


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## AGWhitehouse (Jul 1, 2011)

Gary in WA said:


> "When moisture storage effects are added, things get real interesting. Take a
> brick veneer cladding that experiences rain-wetting followed by exposure to
> solar radiation. The sun tends to drive moisture inward. It makes sense to put
> a vapor barrier behind a brick veneer to stop this inward flow. But what
> ...


I'm wondering when your posts will start with a disclaimer that this information could very well be contradictory to the legally-required building codes...Last I checked this isn't strictly a building science theory forum...


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## Gary in WA (Mar 11, 2009)

quackaddict said:


> Thank you gary, I do appreciate your comments! Your logic makes sense, Do you believe I would need 1.5 or 2 in the cavites to get the vapor retardant or could go with a thinner (.5 ) to seal and still have space for the fiberglass?


--- If you leave the studs at existing 3-1/2" with 2" foamboard inside against the wood boards, and add the 2" f.b. strips to the studs edge inside, fill the cavity with R-13 (3-1/2") asphalt paper-faced, your total will be R-23. Code is R-21 for your location- Zone 7; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_11_sec002.htm

If you want only 1/2" of f.b. glued to the boards, you need to make the wall depth enough for the required total R-value, a problem; need full 6" for R-19 = 6" + 1/2" XPS (R-2.5) = R-21.5 but keeping thicker f.b. on the studs face without sagging of the drywall (past 2" f.b.). You could add 2" f.b to the studs and 1" of wood to get the needed 6-1/2". Getting into long screw lengths for the drywall... while still keeping the studs thermal break.

For any that I confused (sorry), Post 21 and 23 show it is acceptable to use a vapor retarder (kraft paper) and foam in the same wall. As per code; Class 1 or 2 for location Zone 7; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_6_sec001_par003.htm
And paper faced is a Class 2; http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_6_sec001_par005.htm

Gary


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## Gary in WA (Mar 11, 2009)

AGW, please elaborate: " Gary doesn't like code in some instances, and this is one of them." 

Gary


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## AGWhitehouse (Jul 1, 2011)

Gary in WA said:


> Post 21 and 23 show it is acceptable to use a vapor retarder (kraft paper) and foam in the same wall.


Above is such an example. Post 21 and 23 do not mention any lawful building codes I'm aware of, yet you suggest building methods that are generally against code practices. As a comparable example: In the Massachussetts code (A zone 5 state) it states that the exterior "barrier" must be 10x more permeable than the interior "barrier". These are the things people NEED to know. Building science is great for what it is, but when it comes to giving REAL people REAL advice, the theory is only as good as it applies to the building laws that govern.

Unless of course you're a government bank builder:whistling2:


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## Gary in WA (Mar 11, 2009)

If you added that (MA has it's own energy code, or similar, as I would have said) to the discussion at the appropriate time rather than the way you delivered it (two posts accusing me of not following code without an explanation), I could understand your two posts better; but, everyone is different. I was thinking the paper-faced was required per IRC all along (and glad you mentioned it), not even remembering that MA had their own code, rather than prescriptive. We have our own code here, also, much more strict in certain things. Hence the reason I usually say "check with your local AHJ" as you have seen many times before. The links I supplied are showing the vapor retarder (code required) is fine to use with foamboard toward the outside. But not the permanence ratings, as you said. We all need a "check locally" --- unless one checks each members residence/local code/local amendments to prescriptive codes. The science is accurate (IMO) and we will be seeing more states changing their views on what is now written as code, in the near future. Dept. of Energy has used BSC for many of their guidelines as you have seen. The "Build America" series is an example. The codes are slowly catching up to the science. We can advise members "how to" insulate/tile/build/etc. but it is up to them to get required permit, engineering, etc. All members (including moderators) advice comes with a warning; "There are no warranties, either expressed or implied, that the information on this website is accurate and reliable." found under "Terms of service" at bottom of every page. 
"Massachussetts code (A zone 5 state) it states that the exterior "barrier" must be 10x more permeable than the interior "barrier"." if you would, give me a link to that please? 

Gary


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## Gary in WA (Mar 11, 2009)

Well, I have searched for that to no avail, must be looking in the wrong places. I found; MA is under 2009 IECC with amendments; http://www.reedconstructiondata.com/building-codes/massachusetts/

The amendments don't apply to this discussion, unless I missed it; http://www.mass.gov/eopss/consumer-prot-and-bus-lic/license-type/csl/8th-edition-base-code.html

Amendment of air barrier, but not perm ratings; pp.3,4; http://www.mass.gov/eopss/docs/dps/8th-edition/13-energy-efficiency.pdf

Where did you find that at, could you post a link?

Gary


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## quackaddict (Jan 23, 2013)

Quite the first thread, seems I have found a bunch of good info and hit a nerve, appreciate the input both of you have given. I will be looking into MN's code before going forward (still demolishing) and making a decision from there.


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## AGWhitehouse (Jul 1, 2011)

Gary, I've never doubted the science and it's future implications on building practices/code. I just ask that you state that it may NOT be code compliant right along with your suggestions. Buildingscience.com is the study for future code suggestions, not actually current lawful code. You and I know not to take what is typed here and directly apply it without the right approvals/permits, but many don't. They very well might read these posts (may not even be the OP, just a random googler) and apply the methods in real world scenarios, only to find out none of it is code worthy. That's all...

I'll look for that code piece tomorrow, don't have the books now...


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## Gary in WA (Mar 11, 2009)

quackaddict said:


> Quite the first thread, seems I have found a bunch of good info and hit a nerve, appreciate the input both of you have given. I will be looking into MN's code before going forward (still demolishing) and making a decision from there.


--------------------------------------------- have any more questions, you know where we are....

Gary
PS. let us know what the inspector says...


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## AGWhitehouse (Jul 1, 2011)

Turns out the MA job I did where that code section was applicable was long ago enough to be within the expired version of the MA Building Code. They now follow IBC 2009 with amendments. Anywho, here's the section:

Excerpt from the Massachussetts Commerical State Building Code 780 CMR - Seventh Edition (Effective 9/1/08):

"*1304.1.2 Moisture Control.* The design of buildings for energy conservation shall not create conditions of accelerated deterioration from moisture condensation.
A vapor retarder shall be installed on the winter warm side of walls, ceilings and floors enclosing a conditioned space. Batt/blanket insulation with a vapor retarder attached shall be attached to the winter warm sides or faces of the wall studs, sole plates, top plates, lintels and headers at intervals of eight inches on center to prevent convection loops through the insulation. Where batt/blanket insulation is of a "friction fit" design and a sheet vapor retarder is employed, the vapor retarder shall be affixed to the interior face of the wall studs, sole plates, top plates, lintels and headers winter warm side in accordance with the vapor retarder manufacturer's recommendations.
All other envelope building materials and finishes installed towards the cooler, exterior side of the wall shall have water vapor permeance at least ten times greater than the interior vapor retarder material."

The exceptions for this section allowed for a prescriptive approach to bypass the 10X requirement, but it would have to be broken down well for any building official to have considered it. But, given that this is old code now, it's all irrelevant....


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## Gary in WA (Mar 11, 2009)

Yes, I thought it might be an older code, revised in 2007 because of the new (then) usage of exterior foamboard sheathing eliminating the (low perm) inside vapor barrier/retarder, and most of the permeability ratios (wall drying to exterior) associated with it. One still has to figure out the perm ratings of each wall, for each location- both commercial and residential -especially when not using foamboard. That’s why I needed the link from you to verify MA wasn’t under that code ruling anymore (and; that I may have missed one). 
Here is a good link showing how the “I” Codes revised their definition of a vapor retarder due to BSC’s scientific research; http://www.buildingscience.com/docu...ulating-sheathing-vapor-retarder-requirements

When using foamboard, be sure of the interior perm rating (Class); the detailed science behind it; http://www.buildingscience.com/docu...ld-weather-condensation-using-insulation/view

So after all the drama, the advice I gave earlier meets current code but not a 4+ year-old commercial code that is no longer applicable. That’s the problem with memories, wait to you get over 60 years old, you’ll be glad if you remember why you opened the refrigerator door…. lol. I was wrong once, too. Then I double-checked --and I was wrong; that I thought I was wrong… then I typed this wrong because of wrong punkshation – and spelling... lol
Maybe tomorrow I will be correct, if I don’t get up….

Gary


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## AGWhitehouse (Jul 1, 2011)

.....


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## Gary in WA (Mar 11, 2009)

I don’t see how I am suggesting *an exception* to the code, MA calls for an interior Class 2 vapor barrier (asphalt paper-faced fiberglass insulation). I agreed with you and showed how paper-faced is more permeable when needed and can use it with foamboard in a heating climate (Posts 21, 23). With a Class 2, you can fill the wall cavity with whatever acceptable type you want as long as it meets the minimum R-21 requirement. 

Then in Post 28; “ --- If you leave the studs at existing 3-1/2" with 2" foamboard inside against the wood boards, and add the 2" f.b. strips to the studs edge inside, fill the cavity with R-13 (3-1/2") *asphalt paper-faced*, your total will be R-23. Code is R-21 for your location- Zone 7; http://publicecodes.cyberregs.com/ic..._11_sec002.htm

For any that I confused (sorry), Post 21 and 23 show it is *acceptable to use a vapor retarder (kraft paper) and foam* in the same wall. As per code; Class 1 or 2 for location Zone 7; http://publicecodes.cyberregs.com/ic...001_par003.htm
And paper faced is a Class 2; http://publicecodes.cyberregs.com/ic...001_par005.htm

Another link showing the variable permeability of the kraft-paper; http://www.buildingscience.com/documents/insights/bsi-060-joe-south-assemblies

Or are you talking about something else here? He is covering it with ½”drywall, I hope that goes without saying.

Am I missing something, or are you? What exception to the code am I encouraging? 

Gary


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## AGWhitehouse (Jul 1, 2011)

Can't find anywhere the new code (or old the IRC even) specifically states against double vapor retarder (Class I or II) walls, but I still will never recommend doing so....Kraft on the inside (class II) and 2" XPS foam (class II) against the sheathing is not recommended by me. Do as you wish, but that's my professional opinion.


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## Gary in WA (Mar 11, 2009)

AGWhitehouse said:


> Can't find anywhere the new code (or old the IRC even) specifically states against double vapor retarder (Class I or II) walls, but I still will never recommend doing so....Kraft on the inside (class II) and 2" XPS foam (class II) against the sheathing is not recommended by me. Do as you wish, but that's my professional opinion.


------
Thanks for the feedback! I agree; a Class 1 (plastic, foil-faced, etc.) on the inside with *thicker *foamboard on the outside is very dangerous. http://www.buildingscience.com/documents/insights/bsi-058-parthenon/

Using the "dry cup" measurement for any of the standard Class 2; kraft-paper, plywood, OSB, and 30# builders paper, can be used with foamboard. Using the "wet cup" perm rating, (because) they all allow more moisture to pass, the wetter they get- changing from a Class 2 to a Class 3. Standard framing in cold climates; builders paper, foamboard, OSB, fibrous batt insulation with paper facing, drywall. Perhaps the different permeability rating cup-- "use" is confusing, they tend to mix the two testing types in the same sentence, lol.
http://energy.gov/energysaver/articles/vapor-barriers-or-vapor-diffusion-retarders

Gary


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## Nailbags (Feb 1, 2012)

Just to stir the pot a little bit more. I just got done doing a mold abatement on a newer home built 12 years ago. Mold behind the sheetrock on all exterior walls. 3 Mil plastic vapor barrier over unfaced FG. now fast forward to today not to use plastic 3 mil as a vapor barrier just sheetrock and go over with mud tape and texture then two coats of PVA as the vapor retarder. Go figure?


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