I am residing a 100 yr old balloon framed house.  There was no sheathing and I am less than 5' from the property line.  So I am required by the city to add structural sheathing and gypsum sheathing.  After that I am looking at some sheathing insulation. 


I have come up with a detail to bridge the extra thickness from the sheathings, and now have to contend with the foam.  I have at most 1/2" available. 

I am planning to use DC14 drainage plane.  http://greenguard.pactiv.com/reside...ainage-mat

This would naturally be on top of the tyvek. 

I have considered a second layer of 1/4" fan fold to add a little extra since I think it will all fit. 

I am wondering if it is worth the extra trouble and if the regular fan fold should go under or over the tyvek.

I am in Richmond VA, there is batt insulation R13 in the cavities, and I am sealing interior wall penetrations from the outside as I go.

The top layer will be hardie plank siding.

Thanks

More insulation is better, so that would suggest 1/2" ISO insulation. Consider "Tyvek StuccoWrap". Rigid insulation next to the studs will do the most to stop thermal bridging.

If the batts have Kraft facers (~0.4 perms) putting foil-faced iso on the exterior is going to severely hamper the drying capacity of the wall assembly.

If the structural loading is such that you can use 1" Dow SIS as the structural sheathing you'll have R5.5 foam on the exterior, and ~ R15 for a whole-wall R (comparable to 2x6 framing.) SIS is still relatively low-perm (< 0.3 perms), but WAY more vapor permeable than foils, which are well under 0.1 perms. The risk of creating a moisture trap in the wall cavity much lower using SIS than than iso. With SIS you can also skip the drain mat.

Quarter inch fan fold is about R1, but it always has a sub-1 perm facer on it (0.6-0.8perms depending on vendor.), which in combination with the 1.1-perm Pactiv drain mat would bring the total exterior vapor permeance down to something similar to a kraft facer. If the batts are unfaced, or the facers are on the exterior, the fan-fold and drain mat combined don't have sufficient R to fully protect the stud bay from wintertime moisture accumulation during a colder than average winter, but it'll usually be OK. R3+ on the exterior would be more protective (but not required by code.)

Thanks to you both for the replies.  I lurk here alot and learn from you both.


I pulled the permit, and am not savvy or determined enough to fight with permit dept.  So I went with what they wanted.  The plywood is extraneous, building went 120 years without it.  But, it is almost all on now.  Gypsum I am installing  this weekend I hope. 

There is no room for an additional 1" without making more radical changes to the original windows than I am willing to do.  I walk the line between performance and history and find comprimises I can live with.

The batts have kraft face to the interior side, though some of it has been installed from the outside, so it does not lap the studs. 

I can get away with 1/2" total foam, I planned 1/4 of that to be pactiv.  With kraft to the interior is it worth the trouble to add the extra 1/4 of foam?  Sounds like drying wise it is something of an issue?  I cannot decide if it really adds much of anything temp wise.

Another option could be  Owens Corning foamular, which  shows a perm of 1.1 with no foil facing.  I could use that with the stucco wrap if it is really better, but I am not sure if that is near a moisture issue.

I was planning on using the pactiv as I already have a roll of tyvek, but am willing to change methods if it is worthwhile, ie stucco wrap.

Foamular is 1.1 perms at 1", and about twice that at 1/2", which is good.

Pactiv also makes half-inch insulating sheathing under it's own "GreenGuard" name as well as some box-store branded names. Their half-inch goods deliver R3, which would about double the R-value at the thermal-bridging of a 2x4, bringing the whole-wall up to ~ R13 from ~R10, and is enough to protect from interior moisture drives:

http://greenguard.pactiv.com/productfiles/product-downloads/product-sheets/greenguard-slx-sheathing.pdf

http://greenguard.pactiv.com/ProductFiles/PRODUCT-DOWNLOADS/Data-Sheets/insulative-sheathing-data-sheet.pdf

Note that they don't spec permeance for this product line, and it has facers as their fan-fold goods, so I'd hazard that they're around 0.8perms (but not 2), which would be fine. It's probably not much difference from their 0.8-perm R3 half-inch residing goods:

http://greenguard.pactiv.com/residential.aspx?xps-siding-underlayment#xps-siding-underlayment.xps-residing-board

Using that instead of the drain-mat is probably the better approach. The Hardie goods will still dry sufficiently, and 0.8 perms will protect the studwall from high moisture drives of the cement-fiber siding, while still allowing the whole assembly to dry.

BTW: Kraft facers still behave as vapor retarders even if they're not lapping the studs, and even if they have lots of nicks cuts & slashes, so you still have ~0.4 perms, but that's enough to allow seasonal drying (even when stacked up against 0.8 perm GreenGuard), whereas sub-0.1 perm foil or poly facers do not.

Take the time to split the batts around any lateral electrical or plumbing and detail it carefully for a complete, snug fill, no gaps or compressions, otherwise performance takes a real hit from convection currents inside the stud cavity bypassing part of the insulation.

Guess I should have checked on this earlier.  The greenguard products are all special order in this market as far as I know.  The gypsum goes up this weekend, so I need to get it covered, which leaves me at the big box for the foamular. 

I imagine even with standard tyvek, I want the insulation board under the tyvek correct? 

I will probably see about the stucco or drainwrap, not using the DC14 I would like some additiional method for draining.  Spent so much time getting the structure fixed from water and termites I want to give it the best possibility to dry.

Thanks again for all of your help.  The kraft paper info was very informative, I have some areas with a few splits and had wondered how bad that was.  I have been fixing all of the places the insulation had been just shoved into the cavity.

Whether the housewrap goes under vs. over the insulation board is a function of how you're flashing the windows.  See:

http://www.greenbuildingadvisor...usewrap-go

Kraft facers are usually pretty crummy air-barriers anyway, but air-tightness of the assembly is generally more important than vapor-tightness from a moisture migration point of view. Caulking the seams & edges of the structural sheathing and staggering it's seams with the seams of the gypsum sheathing go a long way toward air-tightness. Taping the seams and foam-sealing the edges of the XPS helps too.

XPS is very good at repelling liquid water, and will reject the high vapor drive of fiber-cement cladding as well, even with a minimalist drain cavity. A 3/8" full rainscreen cavity would be ideal, but with 0.5-2-perm XPS and a housewrap it'll be fine- the fiber cement can tolerate a slower drying process far better than wood siding. Making sure you get the flashing details right and making the wall assembly air-tight from the exterior will be HUGE compared to what was probably there when the termites took hold.

Greetings,

I realize you had to endure a forced solution by people who are ignorant about the subject. However I just want to post this for others to be AWARE.

Paper VB FG

Here is some info that might interest those of you want to use foil or paper faced batts.

As you know pitch is used to glue the paper to the batts. However there are some major problems with this that the manufacturer does not tell you.

First off, the pitch releases fumes, it’s a petroleum product. Those fumes are driven into the house by the heat and humidity that passes quite readily thru the FG and paper material. Another problem is that the vapor passage increases as the pith losses its oils, etc. After further time the pitch is shrinking and cracking and that causes the paper to crack. So, no vapor protection.. I’ve seen many of these installations over the years and have never failed to see an infestation of silver fish and other little multi legged critters.

This paper application also seems to exacerbates the dry rot problem when using FG insulation. When water vapor inters the FG it is condensate when it hits the dew point in the batt. You now have water, The pitch acts as a water barrier which increase the moisture content of the wood. Generally, anything over 15% is going to cause rotting.

Using these products is false insurance also because the vapor in the room passes thru the wood and then enters the batt. So your spending extra money based on a lie by the manufacturer, ( lie by omission) and probably INCREASING the amount of retained moisture as compared to an unfaced batt.

As I was going thru some papers a few days ago I came across an old test result chart that was part of a document put out by Owens Coring. I won’t try to describe all the results. But one stands out.

They tested, in their own lab, under “installed conditions”, various insulations under deferent construction methods. The one that stands out is the 2x4” /brick veneer wall sys. When you subtract the wall components “R” values the 3 ½” batt comes out at about R 5.73.

There has also been an on going battle over classifying pitch as a VB material. The older construction manuals and other related sources said it wasn’t. How ever the FG people kept putting it as a VB on the label. At some point the FTC said you can not use the term VB you can use vapor retarder. Well wouldn’t you know, not to long after the FTC reversed its self.

Thank you for considering my opinions.

Thanks again everyone for all the help.

Posted By Dana1 on 31 Aug 2011 12:25 PM
If the batts have Kraft facers (~0.4 perms) putting foil-faced iso on the exterior is going to severely hamper the drying capacity of the wall assembly.

If the structural loading is such that you can use 1" Dow SIS as the structural sheathing you'll have R5.5 foam on the exterior, and ~ R15 for a whole-wall R (comparable to 2x6 framing.) SIS is still relatively low-perm (< 0.3 perms), but WAY more vapor permeable than foils, which are well under 0.1 perms. The risk of creating a moisture trap in the wall cavity much lower using SIS than than iso. With SIS you can also skip the drain mat.

Quarter inch fan fold is about R1, but it always has a sub-1 perm facer on it (0.6-0.8perms depending on vendor.), which in combination with the 1.1-perm Pactiv drain mat would bring the total exterior vapor permeance down to something similar to a kraft facer. If the batts are unfaced, or the facers are on the exterior, the fan-fold and drain mat combined don't have sufficient R to fully protect the stud bay from wintertime moisture accumulation during a colder than average winter, but it'll usually be OK. R3+ on the exterior would be more protective (but not required by code.)

It is the better answer i found above all.

Greetings,

You are going to have moisture problems with FG no matter what you do PERIOD

Having foam on the outside can also cause moisture problems because once your vapor liquefies you have moisture against the building material.

You all add up "R" factors like it was a game. You do not have the slightest idea of what is going on in your walls or how they function. You know nothing of the material characteristics and then wonder why you have high bills. And the amazing part of it is you spend a fortune to get these problems.

If you insist on using a problem generating, inferior product like FG then use UNFACED. Attach 1/2" x1 1/2" furring strips to the out side wall, install a PERFORATED RB which will save you more energy than the FG. And how are you going to control the mold in your walls? Mold starts growing on FG as soon as it is installed. Since the RB will become your primary insulator the temperature of the FG will rise and eliminate or minimize condensation.

Don't use vinyl siding as that is another inferior product and leaks air like a sieve. Use Hardi panel or steel siding.

With all the FG generated moisture you will still attract termites. I have never heard of af a RB house attracting termites. Since there is no condensation the framing stays DRY as it should, which will be less likely to attract termites,

Run a WUFI sim on the stackup, and you'll find that exterior foam at IRC minimum levels or higher for allowing standard latex to serve as the interior vapor retarder are quite protective from interior moisture drives, eliminating condensation issues. While low fiberglass batts are low performance, if detailed reasonably and if the sheathing &/or exterior foam are detailed as air-barriers, the performance is at least "OK" (way better than nothing.)

Putting a semi-permeable hydrophobic layer such as XPS between the fiber-cement and the structural sheathing protects the sheathing from the high moisture drives of sun on dew/rain wetted siding far better than high-perm perforated radiant barrier, yet provides very reasonable (and protective) outward drying rates as long as it's >0.5 perms. Perforated RB on the exterior of the drain plane/housewrap can enhance cooling season performance as long as there's at least 1/4" between the RB and the siding (and preferably the interiorside as well) but do a cost/benefit analysis using a credible energy-use simulation tool (DOE2 or better) to see if it makes sense. Using a high solar reflectivity high emissivity exterior paint (most standard white or light colored paints are pretty good- it doesn't need to be anything too special) can be as or more effective as a high-reflectivity low-E radiant barrier with minimal or no gaps behind the siding. Using RB alone (no fiber or foam) would be a code violation.

Where there's space to design in a rainscreen cavity between the siding and the foam and the R value of the foam meets or exceeds the IRC min as long the interior side of the assembly isn't less permeable than a kraft facer, using foil-faced iso or EPS rather than unfaced rigid foam delivers most of the benefit of a radiant barrier, but s.kelly doesn't have the space.

Whatever it's other drawbacks/issues might be, vinyl siding is inherently backventilated, which is a GOOD thing, since it enhances the drying rates of the assembly (nearly as well as a rainscreen cavity.) Siding can/should never be counted on as a primary drain-plane or air barrier, and air leakage at the siding layer is a GOOD thing as ong is it's still properly shedding liquid moisture to the exterior under most conditions (if not hurricane force winds.) Providing & directing that air-leakage by designing in a top/bottom vented rainscreen cavity is even better, but vinyl siding isn't too bad in that regard.

Greetings,

I don't understand, the so called experts on this forum are advocating kraft paper FG which is known in the construction field for it's extreme problems. Makes you want to question their professionalism and knowledge.

I guess it just goes to show the predatory mineral wool manufacturers call sell any thing to anyone and not only will the public buy it, they'll defend it.

QUOTE> Run a WUFI sim on the stackup, and you'll find that exterior foam at IRC minimum levels or higher for allowing standard latex to serve as the interior vapor retarder are quite protective from interior moisture drives, eliminating condensation issues.

I don't know how to tell you this again in a kinder more gentle way, BUT if the dew point can't draw moisture from the inside it will draw from the outside. With RB you don't have to worry about moisture in the walls. Why do people insist on using sub standard prods and then spend all kinds of of time and money on trying to fix the problem?

QUOTE> Using a high solar reflectivity high emissivity exterior paint (most standard white or light colored paints are pretty good- it doesn't need to be anything too special) can be as or more effective as a high-reflectivity low-E radiant barrier with minimal or no gaps behind the siding. Using RB alone (no fiber or foam) would be a code violation

Not necessarily. It depends on several factors as to whether a light paint is sufficient. If your going to use a paint then use a paint with no carbon in it and add ceramic beads. They are almost as good as RB, and they will help your paint to last longer.

The siding spaces are usually enough to give very good performance. You know you shouldn't be making comments about a material you know nothing about. I get tired of having to constantly correct you. If you can't get your facts straight then please refrain from making them.

Where does it say in any code that you can't use stand alone RB on the exterior? In 30 years I've never ran into this.

Ceramic bead additives have little to no effect on the reflectivity, emissivity or ASTM C 518 R values. As exciting as that stuff is at the microscopic level, the anticipated thermal performance has not been borne out in real-world construction materials & coatings, despite the marketing hype (and occasional test data cherry-picking) of manufacturers.

Building codes still specify an ASTM C 518 R value for cavity fill on stick built structures (which everyone knows isn't perfect, but it's the code), and are silent on the use of radiant barrier in wall assemblies. They are not silent about placement of low-perm and ultra-low perm vapor retarders.

WUFI is a well documented extremely well founded model of moisture migration through building assemblies for specific assemblies, locations, and climates, amply vetted by empirical results.

Building codes are also not silent about exterior foam cladding vis a vis interior vapor retarency on stick built structures in s.kelly's climate and specifies a minimum ASTM C518 R value or construction type (vented cladding), at or above which the interior vapor retardency requirements can be relaxed. In s.kelly's climate R2.5 is the code min for foam-R if the siding is unvented, but vented siding alone is also sufficient. R2.5 of unfaced XPS is semi-permeable, but waterproof. As long as it's air-sealed on the exterior any XPS will be protective from exterior moisture drives, whereas perforated RB will not.

Dew points don't "draw moisture"- it's simply a temperature. If humid summertime air is allowed to leak from either side into the cavity through perforated RB or un-detailed sheathing to reach the inside face of the interior walls of an air-conditioned building that happens to be below that dew point it can condense (but more often it's simply absorbed if it's lath/plaster or paper-faced wallboard), sure. But via vapor permeation alone it won't get ahead of the air-conditioning's drying effect through 0.5-2 perms of foam.

Summertime vapor pressures from outdoor air are small compared to the vapor drives of sun on rain-wetted porous moisture-reservoir siding such as fiber cement. Those high drives are fleeting, but not so short that they can't deliver significant moisture through 5-10 perm+ air-permeable RB. But as long as the sheathing is air-tight not much moisture gets through ~2 perm foam and even less gets through 1 perm or 0.5 perm foam. If the fiber cement siding is back vented, it's even less of an issue. (You can run the WUFI sim on this if you like, but it'll only confirm the IRC's prescription. Oak Ridge Nat'l Labs has a freebie downloadable version: http://www.ornl.gov/sci/btc/apps/moisture/ibpe_sof161.htm )

Nobody here has been advocating for fiberglass or mineral wool industry, nor has recommended kraft facers, but it doesn't take a lot of searching to discover that neither is even close to being the case. Designing stackups for low air leakage, maximum drying capacity, and minimum condensation/absorption at better than code real-world (not ASTM C 518 test chamber testing) R values would be a better descriptor of my approach to stick built assemblies. It doesn't take rocket science math to get there, and IRC 2009 has made it simple even for those who can't be bother to do the dead-simple arithmetic modeling. Moisture issues in building assemblies isn't mysterious stuff. The first 90% is getting the flashing right, the second 90% is getting the air sealing right, but the last 90% (for a nice round 270%) ;-) ), is the vapor permeation stuff. But the last bit is still important- while it's not possible to make an assembly too air-tight, it is possible to make it too vapor-tight. But using some fairly simple standards for where the air-permeable insulation needs to end in the stackup it's usually possible to retain ~1-2 perms of drying capacity in either direction without risking condensation, and the wood stays drier.

Kraft facers are too low-perm for my liking and batts (of any material) are pretty wretched stuff compared to blown, but can be made to perform acceptably with a lot of detailing. Low density batts are the worst (on this forum I've even referred to R22 fiberglass as fraudulent labeling , since when installed perfectly per manufacturers' instructions they only perform to R19, and then only under the most modest of temperature deltas.) In s.kelly's stackup 3lbs cellulose, 1.8lb Spider or Optima would all work just fine in conjunction with 1/2" exterior XPS, and far better than the R13 batts he's already committed to, but that's outside of what was asked.

I'm not easily insulted, neither by "corrections", nor incredulity, nor admonishment for me to STFU. I'll continue to call 'em like I see 'em. I put a lot more stock in those who actually measure stuff than those make assertions or "corrections" without data to back it up. Clearly YMMV.