I'm trying to decide between using a vapor retardant primer, or putting up poly in a dense pack (dry) cellulose assembly.
The structure is this Vinyl->Housewrap->XPS (anywhere from 1/2" to 1")->Housewrap->OSB->Densepack->1/2" Drywall. I know its an odball assembly, but much was done before I took over the project. Because I live in Milwaukee, WI, I'll need to put up a Class 1 or 2 vapor retarder. I'm using airtight jboxes, cans etc.
Any opinions?

NEVER use poly vapor retarders with cellulose in a sub-9000HDD climate! Poly far more likely to create a problem than to solve one. Any impefections/penetrations of the poly allows air-transported moisture in where it will be buffered/stored by the cellulose, but poly doesn't allow sufficient drying capacity for the moisture to get out. Moisture gets in by any number of factors, but leaves primarily via vapor diffusion. A sheet of poly with a bunch of holes in it is still an effective vapor barrier for blocking drying, but the moisture can & will get in via air leaks.

Whether you need to use vapor retardent latex vs. standard latex (Class-III vapor retarder, which would be preferable), depends on the relative R-values of the exterior foam, and whether there is a rainscreen air gap between the vinyl and the housewrap. See:

http://www.greenbuildingadvisor.com/sites/default/files/Table%20402.5.1.jpg

http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barriers

If you provide a 3/8" air gap/rainscreen between the vinyl & wrap, that qualifies as a "vented cladding", and any foam you install is just gravy, from a moisture tolerance point of view. Vented cladding maximizes the ability of the assembly to dry toward the exterior, and minimizes the risk of rain-intrusion. An inch of XPS raises the temperature of the structural sheathing, minimizing the number of hours it spends below the dew point of interior air, and is still vapor-permeable enough to dry toward the exterior.

With only an inch of XPS there will be some amount of time where the sheathing will be below the dew point of the interior air, but but the buffering capacity of the cellulose would take care of that if it isn't locked-in by a highly vapor retardent interior or a minimially-drying exterior. The number of hours in a season where that condition is true are limited though. See Figure 10 in this document- assuming your studs are 2x6" your stackup is probably closest to example 2a, but with high-buffering low air-movement cellulose rather than breezy standard-density R19 batts:

http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

The simulation is for Minneapolis, which is ~10F cooler than Milwaukee in January/February, so move your curve up 10F, leaving the interior-air dew point alone.

Using dense-pack cellulose as a buffer and adding a rainscreen gap to maximally dry the sheathing should be more than enough, but the XPS gives it even more margin.

Print out the articles (including the exceptions list for climate-zone 6) if you need to make the case to the code inspectors. Whenever it can be built/tweaked in such that you can get by with a class-III vapor retarder your studs & sheathing will thank you 50 years from now. If the vinyl isn't already up, add a rainscreen gap. But worst-case, stick with the vapor retardent latex, stay away from poly or you'll be asking for it.

You might find the comments about vented cladding here useful:

http://www.finehomebuilding.com/item/11858/the-best-way-to-ventilate-siding/page/all

"Ventilate to dry your siding and sheathing

The reason to vent is not primarily, or even tertiarily, to deal with rain penetration but to encourage the through flow of air, ventilation, which encourages drying. Ventilation of claddings over moisture sensitive substrates and/or moisture sensitive claddings (eg wood!) is a critical aspect of durability and in-service performance.

There is no evidence in physics or field experience that supports the contention that vents at the top of walls under soffits or projecting parapets cause any performance reduction. In fact, all of the evidence supports the opposite. When we did a big study for ASHRAE a few years ago, we had two identical walls, one with vents top and bottom and one with vents only at the bottom. The top and bottom vent was consistently drier, and was able to dry out injected water much more quickly. Skeptics said "that is because your walls may not have been built identically the same". So we open the top vents on the bottom vent only wall, and sealed the top vents of the top and bottom vent wall for the 3rd year of the study. The results were absolutely replicated: vent top and bottom far superior to vents only at bottom. The physics are obvious. You MUST have through flow to get the benefits of venting. Venting at one vertical or horizontal line does not provide any significant benefits. Widely spread vents provide significant benefits."

Other comments from other sources point out that vinyl siding is inherently vented, with sufficient backside air space and air movement that it doesn't need additional rainscreen, so you're probably good to go without adding any space- you already meet the allowable exceptions to the Class-II vapor retarder requirement.

Perfect. The vinyl is up, and so I don't think I'll be able to get the inspector to bite on a class III retarder.

One question, do different paints/primer brands that are classified as retarders have different perm ratings? If so I could purchase one with a bit higher permeability.

They do have different perm ratings, but that rating is only valid for the specified thickness. To meet the letter of the code class-II begins at 1.0 perms. Applying it at the thickness where it meets the 1.0 spec is hard to verify without cutting a sample and actually measuring it carefully, but the average can be estimated by gallons vs square feet of coverage. Try to keep it above 0.5 perms if you can. At least with back ventilated siding on the exterior your exterior drying capacity will be pretty good, and with the foam & cellulose the amount of water making it to the sheathing from either direction will be pretty limited.

Dana
You seem to have a better handle on this issue than I do...by alot. lol On this topic I would like to give you my stackup and see what you think. Its somewhat similar in ways.
My climate is central Maine.
My walls are outside a post and beam frame.
My walls are 2x8 w 2x4 staggered studs on 12
My interior walls are rough shiplap pine-no way to make it very tight. (like sheetrock would be for instance)

Im putting from inside out--- shiplap-vapor barrier-staggered studs with dense blown cellulose-osb-housewrap-cedar clapboard

Im thinking the vapor barrier inside is going to stop the air-heat leakage/condensation issue and the housewrap will let the wall breath some. ? Nuts ? OK ?

Anyone............

Mark

As a side note...I had a discussion with a guy last month whos up on this stuff who said replace the outer house wrap with good ole fashioned #15 tar paper to allow the outer wall to breath a little better which made sense to me.
I wonder what others think on that too.

Mark

Posted By EastMark on 26 Aug 2010 05:41 AM
As a side note...I had a discussion with a guy last month whos up on this stuff who said replace the outer house wrap with good ole fashioned #15 tar paper to allow the outer wall to breath a little better which made sense to me.
I wonder what others think on that too.

Mark

But he's not exactly correct.  15lb felt has a permeance of ~ 5 perms (slightly vapor retardent) when relative humidity is low, and increases with increasing humidity to ~50-60 perms (highly vapor permeable).  It also wicks and stores moisture (albeit not quickly), and cannot be made air-tight.

Except for those that are designed to be vapor-retarders, housewraps start at ~30 perms (highly permeable) on the low end, and many products are higher.  Housewraps don't wick or store moisture, and can be made into a reasonable air-barrier when detailed properly. Some may pass liquid moiture if contaminated with surfactants, but most won't pass much.

While there's nothing wrong with 15lb felt and it works well, there's no evidence that it's a superior product from a vapor diffusion/moisture control point of view.  There have been rumors for decades about material/chemical incompatibility issues between Tyvek et al and siding or sheathing leading to failure of the housewrap/siding/siding, but I've never seen it, and tend to view those as urban-myth & mis-attribution.

I'm not convinced that housewraps are inherently better either in "typical" installations.  The success or failure of either depends on the quality of the installation, and the roles it's designed to play in the structure, but on it's ability to release water vapor from the assembly (breathability) it has a slight edge over #15 felt except when the sheathing temperature is near the dew point of the outdoor air (when drying is already much hindered for both.)  Under average/moderate humidity housewrap wins the vapor-permeabilty contest.

Still, the presence or absence of back-ventilation of the siding has a FAR greater effect on the capacity of the assembly to dry than whether the drain-plane layer is housewrap vs. #15 felt. 

I certainly wouldn't "...replace the housewrap..." in this house, which already has the siding installed!

One (of many) web references on the topic from people who actually measure & test stuff:

http://bct.eco.umass.edu/index.php/...-barriers/

Good stuff Dana ! I'll stick with the wrap for sure.
That being said does the stackup look best with the vapor barrier to stop heat escape thru the shiplap ?
I realize thats a trade off where I loose the cellulose ability to help control moisture in the wall itself.
Whats your thought? Im afraid there could be air/heat leakage without it that could cause condensation.
 Im refering to the actually barrier behind the shiplap which is the interior sheathing.

Mark

You've kinda hijacked the thread, but...

OK...

OSB sheathing will run 2-5 perms, variable with humidity. In a vapor flow-through design with no exterior foam to raise the temperature of the studs, you'd want the interior to be air-tight, and with a perm rating similar to or lower than the sheathing, but not so low that it limits the ability of the assembly to dry toward the interior. Interior poly is too-tight and counter to the recommendation of some cellulose manufacturers (they won't warranty the stuff if you use vapor barriers.) You might use housewrap on the interior under the shiplap and give it a shot of vapor retardent paint to bring it down to about a perm, and let the cellulose buffer things.

Or, in a double studwall, locating a poly vapor barrier between the two walls will work. The poly would be halfway through the R, and the average winter temp of the poly in a central ME climate would be above the dew point, even though it would drop below that during cold snaps whenever the daily average temp was 0F or below. The outer studs & sheathing would be isolated from the humidity of the interior air, and dry toward the exterior, and the interior studs would be warm enough to be protected (with a little help from the cellulose.) Even with dense-pack you'd still need an interior air-barrier though- shiplap will be too leaky, and impossible to seal. A housewrap between the studwall would be one easy method of achieving that. If you can find one with a perm rating under 10 (but not lower than 1) the humidity-cycling of the interior cellulose would be lower. Dense-packing around a sheet of poly between the studwalls could be tough, but if you mounted some bag-blowing-mesh in there it would be strong enough to keep the poly from getting blown out. (Wet-spraying at lower density might be easier/cheaper.)

Dana1
"There have been rumors for decades about material/chemical incompatibility issues between Tyvek et al and siding or sheathing leading to failure of the housewrap/siding/siding, but I've never seen it, and tend to view those as urban-myth & mis-attribution."

I have seen it as have many others; it is certainly NOT an urban myth. It seems to occur when unprimed wood siding is installed over Tyvek when the tannins in the wood apparently eat away at the chemicals in the plastic wrap. I have talked to Tyvek reps about it; they are very aware of the problem. Some have claimed it has "been fixed"; I don't know that to be the case. Doesn't happen with 15# felt. It is not an issue (as far as I know today) when the Tyvek is separated by a layer of foam. I don't know if it is an issue with a strapping rainscreen installed over the Tyvek, but since in that case the water would not be trapped and could run off, should not cause the problems we have seen.
Bob

That's the best explanation I've read yet- thanks for jumping in!

Backpriming siding has been pretty much standard practice for at least a couple of decades now, which may be how I've missed it. To be sure with a rainscreen there's no leaching of tannins, even if the siding were left bare, since there's no direct contact. (But wood siding should still be at least primed IMHO, even with rainscreen, to promote even drying and minimal warping.) If tannins are the issue there's probably some risk of interaction between sheathing and housewrap as well. Have you seen that? (To be sure there's a higher likelihood on the siding side of the wrap.)