Where is the project located?

How cold was it and what was the likely source of moisture for the frost in the wall cavity? How large an area does the frost cover?

A kraft paper facing is not always as effective as polyethylene because the paper must be sealed along all of its edges and overlapping the edges on the studs is sometimes a poor substrate for drywall.

Here is a link that might be useful: good link

Most winter builds try to get the building insulated as soon as possible, for several reasons, most notably, to salvage the temporary heat for workers comfort.. Typically, when they need to be heated to be worked in, it is done with some sort of auxilliary heat source, which can be a temporary furnace, either electric or gas, or some other kind of fuel based open burner. This exacerbates the frosting in the stud cavities and rim joist cavities. The lower on the structure, the more frost you will see..

Ok, you know why it happened, whats the fix..

Other than tearing out every bay, sweeping out the frost, and reinstalling, there is no fix.

As Renovator said, kraft facing is a poor vapor barrier, In fact, its not a vapor barrier at all.. its a vapor retarder. But then again, so is a sealed perfectly installed polyethlene sheet.... a vapor retarder..

Most of the builders here, if using structural sheathing in the outside of the building, are installing an inch of foam onto the sheathing initially..

The project is located in Northern Montana so temperatures are very cold (daytime highs in the 20's). The frost occurs because the warmer interior air hits the cold osb and condenses. It covers the entire bay where I have a batt. I have a handful of bays that only have fiberglass through 2/3 of the bay and whats NOT insulated is frost free and dry. Interior humidity levels are monitored and are consistently between 30-40%, so we dont have an unusually high level of moisture inside. The Kraft paper is overlapped and stapled but not sealed with anything additional.

Ok so I have resigned myself to taking it all out and letting it dry. We are burning a Quadrafire 7100to keep warm. I have no siding on yet, so if I tear the fiberglass off and dry it out will foaming the outside keep itfrom happening again? Can I put the foam over my Tyvek? I have NEVER seen this done on any project around here.

You are in Montana and have never seen rigid foam panels used on the exterior side of house sheathing? How are typical houses insulated and to what value? Just curious.

Thermal bridging is a significant issue for a properly insulated house, I believe, for which the external sheathing, along with house wrap, is a major remedy.

Your comments make me curious about how houses are built in your area.

Good luck with your project.

I meant they were spraying 1" of foam on the sheathing/studs, not applying sheet goods.

When you strip the insulation, I would immediately brush/sweep out the stud cavity and remove the residue. If you dont, then what will happen, is the frost will melt as the condensing surface warms, then it will be driven even deeper into the sheathing thru diffusion and absorbsion.

The residue left on the floor, will only have one way to exit that house, and that will be to return to a gaseous state, and either find another condensing surface inside the structure, or you flush the house every couple hours and send the moisture vapor outside.

its a viscous circle sometimes, keeping moisture under control throughout a winter build. I would definitly do a sheduled "flush" of the air during the drywall process. It only takes a few minutes..

Use the 20 degree outside air to dehumidify. its the best mechanism you can have.

Dont worry if your outside RH is 80%, thats at 20 degrees(or air temp) , when that air warms, it will be 10%rh or less, and facilitate your drywall mud curing much better and more efficiently than adding heat or dehumidifyers..

Unless there is another vapor barrier (and there should NOT be one) outside the sheathing just leave it alone.
The frost will eventually melt or sublimate and escape outwards through the siding.

In a cold climate--and that's what all of Montana is rated at by Building Science Corp.--you don't want anything other than a vapour retarder on the interior.

Of course, exterior XPS board of 1"-2" or ccspf on the inside of the sheathing will virtually eliminate condensation in the wall cavities.

As pointed out above, temporary heating, typically by moisture laden propane, will increase the humidity in the house under construction, leading to the frost you're finding. Plus that's exacerbated by the fact there is no drywall on yet.

I suppose if I had checked all the homes I built in winter for the same frost I would have panicked too. But there were never any subsequent problems that I knew of from the bits of moisture "trapped" in the wall cavities.

If you read to the bottom of renovator8's link you will see this comment by building scientist Dr. John Straube: "The question comes up, have we seen diffusion-related building failures? And the answer is, very few--maybe in rooms with a swimming pool. Assuming that the vapor came from the inside, you would have to have a very high load before you would see a problem. I think that solar-driven vapor is much more important. The moisture is coming from the other side of the assembly."

Here's the way I build with XPS on the exterior in a cold climate that also becomes a hot climate with summer temps in the low 30s C. (I spend more for AC than heating.)

Here is a link that might be useful: Building in a Very Cold Climate

I tend to agree with Worthy, (how do you address the structural bracing in the walls with foam sheathing? Metal braces? 1x4 let in braces?? Solid sheath the corners with 1/2 and then add 1/2 foam over? Whats your SOP worthy) If you hadnt stripped the batt, you wouldnt have known any better.. Im guessing there are thousands of frosted stud cavities across the heating belt right now, for one reason or another..

Best structural bracing is 3/8" plywood everywhere nailed for shear. OSB is another reasonable substitute. A lot depends on what seismic zone you are in and/or the degree of unified bracing desired.

After making a lot of calls all day and talking to practically every inspector and builder in our builder's association, I have come to the conclusion that I just happen to find something that likely happens in every winter build. I reacted a bit hastily and tore our about half the house to dry out the bays and fg. I'm coming off a week of paying and arm and a leg for a basement remediation company to encapsulate our crawler, so I think I was just a bit paranoid about ANY moisture.

Thank you for the link Renovator8! I had read a few other articles that guy has written and he makes a lot of sense. I definitely fall into the old school category. I have some major concerns about sacrificing the health of your structure simply in the name of energy efficiency. I just don't think having an energy efficient home should trump all else requiring expensive air exchange systems to fight what a well designed and considered home is capable of doing on it's own. Every mold mitigation company rep I spoke to today says they only really see mold issues in homes that are way too tight, and most importantly ones that have vapor barriers in the ceiling.

Virgilcarter- Every house I have worked on in Montana or Wyoming has basically the same insulation design. Usually about R20 batts, OSB sheathing (plywood only on really high end stuff sometimes), and Tyvek wrap. Usually no vapor barrior inside if using faced batts. Only one guy today has heard of anyone ever using rigid foam on the exterior and that was in a town on the other side of the mountain two hours away that has horrendous and constant wind, and not in an area known for their quality builds.

worthy-I too have the same question as rollie? Where do you get your sheer from? Are you putting an additional sheathing over the XPS? What do you do around the windows? Envisioning and inch or two of XPS then sheathing...would I have to use a 6" screw to hold my 2" window trim? Or bucking out your window to sit flush with the exterior?

You may have the wall assembly backwards, if I understand your description above. While there are many options for energy efficient exterior stud walls, here's one of the common "high performance" assemblies:

Wall sheathing (ply or OSB) is fastened directly to the outside face of 2 X 6 studs and nailed according to desired/required shear values. The entire structure is fully sheathed this way.

Stud cavities are filled with one type of insulation or another (fiberglass batts, celluose, or spray) and the interior finish gyp board (or other material) is installed on the interior face of studs. Latex or enamel paint acts as a vapor barrier.

House wrap is then applied with taped seams to the exterior face of the sheathing. This forms the moisture line for water that may get behind the exterior finishes. Rigid insulation panels are fastened to the sheathing exterior to the house wrap. Some sources recommend double layers of rigid insulation with staggered seams (this seems overkill to me unless in very cold zones where thicker rigid insulation is needed).

Depending on thickness of the rigid insulation, strapping may be attached thereafter for use in installing nailable siding. Strapping, if used, also creates an escape path for water which may penetrate the siding, and allows for drying of damp siging.

The external rigid insulation helps stops thermal leakage (along with proper sealing of sill plate, top plate, rim joists, etc).

Just one approach, among many, to a "high performance" wall assembly.

I'm sorry my response was not clear, but that wall assembly is exactly what I do, except for the rigid foam. I am intrigued by the idea and intend to research it a lot more for the next project. My instinct tells me that the foam would prevent any moisture or condensation that may occur (because interior vapor barriers will never be perfect)in your walls from escaping. Thank you for the responses!

Here's one good reference source for high value exterior wall assemblies from the buildingscience.com:

http://www.buildingscience.com/documents/information-sheets/high-r-value-wall-assemblies/high-r-wall-02-advanced-frame-wall-construction?topic=resources/high-r-value-wall-assemblies

As pointed out above, shearing methods depend on local seismic/wind conditions. I've used 2"x6" bucks at the midpoint of the framing around both floors. Thermally, it would be more efficient to have used OSB sheathing under the XPS.

I've been seeing XPS sheathing here for about the last 20 years. Subdivision builders have been using it extensively to cut costs. With exterior sheathing, they can use 2"x4" dimensional lumber and high density batts to achieve ever increasing Code minimums. Oddly, it's still unusual to see infill builders, even of homes in the stratospheric heights of $10 million +++, use XPS.

What do you do around the windows?

Move them out far enough to accommodate the thickness of the sheathing. With just one-inch of foam and no OSB, the 6" jambs from the window suppliers fit fine. The masons have to use slightly longer nails to secure their ties into the framing. One thing: you have to take care on starting the first baseplates to leave enough room for the foam. I would make this a prominent part of the architectural drawings so it's not missed by the carpenters on site. (Don't ask!)

The following from Building Science Corp. illustrates four common ways to layer the insulated sheathing. As I mentioned in my first post, I don't use housewrap, but let the masons apply their brick paper as per usual over the foamboard. This is behind masonry with a gap of at least one inch.

Here is a link that might be useful: Guide to Insulating Sheathing

This post was edited by worthy on Mon, Jan 7, 13 at 19:11