I just finished reading the article "Can Exterior Foam Insulation Cause Mold & Moisture Problems" from GreenBuildingAdvisor, and am now concerned about the home I am finishing. I purchased this home with the structure nearly build. The wall construction is 1" of XPS (3/4" in some spots)->Tyvek House Wrap-> 3/4" OSB. I intended to insulate it by spraying 1" of ccSPF (Medium Density Icynene) and either batt or cellulose. For the exterior, I was going to wrap it again with housewrap, as the foam has degraded in the sun a bit, then side it with vinyl. After reading the article, I'm concerned that moisture could be caught, and not be able to dry, as there will be XPS one side, and ccSPF on the other. Am I wrong here? I should mention that I'm in a Cold Climate.

Don't sweat it- the vapor retardency of 1" of 2lb Icynene is over 2 perms, as is the OSBitself, and 1" XPS is only about a perm. Putting the 1" XPS on the exterior keeps the OSB warmer treating it to conditions almost a full climate-zone more mild, so it spends fewer hours below the dew point during the winter. The whole XPS/OSB/Icynene stackup is still over half a perm, which is permeable enough to allow stud cavity to dry toward the exterior. Housewraps are air-barriers, but highly vapor permeable, and have no effect on the vapor profile of the structure. Vinyl siding is neither a vapor or air barrier, with no moisture trapping effect.

The article was talking about foil-faced iso , which has a perm rating of about 0.03 (less permeable than 6 mil polyethylene), which effectively blocks drying toward the exterior, and requires a drying-toward-interior stackup. To get away with drying toward the interior in cold climates the R-value of the exterior foam has to be high enough that the OSB sheathing stays above the dew-point of the conditioned space air during the winter. If one assumes the interior air is maintained between 30-35% relative humidity and 68-70F, the dew point temp is ~ 37-40F. Designing the stackup for a ratio between exterior R and stud-cavity R such that the cold edge of the studs are above 40F on heating "design day" (the typical coldest 1-3% of heating season binned hourly weather data to which heating systems are designed), the moisture of the OSB will stay reliably out of the rot & mold zone without an inteior vapor retarder. eg. If in your local climate heating systems are designed for -10F, the 40F point within the assembly with 70F interior air is half the way through the insulation, so 50% of the total R has to be outside the sheathing if it is foil or poly faced.

With XPS at 2" or thicker it drops well below 1-perm, and you have to balance the vapor retardency of the interior to keep the sheathing dry. But at only an inch, you're good- the sheathing willl dry toward the exterior.

With batt or cellulose on the interior side of 2x6 studs with an inch of Icycnene you're looking at 4.5" of fiber, or ~R15-R17 of fiber, with ~ R5+R5 of foam to the exterior for a total center-cavity R of ~ R25-R27, or 37-40% of the R to exterior of the fiber. If the interior surface of the Icynene is below 40F at heating design temp at your location, with batts you'd need to be very careful to air-seal, and to have kraft-facers or other vapor retarderslike poly sheeting to make it substantially more vapor-retardent than the exterior stackup, so that the humidity within the cavity stays below that of the conditioned space air, or you will suffer periods where the outermost fiberglass get condensation, during which their R-value drops dramatically. If the number of condensing hours are substantial you could also run into mold issues with water dripping/accumulating at the bottom of the stud bays.

With cellulose you can tolerate more condensing hours at the fiber/Icycnene interface, since micro-condensation gets wicked harmlessly into the cellulose & redistributed, without losing R or creating mold conditions. Because it holds onto the water over time, it's important not to use a strong vapor retarder like poly sheeting on the interior in your stackup, to allow the cellulose to release the seasonal buildup toward the interior as well as the exterior during the shoulder & cooling seasons, but some amount of vapor retarder is useful. In much of the lower 48 painting the interior with standard latex (2-3 perms) would be sufficient, but in the coldest parts of the upper midwest vapor-retardent latex primer (~0.5 perms) might be called for.

You've given me a lot to think about. And here I thought that the walls would be easier to get a handle on than the roof.

This house is such an oddity, as so much of the work/decisions were done before I got there. The XPS was up, but for a very long time, unprotected. The builder used 1" in some places, 3/4" in other places. There is a bump out that has none at all! The panels are weathered and degrading. I don't want to have to count on them for anything at all at this point. I'm treating them simply as bonus R.

From reading your post, it sounds like the the XPS should not greatly inhibit outward drying, which is nice. The part that confuses me is the potential issue with interior moisture. You mentioned that there is a chance that moisture could condense on the interior side of the Icynene. Would that not be able to dry to the exterior?

It seems that my choices here are keeping the moisture out of the wall cavity, or ensure that if it gets in there, that there not condensation issues. I guess I just don't know what the likelihood this situation is, in practice. I'm using (close to ) air tight j-boxes, airtight can light housings, etc. I wasn't planning on using a vapor-retardent latex primer.

Being that the whole point of me doing a "scratch-coat" of ccSPF was to seal air leaks, maybe doing a scratch coat of open cell would be better, since it is more permeable? I know there must be a reason why I didn't consider that originally, but It escapes me now.

Posted By beckkl on 28 Jun 2010 12:55 PM
You've given me a lot to think about. And here I thought that the walls would be easier to get a handle on than the roof.

This house is such an oddity, as so much of the work/decisions were done before I got there. The XPS was up, but for a very long time, unprotected. The builder used 1" in some places, 3/4" in other places. There is a bump out that has none at all! The panels are weathered and degrading. I don't want to have to count on them for anything at all at this point. I'm treating them simply as bonus R.

From reading your post, it sounds like the the XPS should not greatly inhibit outward drying, which is nice. The part that confuses me is the potential issue with interior moisture. You mentioned that there is a chance that moisture could condense on the interior side of the Icynene. Would that not be able to dry to the exterior?

It seems that my choices here are keeping the moisture out of the wall cavity, or ensure that if it gets in there, that there not condensation issues. I guess I just don't know what the likelihood this situation is, in practice. I'm using (close to ) air tight j-boxes, airtight can light housings, etc. I wasn't planning on using a vapor-retardent latex primer.

Being that the whole point of me doing a "scratch-coat" of ccSPF was to seal air leaks, maybe doing a scratch coat of open cell would be better, since it is more permeable? I know there must be a reason why I didn't consider that originally, but It escapes me now.

If the air in the fiber insulation is at the same humidity as the conditioned space air, the closer it is to the exterior the colder it is, but the humidity remains the same.  When the interior face of the Icynene is cold enough, the air in fiber layer will condense there.  It can only dry toward the exterior slowly, since it is only semi-permeable.   If the stud cavity is air-tight with a vapor retarder the interior, the average humidity will be somewhere between the colder-dryer exterior air and the warm humid interior.  It's balancing act between the vapor retardency of the exterior foam & OSB layers, and the interior wallboard/paint/vapor retarder levels.  If the condensation temp of the cavity air happens anywhere from the Icynene out to the exterior it's not a problem, because there's no air from which to condense air, and the vapor permeability of the XPS is high enough that the OSB will dry toward the exterior.  But you can't let condensation happen regularly within the fiberglass without the potential for problems.  Cellulose is more forgiving, since the only effect of a temporary condensing conditions in some layer of cellulose is that the cellulose starts to accumulate moisture. As long as there are far more hours in a month where all of the cellulose is above the dew point of the stud cavity air, the buildup isn't significant- it can dry in either direction.  Cellulose will also wick away any condensation that occurs on the framing lumber, reducing the potential.

An inch or three of open cell is far TOO permeable to protect the OSB from accumulating water over the winter without interior vapor retarders.  Even if you went with a full cavity fill of open cell you'd still need to use vapor retardent interior paint or poly sheeting to ensure that the studs & sheathing stay dry enough.  But poly sheeting can become a summertime condensation issue in an air-conditioned house during extended periods of outside dew points in the 70s.

What is your ACCA or ASHRAE heating design temp?  If warmer than -5F you'll do just fine with either wet-sprayed celluose or dry dense-packed cellulose and standard latex.  In Milwaukee  or Chicago you'd be fine, but Duluth or Fargo, reduce the interior-side vapor retardency to 0.5 or less using vapor retardent paint or other.

According to this page, the design temperature is -15 for Milwaukee.

It sounds like choosing something other than batt will give me some peace of mind (or go with the vapor retardent primer). The problem is that the dense-pack guy doesn't like using the webbing to blow, and I'm not wild on the hole method. I never received bids from wet-spray guys, so maybe I'll take a look at that.


Does anyone ever do 1" of ccSPF, then finish with 4.5" of ocSPF? Waste of money?

IMO, most analyses of wall moisture miss a major point - all houses have a significant amount of air flowing through cracks (not through the materials themselves). Much of the time, this air will contain enough moisture that it will condense out at some point in the wall cavity (do you have Winters below 40F?) . This will happen continuously for months (ie, no chance to dry and long enough for mold to grow). Where does this liquid water go? Note that none of this depends on wall material's vapor or air permeability . Experts say that this issue is 100x more important than water vapor or air flow through the materials (very little of this occurs so why does all the analysis focus on it?).

What reduces this problem?

make sure that the house isn't pressurized at any point (in the Winter)
make the house as tight as possible (1/2 the airflow is 1/2 the water)
keep interior humidity low (30% is 1/2 as much water as 60%)
use materials that aren't damaged by water and don't support mold (like foam), especially on the cold side
absorb it (like cellulose does to some extent)
make sure overlaps and holes dump water in the right direction (away from materials that will mold)

Posted By beckkl on 28 Jun 2010 02:07 PM
According to this page, the design temperature is -15 for Milwaukee.

It sounds like choosing something other than batt will give me some peace of mind (or go with the vapor retardent primer). The problem is that the dense-pack guy doesn't like using the webbing to blow, and I'm not wild on the hole method. I never received bids from wet-spray guys, so maybe I'll take a look at that.


Does anyone ever do 1" of ccSPF, then finish with 4.5" of ocSPF? Waste of money?

The ocSPF has slightly lower R than cellulose, and zero hygric buffering capacity, which means any vapor diffusion that makes it in ends up in the structural wood, not harmlessly stored in the the insulation as it would be with cellulose.  In an already air-tight wall, the extra foam becomes a waste of money if it's more expensive than spray cellulose.

With a design temp of -15F wet spray cellulose, air-tight wallboard and interior vapor retardent latex should get you there.

As jonr points out, air transported moisture is usually the larger issue in most construction.  But the air leakage through an inch of ccSPF is zero, which is at least half the point of flash & batt.  Then it becomes only an issue of how much convective air-transport there is through the fiber from unintended air leaks on the conditioned space side into the wall cavity, not flow-through issues completely through the wall (which would deposit large amounts of moisture at exfiltration points.)  With cellulose that air flow more than a full order of magnitude lower than it is with standard density fiberglass batts, and with dense-pack it's about 2 orders of magnitude lower, at which point even without using air-tight techniques on the interior finish wall vapor diffusion starts to dominate. The convective forces into stud cavities from interior-side leaks in a building with an SPF-sealed exterior are already much lower than the amount of flow-through air on a non air-tight wall structure. 

Doing a blower-door diagnostic & touch-ups with SPF after the flash-foam is installed to ensure air-tightness prior to putting in the fiber insulation is always a good idea.

> air leakage through an inch of ccSPF is zero...air flow more than a full order of magnitude lower
While this may be true at some measurement point, this is never the case for the entire exterior surface - homes using ccSPF are slightly tighter, not infinitely tight (ie, ACH=0 doesn't happen). Air and vapor flow (typically 10,000+ cu ft/hour) in any likely construction will be through the gaps, not through the materials themselves. Figure maybe a liter per hour being left in the walls in cold weather.

Posted By jonr on 28 Jun 2010 09:00 PM
> air leakage through an inch of ccSPF is zero...air flow more than a full order of magnitude lower
While this may be true at some measurement point, this is never the case for the entire exterior surface - homes using ccSPF are slightly tighter, not infinitely tight (ie, ACH=0 doesn't happen). Air and vapor flow (typically 10,000+ cu ft/hour) in any likely construction will be through the gaps, not through the materials themselves. Figure maybe a liter per hour being left in the walls in cold weather.

The locations of the leaks tend to be fairly localized at penetration points for utilties or window/doors/vents, etc, not just random studwall cavities.  Making the walls generally air-tight with SPF dramatically reduces the likelihood of a generalized mold issue, such as were common in '80s construction with mis-applied vapor retarders, etc.  But ACH=0 (or ACH<0.05) is possible (if not usually as a retrofit.)

A -15F design temp is probably on the cool side of reality for Milwaukee, and the 3% binned hourly number is probably warmer than -10F.  According to Wisconsin State Climatology office data for Milwaukee it's only seen temps below -10F three times, -15F but once over the past decade.  But even if the design temp based on 25 or 50 year weather data is truly -15F, were bekkl to go with 2" of 2lb Icynene instead of 1"  there would be ~R15 of exterior side foam to ~R13 of fiber, and slight air-leakage from conditioned space or vapor diffusion into the cavity would have no condensation potential of consequence even without interior side vapor retarders of any type.

Unfortunately, 2" is simply out of the budget. I think 1" of the Icynene and a wet-spray cellulose would be a good mix of price/performance. I just need to find a wet-spray guy.

> and slight air-leakage from conditioned space or vapor diffusion into the cavity would have no condensation potential of consequence

While I'd much rather have condensation (at perhaps 1 liter per hour) occur in foam than something more susceptible to mold, I question where this water goes after it condenses - ie, it may well run back into mold susceptible materials.

Posted By jonr on 29 Jun 2010 11:21 AM
> and slight air-leakage from conditioned space or vapor diffusion into the cavity would have no condensation potential of consequence

While I'd much rather have condensation (at perhaps 1 liter per hour) occur in foam than something more susceptible to mold, I question where this water goes after it condenses - ie, it may well run back into mold susceptible materials.

Air doesn't penetrate the foam. Condensation that occurs within a cell of closed-cell foam pretty much goes nowhere- it stays put until it's warm enough to re-vaporize & escape.  The rate at which the moisture finds it's way into/out-of foam is what it's perm rating is all about, but in no way will you get a liter per hour (or per YEAR) of air through closed cell foam. (The stuff barely passes H20 vapor, let alone ~2x larger air molecules.)

And if you only have 10 hours/decade of condensation potential at interior surface of the foam, you don't have a mold problem, even if it's leaking 100 liters/hr per bay into the fiber.

OK, "condensation within a gap that is surrounded by foam" is clearer than "condensation in foam". The vast majority of air and water vapor movement occurs through cracks and gaps, eventually making it to the outside. At some point on this path, about 1/2 of the moisture will condense out - for months on end.

I agree, ignore much of the stuff about air or water vapor moving through and condensing within the materials themselves. Relatively little of it happens and in many cases, for very limited periods.

Jonr is right. The builder here of external sheet foam homes has only had issues at leak points.

Posted By jonr on 28 Jun 2010 02:39 PM
IMO, most analyses of wall moisture miss a major point - all houses have a significant amount of air flowing through cracks (not through the materials themselves). Much of the time, this air will contain enough moisture that it will condense out at some point in the wall cavity (do you have Winters below 40F?) . This will happen continuously for months (ie, no chance to dry and long enough for mold to grow). Where does this liquid water go? Note that none of this depends on wall material's vapor or air permeability . Experts say that this issue is 100x more important than water vapor or air flow through the materials (very little of this occurs so why does all the analysis focus on it?).

What reduces this problem?

make sure that the house isn't pressurized at any point (in the Winter)
make the house as tight as possible (1/2 the airflow is 1/2 the water)
keep interior humidity low (30% is 1/2 as much water as 60%)
use materials that aren't damaged by water and don't support mold (like foam), especially on the cold side
absorb it (like cellulose does to some extent)
make sure overlaps and holes dump water in the right direction (away from materials that will mold)

I'm interested to understand this issue better.  Beginner question here -
What would make a house presurrized? Can you explain this more?
Thanks.

Wind pressurizes one side of the house and pulls on the other. Stack effect (warm air rises) causes the upper floors to be pressurized and the lowers floors to be depressurized. Dryers, vents and exhaust fans cause depressurization. Unbalanced supply/return ducts can cause either.

Luckily, homes (like mine) tend to have things that depressurize in cold climates during the heating season - pulling dry air in is ok*. I suppose one could have pressure relief dampers that would limit maximum pressure - or self balancing ERVs that sense pressure and control in and out fans to keep it neutral.

I have not found a good complete reference about this issue - just pieces here and there.

* - except when it is raining - then it may pull rain water into the wall cavity.

Commercial buildings commonly deal with pressure issues - not sure why houses don't:

http://www.trane.com/commercial/library/vol31_2/index.asp#why