Replies

1. GBA Editor
   Martin Holladay | Jan 18, 2015 06:15am | #1

   Brian,
   The symptom you describe is unusual and worrisome. My guess is that (a) the spray foam installer did a sloppy job, and some areas of your wall sheathing weren't properly insulated, and (b) your wall system has air leaks that pull humid indoor air into the wall system, where the moisture in the indoor air is able to condense when it contacts the cold wall sheathing.

   This diagnosis is a guess. Something else could be going on, of course. Are there any plumbing pipes in these walls?

   In any case, I would open up the walls if this were my house, so find out what's going on.

2. 
   charlie_sullivan | Jan 19, 2015 04:23pm | #2

   Any correlation between when the moisture appears and the weather?

   Another piece of information that might help is the indoor humidity level--you can get an electronic temp/humidity meter for $10 at a big box store if not your local hardware store.

3. 
   wjrobinson | Jan 19, 2015 07:03pm | #3

   Your construction heater and your water coming out of the drying two coat plaster are condensing in your wall. Your humidity level is way way too high.

   Solution is to dehumidify. Open windows. Do not heat with propane construction heaters. Use a standard left over furnace vented out temporarily thru a window. Make sure the unit is a direct vent unit which is one with a fan on the exhaust.

   Once you way lower the construction moisture all should be OK. That should be the problem. If it is not then you need an expert onsite to figure out what's up.

   With the very cold conditions of late my bet is there you all are trying to save all your construction heat and have the place tightly closed up, no open windows etc which has to be done to rid the home of construction moisture. In cold weather rent a commercial dehumidifier, deal with the loss of heat that opening windows or running bath fans creates. You have to get rid of the moisture you are adding. Better timing for when doing such work is the only other option.

4. 
   davidmeiland | Jan 19, 2015 08:30pm | #4

   I will tentatively agree with AJ that this problem can be dehumidified away, but I would want verification that there isn't an ongoing problem, and that will be difficult to obtain without some diligent effort.

   Once the moisture from the plaster and paint has been removed, you need to be sure that condensation is not continuing to form on the surface of the foam (or on the plywood in places where there may be gaps or fissures in the foam). The only way I can think of to do this semi-non-destructively is to use a moisture meter with wall probes to take readings in the bottom plate, and/or in studs near the bottom plate. You would need a baseline moisture reading and you would need ongoing readings to get a picture of what is going on inside the wall. To get reasonable readings you need normal indoor humidity and a mix of average and extreme cold outside.

   If they haven't installed baseboard yet, perhaps there are opportunities to get some readings without poking holes where they will be seen.

   I like infrared but would be skeptical of it in this situation, because condensation will tend to run down the foam to the bottom plate without wetting the fiberglass much.

   A direct visual inspection would be much better, but that means removing plaster. Again you would need normal indoor humidity and cold weather outside. Cut some inspection holes, get the fiberglass out of the way, and see if there is moisture on the surface of the foam.

   Any time you undertake an investigation like this you risk false negatives. You could find no problems in a given area when there are problems in other areas. The desire to do little or no damage makes things difficult.

   I'm skeptical of spray foam applied to very cold surfaces, and also skeptical of flash and batt in general, because it's hard to imagine there aren't significant air channels between the batts and the irregular face of the foam.

   I'm surprised that Martin didn't question the 1"-2" thickness of the foam. Seems like 1" is just not enough in your climate.

5. GBA Editor
   Martin Holladay | Jan 20, 2015 07:55am | #5

   Brian,
   David brings up a good point when he chastises me for failing to mention the fact that your spray foam layer is too thin. In your climate zone (Zone 6), a 2x6 wall with flash-and-batt insulation needs a foam layer with a minimum R-value of R-11.25. (For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

   A full 2 inches of closed-cell spray foam is close to the minimum you need, but 1 inch definitely won't do.

6. Expert Member
   Dana Dorsett | Jan 20, 2015 03:11pm | #6

   No, a flash' n' batt absolutely does NOT need to be R11.25 with the foam it's on the INSIDE of the sheathing, (unless you're adding more thickness to the fiber layer to make it a full 5.5" deep. )

   The IRC is silent on flash'n'batt in wall assemblies, and the R11.25 prescriptive for 2x6 walls only applies to wood-sheathed walls with the continuous insulation exterior to the sheathing, with air permeable insulation in the cavity.

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

   Note, for unvented ROOF assemblies the IRC DOES weigh-in on flash'n'batt. Unlike walls, roof sheathing can't dry toward the exterior, and gravity would pull liquid condensation directly onto the sheet rock facer through the fiber layers rather than let it condense & evaporate clinging to the foam face as it would with minor condensation events in a wall assembly.

   But in theis wall if foam is 1" (R6-R7) and the the fiber is 4.5" thick (about R20 for an R24 batt compressed to 4.5"), that's still a bit of a problem for zone 6.

   It's the ratio of the foam-R to the total R that matters, since that's what determines the average temp at the foam/fiber interface (or sheathing, in the exterior foam instance) where any condensation/adsorption would be taking place.

   Putting R11.25 on the exterior of a 2x6 wall you have no more than R24 fiber, for a total of R35.25, and a foam/total ratio of 11.25/35.25= 32%.

   That means as long as the foam-R is more than half the R-value of the fiber layer, you won't end up with copious liquid condensation. With 1" lflash-foam / R20 fiber you have R6-R7, for a center-cavity R of about R27, and a foam / total ratio of only 25%, which isn't quite good enough.

   That's still not a disaster likely to create liquid seepage (after the plaster dries and the house is fully conditioned), but it has the potential to create springtime mold issues inside the wall cavities. As long as you keep it under 35% RH in winter those problems are unlikely. Even at higher interior RH the sheathing would be fine, since it is protected from interior moisture drives by 1" of foam (which runs 1-1.5 perms) and will still dry toward the exterior. But at higher than 35% interior RH you would end up with some moisture accumulating at the foam/fiber boundary- enough to keep it damp-ish, if not actively dribbling.

   At 1.5" you'd have about R9 foam / R18 fiber, and you'd be at roughly the code- prescriptive R-ratio for exterior insulation. At 2" you would have decent margin and virtually no moisture accumulation.

   So, it turns out that it actually matters just how thick the "... 1-2" of closed cell foam..." really is. If it was the full 2" it would have been easier to install R15s in to that 3.5" remaining depth, but if in fact he was able to compress R24s in there it was almost certainly less than 2" and could be more like 0.5-1". Anything less than 1" COULD be a problem in the longer term in a zone 6 climate.

   A drill and several exploratory holes to measure layer thicknesses could tell you where it really lives.