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Spray foam hybrid flash and batt help! wet wall cavity

We have new construction and have dry wall up and painted. Have spray foam 2" on exterior wall and flash and batt in wall cavity. Had a cold snap and with humidity in house the vapor went through wall on north and east side of house, hit the spray foam and condensed. We have heat on 62 and have 2 contractor size dehumidifiers running. The humidity level now is 50 , basement is 55. Has been a whole week. How long to dry. It is maine, and rainy out for next 3 days so feel that high humidity outside may help to drive vapor back inside. How long will this take or will we need to remove drywall? Had a professional company do the insulation and it seemed to be 2 " every where. Feeling pretty upset.

Asked by James Bernier
Posted Jan 12, 2013 4:51 PM ET


13 Answers

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Just to confirm. You do not have 2 inches of foamboard on the exterior of your homes sheething AND another 2inches on the interior. Correct?

Answered by stephen edge
Posted Jan 13, 2013 6:33 AM ET


Laurie and James,
Although your post is ambiguous, I'm going to assume that the only foam on your walls is 2 inches of spray foam on the interior of your wall sheathing. I'm going to assume that you don't have any rigid foam on the exterior of your wall sheathing. If I have misunderstood, you will have to clarify the issue.

You didn't mention your climate, so we have no way of knowing whether the foam on your walls is adequate to avoid problems. If you read the following article, you can learn more about foam thickness requirements: Calculating the Minimum Thickness of Rigid Foam Sheathing.

You also didn't mention whether your contractor used open-cell foam or closed-cell foam. I certainly hope you have closed-cell foam -- because that's the only type of spray foam that is appropriate for a flash-and-batt job.

You also failed to tell us whether you have 2x4 walls or 2x6 walls.

Assuming that you have closed-cell foam, it has an R-value of about R-13. That's adequate for a 2x4 wall in all U.S. climates; but if you have 2x6 walls, it's not quite enough for Climate Zones 7 and 8.

Here's the last question: If your walls are covered with gypsum drywall, how do you know about this condensation? Are you getting so much condensation that liquid water is dribbling out of the bottom of your walls?

Answered by Martin Holladay
Posted Jan 13, 2013 8:12 AM ET


Thanks. We are in South west Maine. We have drywall up and painted. My husband just had a feeling, so checked the insulation in basement first at the kneewall where there is no drywall. it was wet toward the sprayfoam side. Then made small holes in the bottom of drywall on first floor . Wet. Yes, 2x6 construction. Closed cell foam. Yes, my husband can understand from your statement why the 2x4 would be ok. The fiberglass in the cavity is R15. Cold snap a week and a half ago down to -5 in the early morning.

Answered by James Bernier
Posted Jan 13, 2013 9:43 AM ET


Correct. We have just the 2" sprayfoam on the interior of outside wall.

Answered by James Bernier
Posted Jan 13, 2013 9:44 AM ET


Laurie and James,
Southwest Maine is Climate Zone 6, so you should be OK in the long run. Your problem appears to be due to construction moisture.

There are three things you can do: heat your home, ventilate, and run dehumidifiers. Doing all three at once is obviously the best approach. There will be an energy penalty, of course, but eventually you should be able to bring the indoor relative humidity levels down to a safe level.

Answered by Martin Holladay
Posted Jan 13, 2013 10:08 AM ET


Doing all 3. Have mechanical ventilation up and running the last 4 days. Fans, 2 good dehumidifiers and heat at 62. Thanks . Will try to relax a little.

Answered by James Bernier
Posted Jan 13, 2013 10:49 AM ET


There won't be any problem if this goes on for a few months or similar short period. In the long run, I think you should be using a hygrometer to monitor indoor temperature and humidity, and make sure it is within acceptable levels. You need to do a bit of research and find a decent, accurate unit. It is also possible that your HVAC contractor could install a thermostat with a RH% readout. If your house is tight, you will need to stay on top of this and make sure you're ventilating enough.

Answered by David Meiland
Posted Jan 13, 2013 11:12 AM ET


We just opened up 3" of drywall all along the bottom of exterior on the north and east walls hoping that the warm dry air we have in the interior may assist in drying it out faster. Not sure now if we should just leave the drywall sealed up. Seems to be a bit dryer where we have removed drywall. Assuming from last comment, thanks David, that we will get it dry and likely not have mold issues.

Answered by James Bernier
Posted Jan 14, 2013 8:17 PM ET


Do you have a way to accurately measure relative humidity?

Answered by David Meiland
Posted Jan 14, 2013 9:58 PM ET


yes we are measuring with a humidistat. We tested accuracy. We have moved in basement from 65-50. And first floor from 55- 45. Temp is at 65 interior first floor. A bit lower in basement. 2nd floor also wet. We have opened up 3" upstairs and downstairs to get air in there. Concerned that the wet insulation has lost r value to the point that the warm air is passing through wall and making it worse. It seems worse today. Temp outside has been 25 degrees last night, likely 39 today, then another cold snap coming. We are really appreciative of help here.

Answered by James Bernier
Posted Jan 15, 2013 9:13 AM ET


Opening up the drywall only adds to the condensation, since the warm humid air can now reach the cool surface of the foam relatively unimpeded. With standard latex paint (2-5 perms) the cavities can dry at reasonable rates once you get the humidity of the conditioned space under control.

The warmer you can make it inside, the warmer the condensing surface will be, which will lower the rate at which liquid water accumulates on the foam, and it will cease when the surface of the foam is above the dew point of the interior air. The warmer temps will also drive the adsorbed moisture out of the wood more quickly too. Ideally all of the condensing would be happening on the coils of the dehumidfiers until you bring the store moisture in the construction materials down to some plausible background level.

With new construction the sources of excess humidity are many- the wood framing, concrete etc, but once you've purged it the problem won't recur- you won't need industrial strength dehumidifiers forever. If much of the wood in the house was exposed to rain during storage & assembly it can be several times more moisture to purge than if the materials were kept dry throughout.

The high humidity outside during the recent warm periods is only a remote secondary factor, and should not increase the condensing rates. Higher temps would in fact DECREASE the rate of water accumulation on the surface of the foam, since it would be warmer, closer to (or higher than) the dew point of the interior air. If it only seems to show up during rainy weather, you have a LEAKAGE problem, not a condensation problem.

Answered by Dana Dorsett
Posted Jan 15, 2013 5:56 PM ET


ok, we may close it back up though it is dry where it is open, 3 inches along the bottom.. We are almost ready to bulldose it and walk away.

Answered by James Bernier
Posted Jan 15, 2013 8:03 PM ET


>>Opening up the drywall only adds to the condensation, since the warm humid air can now reach the cool surface of the foam relatively unimpeded

Maybe, maybe not. Although the open drywall lets interior air (and whatever moisture it contains) reach the foam very quickly, it also lets heat through more quickly too, raising the temp of the surface of the foam, perhaps above the dewpoint.

Personally, I think removing the drywall is OK. When disaster remediation companies go in to dry out a house where a major leak has occurred, they remove areas of drywall (not necessarily areas that were water damaged) so they can inject lots of heat and air into the wet cavities.

I was general on a large repair job after a leak recently, and the company that came in raised the air temp to 90 degrees using 240v heaters, along with several dehumidifiers and a couple of "injecta-dry" machines that blew hot air through tubes into framing cavities (we had to repair a lot of small holes in the drywall from that operation). It was very, very effective, and also expensive.

I think your 65 indoor temp is on the low side for a dry-out. Get another heater or two and get it above 70. The dehumidifiers will work faster and the whole thing will be over sooner.

Regarding bulldozing the place, I know how you feel, but the bottom line is that you were unaware that this was going to happen, and unprepared for it. It is normal, although you should have been drying before the drywall went up, as well as monitoring humidity. The first time this happened to me I was surprised, but I was lucky--I found a bit of condensation on a rim joist after insulation was installed. I immediately got a hygrometer and found the RH was off the charts. I then got a couple of large dehu's and dried the house out. I kept them there all the way through the end of interior painting. Now I know.

Final note, I think you should get a blower door test and analyze your mechanical ventilation needs for the long term. You may have a tight house that needs active management of interior air quality. If this is your first tight house, it will be a new experience for a while.

Answered by David Meiland
Posted Jan 16, 2013 12:56 AM ET
Edited Jan 16, 2013 1:01 AM ET.

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