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Originally posted by Scottpat

One thought I have is that the HVAC system is depressurizing the interior of the home and is then pulling the water vapor(humidity through the wall system.

Good point (didn't think of an existing negative pressure condition) however, I'm not sure it would be the HVAC system. Are there any active exhaust fans running that could be causing a negative pressure? Is there a MUA unit that should be running that isn't? If the building is in a negative pressure then Jim's suggestion would certainly be a good idea.

Also, you can check building pressure (or lack thereof) with a Magnahelic. Might be a good (cheap) first step in determining what the existing condition of the building is.

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Originally posted by Jim Katen

Originally posted by kurt

Nope; residential. Gas fired forced air, room supplies & returns, ducts in ceilings & floors.


Just for fun, run a 12" duct from the outdoors to the return air plenum. Program the system to run the blower at 1/2 speed 24/7 and to ramp up to full speed when there's a call for cooling.

This'll effectively pressurize the whole building.

It's cheap, easy and worth a try.

- Jim Katen, Oregon

NO NO NO NO A thousand times NO. That will create a mold farm. You'd be bringing in huge amounts of humid outdoor air that WILL cause mold to run rampant. You would have to condition the outdoor air you're bringing in (at least at this time of year.)

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That's why this is intimidating; I don't know which direction to go.

Is there an icynene product, or soy based insulation anyone is aware of, that could be retrofit into the wall cavities w/out tearing out the drywall? I'm doing some research on the soy based, but it would be great if someone had first hand knowledge.

And, I thought icynene allowed moisture to pass through it; it's an air barrier, but not a moisture barrier, correct?

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Hi Kurt,

You're right, icynene is vapor permeable, but it's not the vapor but the temperature drop that's causing this. I misunderstood you before and thought that the condensation was occurring on the inside surface of the plastic on the backside of the drywall. It's obvious now that what's occurring is just what happens in North Carolina or Virginia when a builder places the vapor barrier on the wrong side of the wall.

It's air conditioned and you've got the vapor barrier where one would expect to find it in a northern climate, but you've got an unusually hot summer with the A/C running all the time, as you'd have it in Virginia, North or South Carolina this time of the year and with the high humidity, like the mid-Atlantic states, you've emulated a wrong-side vapor barrier situation. Vapor is trying to move from warmer/humid to cooler/less humid and is encountering that plastic behind the drywall - plastic that's been cooled by the AC to below dewpoint. If all you had was plain drywall, I bet that the vapor could have safely diffused to the interior where the A/C system would have removed it.

I think you'll find that if you eliminated the plastic and used foam in that wall that diffusion to the interior will occur naturally without any condensation forming, because the cooling will be extremely gradual as that vapor moves through the insulation and, because it can't accumulate anywhere and will diffuse through the drywall to the interior, the wall will remain dry.

However, if you leave the plastic on the backside of the drywall and shoot the wall, you may find that the icynene just behind the plastic will end up damp because diffusion can't take place. If memory serves, Icynene says that one should not use a vapor barrier at all in their wall systems. I suppose you could experiment with two different wall panels to find out though.

Oh, the other question. Yes, Icynene does make a slow-cure formula that expands far slower than the regular formula. It is designed for use in retrofit situations where a hole is drilled in the wall and the foam is shot into the wall cavities. It generally takes overnight to fully expand and cure.



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Yes. You said it back to me exactly as I'm seeing. I don't think I was explaining this succinctly in my previous posts.

I'm thinking that this is a very strange operation. We just don't get that kind of weather very often; like about 5 days a year. I'm going to simply monitor this for a while & see what turns out.

I also think you're right about retrofit foam; if I shoot the wall up w/out removing the vapor barrier, I think I could make it much, much worse.

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One thing; I thought icynene was an air/thermal barrier, not a moisture barrier, i.e., water can pass through it(?)....

Good catch Kurt. In my hastily typed reply I misspoke (can you do that on a message board?). I should have written "fill the cavity with a sprayed foam insulation like icynene that makes its own continuous air barrier and vapor retarder. I know that many of you guys know your stuff and I post here because I am learning and glad that I can count on you guys at TIJ to make sure I get it right! I have more time now than I had when I first posted, so here's what I think is happening.

The wall profile you described from exterior to interior is as follows: solid masonry, 1.5 inch stud cavity filled with poorly installed fiberglass insulation, vapor barrier, drywall. Summer conditions: hot humid air on the outside, cooler drier air conditioned air on the inside. The temperature inside the home is below the dew point of the outside air. Heat wants to move toward cold, and wherever it can find air as it moves, it carries the water vapor along with it (as long as it doesn't hit a vapor barrier).

Now you had a prolonged period of hot & humid weather. During these conditions a temperature gradient forms in the wall such that as the air (carrying the water vapor) inside the wall cools down, it passes through the dew point on the exterior side of the vapor barrier, condensing inside the wall cavity. In short, Chicago's climate for a period of time became like the climate for the southeastern US.

The problem now becomes one of limiting air flow inside the wall cavity, and moving the dew point to a location where if condensation forms it cannot damage the building materials and the water still has a way to get back out of the wall so the wall assembly can dry.

Air movement is by far the most prevalent way that water vapor moves through the building assembly, although it can also move by diffusion if the vapor gradient is great enough to overcome whatever is resisting it. Diffusion doesn't usually become a big factor unless you are dealing with extreme cold climates or below grade conditions. Sprayed polyurethane foam blocks air movement and does it well by filling voids and sticking to the building materials, and it has a greater R value per inch than fiberglass.

Icynene is an open cell foam that has an R value of about 3.6 per inch, Icynene does not stop vapor movement but it slows it down (about 16 perms at 3" thickness). A good thing about Icynene is that if water vapor NEEDs to move through it, it can, and it can dry out if it does get wet. There are other polyurethane formulations that can bring the R value up to about 7 per inch, and/or are more effective blocking vapor or moisture movement. If you go the foam route, I recommend you consult with a contractor that specializes in spray foam insulation to help you select the formulation (density and open vs closed cell) that is most cost effective to meet your needs.

Injection into the cavity could be done but I think you want to take the existing fiberglass and vapor barrier out before you put in the foam.

Since you would be tightening up the building envelope, you'll need to be sure that moisture generated inside the home has a way to be removed other than by the air conditioner (because it doesn't run in your climate throughout the year), and that the mechanical ventilation systems do not cause back-drafting on any of the vented fuel burning appliances.

Pressurizing the house might work, but as Mark mentioned you'll need to remove the humidity from the air that you bring in from outside. It might also force more cold air into the wall cavity making it even more likely that condensation would occur within the wall. It will also force the cold air to go into places where it hasn't been before, causing unknown effects. You'll also need to look at the effects it would have on the various venting systems in the home. If it does work, it is an energy intensive solution, so it may not be cost effective if they plan on staying in the home for a while.

Chad's suggestion might work, but you still have that chase and stack-effect going on inside the wall cavity, you are now dealing with multiple vapor barriers, and you are moving the inside finished surface of the wall in a few inches.

Whatever method you choose, flip the gradients around the other way for typical winter conditions in your climate (warm humid air on inside, colder drier air on outside). Do you foresee it causing any problems?

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Originally posted by mcramer

NO NO NO NO A thousand times NO. That will create a mold farm. You'd be bringing in huge amounts of humid outdoor air that WILL cause mold to run rampant. You would have to condition the outdoor air you're bringing in (at least at this time of year.)

I don't think so, not in the summer anyway. You'd be bringing in lost of humid outdoor air but it'd be mixed with the indoor air. The net effect will be to pressurize the house and raise the indoor humidity. Neither of these things will cause condensation or mold growth.

If you're concerned that the indoor humidity will be too great, you could install a humidistat to control the AC system. This would let your AC function like a dehumidifier.

This approach has the advantage of being very cheap and easy to try. If it doesn't work, you haven't lost much.

- Jim Katen, Oregon

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Hausdoc said: "Vapor is trying to move from warmer/humid to cooler/less humid and is encountering that plastic behind the drywall - plastic that's been cooled by the AC to below dewpoint. If all you had was plain drywall, I bet that the vapor could have safely diffused to the interior where the A/C system would have removed it."

I don't see how that can be, that would cause moisture buildup in the wall cavity but not on the exterior of the drywall. Seems to me the humidity problem is in the interior, not the exterior. I agree with Chris, dehumidify the interior air (although who knows what the result will be at the interior of the wall cavity).

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No, if one has condensation on the "outside" of a vapor barrier installed on the interior, that means moisture is trying to migrate to the interior & it is being stopped/condensed on the wrong side of the vapor barrier.

This last week, which was just beautiful weather, the phenomenon didn't occur. It only happened when we had a "southern" climate, i.e., when it was 100degF & 95% humidity.

I think this whole thing was a mini-experiment in building science; we had a couple micro climates in the space of a week, & could see how it worked in both.

Basically, it was a fluke.

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We don't use any vapor barriers here in Washington's temperate climate. When it's hot enough to turn on AC, vapor simply diffuses through drywall to the interiors and is removed via the AC. In winter, vapor migrates outward through the drywall and wall cavity to the exterior. No wet wall cavities unless someone was dumb enough to flash windows and walls wrong or put plastic into the walls.

OT - OF!!!


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