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Tricky Fix


kurt
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Well, we've figured out what's happening; the problem is how to fix it. I'm looking for ideas.

Solid masonry structure; 3 stories (urban thing). Outer wall is face brick; inner wall is CMU. Flashing, wicks, etc. The wall doesn't leak; this is about vapor migration.

MAJOR condensation on the outer ("warm") side of the vapor barrier; VB is located right under the drywall. I mean the damn wall was sopping wet. This occurred last week w/the 100degF temps & >95% relative humidity.

I think the building temporarily suffered the same as Southern buildings, i.e., in the south the vapor barrier should go on the exterior so the wall can "dry" to the interior. Stains on the framing indicate it happens on an irregular basis, which would also make sense. We got a fair amount of stack effect going on w/conduit chase(s) & crappy building practices.

So, moisture gets in, but can't breath properly in these extreme conditions. Customer wants to know how to "fix it".

I have no idea. Anyone?

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Kurt,

I was in a 5yr old homes basement last week where the approx. 4ft of the rear and part of both side walls were above grade. Cedar sided exterior. The plastic vapor barrier on the basement side was soaking wet against the fiberglass insulation. I pulled plastic loose, insulation was wet on the warm side and the cavity was dry. The insulation did not show a history of this having occurred in the past.

Jim in Chicago Suburbs.

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Hi Kurt,

Hi, what do you think is going on? Exterior walls cooling at night and causing all of that damp/warm air to migrate toward the cool exterior and then condense on the plastic when it can't pass through?

In the south, because they've got AC running day and night, they generally put that barrier on the outside of the wall cavity, with the layer of insulation between the vapor barrier and the drywall, in order to keep that humid outer air from diffusing into that cooled wall cavity. What little gets by the barrier diffuses in such small amounts to the interior that condensation can't form in the wall. It leaves the back of the exterior sheathing damp but that dries to the outside as soon as it cools at night and the humidity drops.

It sounds like they're accumulating moisture inside that building all day long and then, at night when the temperature in that outer wall drops, all that interior humidity is trying to migrate warm to cool, hits that barrier, cools to dewpoint, condenses and then ends up soaking into the drywall.

Is there any insulation at all in that outer wall that will insulate the interior walls from that cool outer wall and keep things relatively balanced at night?

Is the building air conditioned? If not, maybe it's time to consider renting a couple of those big honking dehumidifiers - at least to get through the hot/humid patch until a solution can be implemented. I saw one the other day. It took up the space of a copy machine and was sucking huge amounts of moisture out of a home.

ONE TEAM - ONE FIGHT!!!

Mike

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Kurt,

I managed a commercial construction company for nearly a decade and this same condition occurred in an eight-story building we erected. I consulted with two engineers, an architect, and some guy with a fistful of outlandish, esoteric degrees I was referred to. No one could--or would--definitively formulate a plan for mitigating the moisture. There were lots of "maybe"s and "probably"s, but in the end we left it alone. I'm pretty sure the owner of that building still hates my guts. I offered to pay for any reasonable remedy but the remedy was never discovered.

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Mr. O'...

I think you nailed it exactly. It's a tour de force of building science writ large.

Bain....

I think you also nailed it exactly; I'm not sure this is "fixable" without rebuilding the entire structure.

I'm thinking we might upgrade insulation as much as possible w/foam, but after that, I can't imagine any good solution.

Any ventilation in the wall, i.e., vents @ grade or @ the top of the parapet wall, would only exacerbate the stack effect, no?

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I'm a little confused as far as which side of the vapor barrier the moisture is actually condensating on, but I'm going to assume it is condensating on the interior side since you say that there are stains in the drywall.

If this building has a crawlspace, it might be a prime candidate for a 100% sealed and conditioned crawlspace.

(Since a cooling system is also a de-humidification system, that seems to be the only place that level, of moisture could come from?)

If it's a basement, that still might be where the lions share of the moisture is coming from. (earth)

Maybe I've not got a good mental picture of your conditions.

Help me out if I'm way off base...

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I think Mike was pretty close, except that at night the humidity goes up. Today the humidity was around 68% at 97 degrees. Tonight at 11 PM it is 93% at 82 degrees!

I'm going with the stack effect. I have seen this on older stick built homes that are built in the "balloon" framing style on conventional foundations. It was never a problem, until they were updated and A/C was added and then we had the warm attic air, the cool room interior air and the cooler crawlspace air mixing about midway. The result is often warped wood floors and moist walls 3-5 feet from the floor.

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Sure,

The humidity goes up but the temperature begins to drop. Somewhere between 2:00 amd and 5:00 am when the masonry starts to cool and bring down the temperature of those inside walls, that humidity starts condensing on the inside face of that plastic. They probably would have been better off to do away with the plastic and use foil-faced foam insulation. It would have functioned as a vapor barrier yet prevented the sudden change in surface temperature that caused the condensation.

It's just a theory. It needs to be tested.

ONE TEAM - ONE FIGHT!!!

Mike

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Vapor barrier on the inside, right under the drywall.

Condensation on the "outside" of the vapor barrier, i.e., moisture is condensing on the side of the vapor barrier facing the exterior. In these whacko weather conditions (for Chicago), that's the "warm" side, as the interior is nicely cooled w/AC.

No crawlspace; bsmt.

Lotta bypasses, i.e., furring framing channels running top to bottom to hold the drywall, conduit holes, lousy insulation, etc. Lotta stack effect.

About 14 year old solid masonry house in Lincoln Park; approx. 2 million dollar home.

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Like you said, though. The weather is "whacko." It's almost impossible to construct a building that'll perform perfectly in all weather conditions. I had calls last week from recent clients who complained that their A/C systems weren't working in the mid-ninety degree temperatures. I explained that the systems weren't designed to function properly under those conditions because they don't occur often, but I'm not sure anyone believed me.

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Originally posted by kurt

Vapor barrier on the inside, right under the drywall.

Condensation on the "outside" of the vapor barrier, i.e., moisture is condensing on the side of the vapor barrier facing the exterior. In these whacko weather conditions (for Chicago), that's the "warm" side, as the interior is nicely cooled w/AC.

No crawlspace; bsmt.

Lotta bypasses, i.e., furring framing channels running top to bottom to hold the drywall, conduit holes, lousy insulation, etc. Lotta stack effect.

About 14 year old solid masonry house in Lincoln Park; approx. 2 million dollar home.

What would happen if you were to artificially pressurize the entire interior of the building during the summer months?

- Jim Katen, Oregon

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Originally posted by Jim Katen

What would happen if you were to artificially pressurize the entire interior of the building during the summer months?

- Jim Katen, Oregon

Leave it to Katen to ask the question. That is what the contractor asked, & I didn't know the answer, and that's when I started asking in here.

How would one do that in an intelligent & efficient manner?

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Originally posted by kurt

Leave it to Katen to ask the question. That is what the contractor asked, & I didn't know the answer, and that's when I started asking in here.

How would one do that in an intelligent & efficient manner?

What kind of heating/cooling systems are already in place?

If this were a commercial structure, I'd suggest tampering with the economizers.

- Jim Katen, Oregon

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Drywall on 1 1/2" furring, i.e., furring on the interior face of the CMU, poorly installed fiberglass insulation, conduit penetrations/bypasses from bsmt. to roof.

We think there's significant stack effect taking moist air up through the furring "chase"; I suppose it could act as a plenum.

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Tear the drywall off, remove the fiberglass and vapor barrier, fill the cavity with a sprayed foam insulation like icynene that makes its own continuous air & vapor barrier, reinstall new drywall? I don't think you would get any condensation on the drywall or the foam. Any moisture that entered the masonry from the outside needs to have a way to dry to the outside. Make sure the moisture generated inside the home has a way to get out since you are sealing up that stack effect in the furring chase. BTW, this is just an expansion of Terry's post above, I think he nailed it.

(this is more of a idea for discussion purposes than an outright recommendation...)

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Folks seem to all be zeroing in on this from the same angle. I think the air barrier aspect is critical.

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

Thinking this through w/several minds is extremely helpful.

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Originally posted by kurt

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

Perfect.

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

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Although running outside air into the building will help pressurize it I don't think it's the answer.

1st problem is that at the height of summer you'll be bringing in 90+ degree air with a dew point of 70+. This will only exacerbate the condition. I'll dig around for my Trane ductulater to see how many CFM a 12" duct can move.

2nd problem is that your now changing the engineering and dynamics of the air conditioner. Instead of 75 degree return air with 40% RH your now bringing in a much higher return air temperature and humidity. Can the existing air conditioner handle it when the problem is at it's worse?

3rd. I guess the line of thinking is to pressureize the building so it will not allow the wall cavity to transfer the warm, moist air, to the existing vapor barrier. I'm not sure this will work. You'd have to pressureize it enough to allow the wall cavity to force air back out. I dunno.

I still think it's a matter of better insulating the wall cavity as to prevent the dew point condensation from occurring in the first place.

One other thing, the only time I've seen a vapor barried on the outside of an interior wall was in a computer room application. This was to prevent the humidity from migrating through the drywall which it will do. These computer rooms were located inside a conditioned building so I've never run into this type of a problem with a vapor barrier.

For what it's worth.

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Chad, has a good idea. The polyiso board (foil backed) would work as an insulator, and could be drywalled over.

Kurt, I have been thinking more and more about this. Did you by chance take the RH and temps throughout the house. I may have missed it in the post, but is this condition only on the exterior walls? Reason I'm asking is that with the stack effect in the south, you will also find interior walls that are wet.

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. I have seen this on several occasions when an improperly installed vapor barrier(wrong material or just installed wrong) is in place or it is missing entirely.

With this being a CMU home, the walls do have all sorts of little chases as you have said. If it is a interior type stack effect with the furring strips acting like a chase, I don't think it is repairable short of removing the drywall and starting over.

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