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Rim joist, sub-siding condensation


Robert E Lee
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Looking for some thoughts from others of you in heating climates. I visited the home today of a previous client who called with a concern about some mold/mildew stains that was present on the interior of the rim joist and the sub-siding. The attached photo's were taken in a lower level bedroom that had the walls covered with insulation/vapor barrier at the time of the inspection (over a year ago). In the process of finishing this room they noticed a stain on the vapor barrier which led to removal of the vapor barrier and the insulation. While the photo doesn't show it well the rim joist had the greatest amount of stains, followed by the stud bay with the plumbing drain line. I told her I felt the cause was frost/condensation forming on the surface of the rim joist as it had no vapor barrier, and that the heat generated by the plumbing drain line (3 teen agers taking showers) behind the vapor barrier was causing the sub-siding to frost up. The client is quite concerned about this condition and covering it up by the finishing off of the room. My feeling is that once the walls are covered with sheetrock this frost/condensation that has taken place will cease.....any other ideas? By the way do any of you routinely remove rim joist insulation? I didn't in this case as I didn't see anything that drew my attention to the area.

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Thanks,

Robert E Lee

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Okay. Hillbilly (but highly competent) carpenter mode..... I would never suggest this to an inspection customer, but if the person was a good friend, I'd help them with it.

Carefully excise a few strips of OSB out of the offending stud cavities, enough to ventilate the cavity from the exterior. I might even drill some holes w/a Forstner bit & stick plug vents in; in fact, I like that better than the wood strip removal method. Then, insulate & vapor barrier the walls & finish.

Point being, I don't think anything real horrible is going on except the insertion of vapor resistant materials into the mix. Every old house I've ever torn up had condensation in the walls in winter; come warmer weather, it would equalize back out due to natural materials being vapor permeable. Let the house breath a little, and it will probably be fine.

Okay everyone, tear me a new ass----.

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Originally posted by kurt

Okay. Hillbilly (but highly competent) carpenter mode..... I would never suggest this to an inspection customer, but if the person was a good friend, I'd help them with it.

Carefully excise a few strips of OSB out of the offending stud cavities, enough to ventilate the cavity from the exterior. I might even drill some holes w/a Forstner bit & stick plug vents in; in fact, I like that better than the wood strip removal method. Then, insulate & vapor barrier the walls & finish.

Point being, I don't think anything real horrible is going on except the insertion of vapor resistant materials into the mix. Every old house I've ever torn up had condensation in the walls in winter; come warmer weather, it would equalize back out due to natural materials being vapor permeable. Let the house breath a little, and it will probably be fine.

Okay everyone, tear me a new ass----.

Forstner bits are too friggin' expensive to be drilling framing, OSB etc! I'd use a speed bore or a hole saw.

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First off, in the first picture, you need to add another stud next to the one that has almost been completely notched out by the drain piping. How dare they put all that wood in the plumber's way. [;)]

2005318193940_2005317191622_Kraft1.jpg

What I'm wondering is if the back side of the drain line was insulated. I'm guessing that there was no insulation between the exterior and the drain line. Almost as if insulation was placed over the drain, providing no insulation behind. That would cause condensation to form on the pipe on the back side when COLD water was put down the drain in the summer. (Hot water would add heat, thus decreasing RH).

I'd say, be sure to insulate completely around the pipe to prevent condensation. Maybe use foam.

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Originally posted by Corners

. . . What I'm wondering is if the back side of the drain line was insulated. I'm guessing that there was no insulation between the exterior and the drain line. Almost as if insulation was placed over the drain, providing no insulation behind. That would cause condensation to form on the pipe on the back side when COLD water was put down the drain in the summer. (Hot water would add heat, thus decreasing RH).

That's one possibility, but if condensation were forming when cold water went down the drain, it would form on the drain pipe and soak the insulation.

In the winter, hot water running down the drain would, as you describe, warm the surrounding air and lower the RH. But it wouldn't stay that way for long. The warmer, dryer air would soon absorb moisture from all around it as the vapor pressure tried to equalize itself. Then, when the pipe cooled off, you'd have oversaturated air with a relatively high dew point.

I'd say, be sure to insulate completely around the pipe to prevent condensation. Maybe use foam.

I think that this is a simple problem. Fill this cavity with insulation -- any kind, take your pick -- install a nice, clean vapor barrier on the inside, sheetrock over it and paint. Everything will then be fine.

The problem was caused by a half-assed vapor barrier and no drywall that allowed lots of vapor movement into the poorly insulated cavity.

And Robert, no, I don't remove insulation from anywhere unless I have a reason to suspect something interesting is going on behind it.

- Jim Katen, Oregon

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Originally posted by Jim Katen

That's one possibility, but if condensation were forming when cold water went down the drain, it would form on the drain pipe and soak the insulation.

In the winter, hot water running down the drain would, as you describe, warm the surrounding air and lower the RH. But it wouldn't stay that way for long. The warmer, dryer air would soon absorb moisture from all around it as the vapor pressure tried to equalize itself. Then, when the pipe cooled off, you'd have oversaturated air with a relatively high dew point.

That's what got me thinking about the cold water. Vapor diffusion is a slow process, especially in new homes (vapor barrier, vinyl siding, etc…). Therefore, the heated pipe would add heat and not moisture, thus no change in dew point. If hot water were dumped down the drain, the drain would not stay hot for long, not allowing much time for vapor to pass into the wall cavity by diffusion (there would be no standing water in the pipe, and the specific heat of PVC is quite low in comparison to water, plus a lower density). Plus, the RH in the winter is already very low, and heating would only decrease it further, so where is the moisture to come from in the winter with a vapor barrier installed? Cold water in the pipe lower than the dew point in the summer would cause the insulation to become saturated if condensation occurred. Once the condensation has occurred in the wall, the vapor would be pretty well trapped between the vapor barrier and vinyl siding. That’s what I’m seeing in the pictures is the entire cavity with mold; the moisture has spread throughout the cavity via insulation osmosis.

I think we all can say that the wall needs to be properly insulated, especially around the drain pipe. I’ll just reiterate my stance and be sure to insulate behind the drain pipe on the exterior facing portion of the pipe with foam.

Oh, and fix that worthless stud.

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Originally posted by Corners

That's what got me thinking about the cold water. Vapor diffusion is a slow process, especially in new homes (vapor barrier, vinyl siding, etc…). Therefore, the heated pipe would add heat and not moisture, thus no change in dew point. If hot water were dumped down the drain, the drain would not stay hot for long, not allowing much time for vapor to pass into the wall cavity by diffusion (there would be no standing water in the pipe, and the specific heat of PVC is quite low in comparison to water, plus a lower density). Plus, the RH in the winter is already very low, and heating would only decrease it further, so where is the moisture to come from in the winter with a vapor barrier installed?

I agree with all that. But as I understand Robert's post, there was no drywall. Just a poorly installed sheet of plastic. In that case, it wouldn't take much time for the air in the cavity to absorb moisture from without the cavity.

Cold water in the pipe lower than the dew point in the summer would cause the insulation to become saturated if condensation occurred. Once the condensation has occurred in the wall, the vapor would be pretty well trapped between the vapor barrier and vinyl siding. That’s what I’m seeing in the pictures is the entire cavity with mold; the moisture has spread throughout the cavity via insulation osmosis.

That's certainly possible.

I think we all can say that the wall needs to be properly insulated, especially around the drain pipe. I’ll just reiterate my stance and be sure to insulate behind the drain pipe on the exterior facing portion of the pipe with foam.

Oh, and fix that worthless stud.

Agreed.

- Jim Katen, Oregon

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  • 2 months later...

Cut Stud

It looks to be like the cut out stud is not a bearing stud (See the stud to the left and also the right of it). The spacing between the three studs looks about 8" each. That stud could be a extra for nailing a wall at that point or support for the piping.

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  • 3 weeks later...

I am relatively new to the profession, but to get to the root of the problem I would suspect the lack of slope in the waste pipes slowing the flow allowing the warm grey water the chance to warm the outside of the pipe. The first pic looks almost like it has a negetive slope back to the elbows but that could just be an optical illusion.

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