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    Exterior cedar condensation

    Exterior cedar condensation

    New postby worried on Sat Sep 19, 2009 10:24 am

    We recently had our cedar painted and now are experiencing condensation on the siding an exterior windows in the early morning. When it warms up, it disappears.

    Is this normal? Should we be concerned about infiltration to the sheathing and insulation?
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    Re: Exterior cedar condensation

    New postby Jerry Peck - Codeman on Sat Sep 19, 2009 12:03 pm

    A couple of questions for additional information first:

    Was the cedar siding painted before? Or stained before? Or left bare before?

    Have you had this condition happen in similar weather before?

    Here is one possibility based on my presumed answers to the above questions:
    - This is the first time the cedar siding has been painted, it was either bare or stained before, and, this condition had never happened in similar weather.

    That would lead to a presumption that "painting" the cedar siding is the cause of the new condition.

    If the above is presumed to be the case, then what could be happening is that previously the cedar siding was allowed to breathe better, had a much greater vapor transmission rate, and the paint now acts as a vapor retarder, not allowing the same amount of moisture to breathe through the walls as before, so the moisture which can no longer breathe or transmit through the cedar siding now much find another way out.

    This causes more of the moisture and vapor to take the path around the siding ends at the windows instead of going through the siding, siding laps, and siding joints, condensing on the cooler windows.

    Could this be a problem long term?

    Yes it could, but the way to find out would be to have a person knowledgeable in building science come out and do an inspection and possibly even a door blower test (where a blower is installed in the front doorway and air is exhausted from the interior, depressurizing the interior, then the person goes around the inside and outside of the house looking for where the air is leaking in - same results would be if the house were pressurized, however depressurizing is much easier).
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    Re: Exterior cedar condensation

    New postby worried on Sat Sep 19, 2009 3:20 pm

    Yes, it was previously stained/painted.

    I don't recall it ever being so noticeable until now, maybe sometimes on the windows, but not the siding.

    The condensation occurs only on humid/foggy mornings - dew all around. There doesn't appear to be any more condensation on the outside walls than the windows. Since it wasn't noticeable until the outside walls were painted, is it more likely that the moisture is all from the outside, rather than seeping to the outside surface from within the house? If the fresh paint builds a better vapor barrier, wouldn't this PREVENT more moisture from the interior appearing on the outside walls? I'm just trying to figure out why, on a humid outside morning, our freshly painted outside walls would have more condensation than our neighbor's. At 50% measured humidity in our house, is it at all likely that interior humidity would play a big role in outside condensation, or is it more likely to be higher outside condensation due to the surface conditions of the new paint?

    P.S. I also posted on the inspection site (where I found this site).
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    Re: Exterior cedar condensation

    New postby Jerry Peck - Codeman on Sat Sep 19, 2009 6:52 pm

    worried wrote:Yes, it was previously stained/painted.


    There is a difference between being "stained" and being "painted".

    Most exterior stains soak into the wood and do not seal the wood, and thus do not offer the same protection for the wood against weathering as paint does. Likewise, because the stain soaks in and does not lay on the surface, a certain amount of condensation would be soaked in too and not be seen.

    Paint actually lays on the surface of the wood and does not soak in the way stains do, which is why paints offer greater protection for the wood against weathering, and because the paint lays on the surface of the wood, it creates a smoother surface for moisture to bead up on and keeps that moisture from soaking in as it may have had the wood only been stained.

    The condensation occurs only on humid/foggy mornings - dew all around. There doesn't appear to be any more condensation on the outside walls than the windows.


    You probably see the condensation now as it is no longer soaking into the wood, the moisture is laying on the surface.

    Since it wasn't noticeable until the outside walls were painted, is it more likely that the moisture is all from the outside, rather than seeping to the outside surface from within the house?


    Probable and likely, yes.

    If the fresh paint builds a better vapor barrier, wouldn't this PREVENT more moisture from the interior appearing on the outside walls?


    I would use the word "prevent" as the paint is not a vapor "barrier", only a vapor "retarder" which retards the flow of water vapor.

    The paint serves to reduce and slow the transmission of water vapor both from the exterior into the wood, and from the interior through the wood to the exterior. If enough water vapor is being transmitted from the interior through the wall and into the wood without being allowed to ventilate out, you will eventually see blisters forming under the paint, and when you poke a hole in the blisters you will find moisture and even water - only time will tell if that happens.

    Here is a question: When the house was painted, the painter *did not* caulk along the lower edges of the cedar siding where one board laps over the board below, did they? Hopefully not, as those need to be left open to allow the moisture and water vapor from inside to be ventilated out to the exterior.
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