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Uninsulated, solid masonry walls: a humidity problem when adding AC?

JGT615 | Posted in Energy Efficiency and Durability on
I’m working on a renovation of a building that was built in the 1950’s in Tennessee (climate zone 4A). The exterior walls are solid masonry. The structural portion is 12″ thick brick — outside of that is limestone. The interior finish is plaster applied to the brick.
There is no insulation anywhere in the wall and any air space between the brick and the limestone is minimal. There are no weep holes and no vapor barrier or weather barrier anywhere in the assembly. This is a very old fashioned wall.
In the past the building had some air conditioning but probably not up to current standards of comfort. The owner would like to completely replace the mechanical systems and install modern air conditioning. Due to space and budget constraints it is not practical to install insulation on the interior side of the walls. So the walls will remain uninsulated.
The climate in Tennessee is mixed but tends more toward hot and humid than toward cold. It is rare now to get temperatures below 20 degrees, and even then they don’t last for more than a few days. But we have many days of high humidity and high temperatures.
Humidity migration does not seem to have been a problem in this building in the past, but I am concerned that with the addition of the new air conditioning system we may have lower humidity levels and temperatures inside the building than there were previously, and the differential between inside and outside may be greater. Given that there is no vapor barrier or insulation in the wall, should we be concerned about humidity migrating in during the summer, or will the natural breathability of the masonry allow that humidity to escape and evaporate?

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  1. GBA Editor
    Martin Holladay | | #1

    The main disadvantage of the walls you describe is that the walls have a low R-value. From a moisture management perspective, adding air conditioning will improve the situation, not make things worse, because the air conditioner will tend to lower the indoor relative humidity during hot, humid weather.

    You can stop worrying.

  2. JGT615 | | #2

    Many thanks!

  3. JGT615 | | #3

    I have one follow-up question on this. There are some areas in the attic of the building where the gable end wall is exposed and we may want to apply spray foam insulation on those walls -- primarily to block air infiltration but it would provide some thermal benefit as well even those the walls in the occupied spaces below are not insulated.

    I read this article and it seems that especially in a warmer climate this should not pose a problem:

    Given that the wall will be able to dry out to the exterior still and moisture that migrates through will stay on the exterior of the insulation, are there any potential problems with applying the insulation on these walls?


  4. GBA Editor
    Martin Holladay | | #4

    Q. "Are there any potential problems with applying the insulation on these walls?"

    A. The only potential problem is fire safety. Building codes usually forbid spray foam to be left exposed, although the codes are open to a variety of interpretations when it comes to attics. Check with your local building department to see how the codes are interpreted locally.

  5. JGT615 | | #5

    We would be required to apply a "Thermal Barrier" per IBC 2603.4 to the insulation. Something like DC315 or similar. Selection of the specific insulation product, its thickness, and the standards for the coating would be confirmed with local codes authorities (we have discussed this initially with them and they have agreed in principle).

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