• Consider this video.
  • Without a vapor barrier, this home will accumulate black mold on the plywood on the exterior of the home.

My Question:

Does black mold on the exterior of a home necessarily imply air quality problems on the inside of the home?

Reason for my question:

  • I suspect that vapor barriers are a recent invention.
    • And if they are, then I doubt that all homes built without a vapor barrier necessarily have black mold and air quality problems on the inside.
  • Am I correct?

Older homes without vapor barriers may not necessarily have black mold problems, but they probably do not have very good insulation or air conditioning either.

Houston, TX is a humid subtropical climate with average summer conditions of 95 degrees F and 75% humidity, so lack of a vapor barrier on the outside walls of this new home will lead to IAQ mold problems inside if other conditions are correct.

The dew point for 95F air at 75% RH is about 85F. Without the vapor barrier, hot humid outside air in summer will likely make contact with internal surfaces cooled below 85F by the A/C, causing condensation well inside the wall. The better insulated the wall is inside, the farther in the condensation will occur, since the dew point will move toward the direction of the insulation. During summer the exterior wall without the barrier could be wet internally, not just underneath the siding (depending on indoor and outdoor conditions and how the insulation is constructed within the wall).

It might be possible to mitigate this somewhat by making certain the A/C system creates positive static pressure within the house; however, if the A/C system causes negative static pressure in the house, then the problem will be especially bad. I suspect that is how this guy in New Orleans got into trouble: How to solve new condensation in bathroom exhaust fan ducts?

The following link points to a helpful essay titled, "How to Avoid Moisture Damage to Walls from Condensation": http://www.tlpca.org/images/articles_condensation.pdf See the description on page 9 for damage that occurs to a Florida home when there is not a vapor barrier on the outside and vinyl wall covering is installed on the inside.


NO, it does not necessarily imply that there are or will be indoor air quality problems.

First off, your assumption that without a vapor barrier "this home will accumulate black mold" is incorrect, with the word will being the problem. Black mold might accumulate depending on many variables including climate and building construction and condition.

If mold were to accumulate on the exterior sheathing that is not a guarantee that similar growth will occur on the interior parts of the wall.

If mold growth does occur on the inside of the wall, that does not necessarily mean that the effects from the mold will be able to affect the interior of the structure.

Even if mold growth is actually present on the interior parts of the structure, that is not a guarantee that air quality will necessarily be affected.

There are too many variables for anyone to be able to state with such broad assumption that air quality will be affected. Of course, the presence of black mold is not a good thing, but localized mold growth can be abated.


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