September, 2005
You Tell Us
Inspection News and Views from the American Society of Home Inspectors

More on Moisture Control


To the editor:

While Dale Dorman’s “Moisture Control in Homes” (part 2,  ASHI Reporter, June 2005) provides some useful information on moisture, the article also contains some errors. It is common for “building scientists” to refer to the partial pressure of water vapor in air as the “vapor pressure” of water, but this is incorrect. To a chemist or physicist, the vapor pressure of pure water is a physical property of water (like density or conductivity) that depends on temperature only. For example, the boiling point of pure water is defined as the temperature at which the vapor pressure of the liquid is equal to the total atmospheric pressure. Neither the boiling point of water nor its density depend on the water-vapor partial pressure (in other words, the temperature-dependent relative humidity) of the atmosphere. What building scientists refer to as vapor pressure should be called the water-vapor pressure (or partial pressure of water vapor), which is not dependent on temperature.

The article suggests that a dirt floor in a crawl space should be left partially exposed, but this is incorrect, as evaporation of moisture can lead to high relative humidity, condensation and mold. The article also states that attics and crawl spaces “require ventilation.” Recent studies ( have shown that in humid climates, ventilation actually brings moisture into crawl spaces and that airtight crawl spaces, with completely vapor-tight foundations and floors, are very successful in maintaining the relative humidity at safe levels.

The primary purpose of attic ventilation is dissipation of solar heat. The means of attic heat loss in the summer is by convection, because of the large temperature difference (up to 40? F) between the attic and exterior air. In the winter, the temperature difference between the attic and exterior air is usually less than half the summer amount, and can even be no more than a few degrees, in a well ventilated and insulated attic, so air flow by convection may be minimal. Due to the large temperature difference between the habitable spaces and the attic, nearly all winter moisture flow from a home is pressure-driven, via mechanical flow through openings such as bathroom vents (open to the attic), leaky attic pull-downs and hatches, plumbing and chimney chases, and recessed fixtures; vapor diffusion (due to differences in water-vapor pressure) through drywall is negligible. Even well-ventilated attics can suffer significant condensation and mold growth as a result of such moisture entry, so, pointing out potential areas of air-leakage into an attic is at least as important (if not more so) as discussing ventilation with a client.

Jeffrey May
May Indoor Air Investigations LLC
Cambridge, Mass.

Debating underlayment’s purpose

To the editor:
The “Slate Roofs” article in the July ‘05 edition of the ASHI Reporter is both well written and exquisitely designed. My congratulations to the author and to ASHI for printing it.

I do have one concern, though. Mr. Jenkins’ insistence that slate roofs do not require underlayment is assumptive at best, and specious, in my opinion. Underlayment is recommended by all slate tile manufacturers, and the National Roofing Contractors Association Roofing and Waterproofing Manual, and so it follows by the International Residential Code in R905.1 and R905.6. It is also a requirement set by Underwriters Laboratories when assigning a fire rating to slate or any other shingles.

Mr. Jenkins’ statement that “felt is temporary” because it does not last forever can be applied to nearly every other component of the roof and the whole house for that matter. Felt usually deteriorates at about the same time (50-60 years) as the flashing. Is flashing also temporary?

The danger I see in this type of jingoistic rhetoric is that it might just trip up the inexperienced inspector. Mr. Jenkins’ claims would be hardly supportable in a court of law. Let’s hope that these sorts of pronouncements are just a “temporary” part of the ASHI Reporter.

Thank you for your time and consideration,

Aaron D. Miller, CEI, CRI
Aaron’s Home Inspections
Garland Texas

The author replies

To the editor:

I also recommend underlayment, but that’s not the point. The point is that the purpose of underlayment is to keep the water out of the building until the roof is installed. Once the slate is installed, the underlayment is obsolete. Some people want to argue that because underlayment is recommended, it must be an important element of a slate roof and, therefore, the thicker, beefier and more expensive, the better. But the underlayment is important only until the slate is installed. After that, it doesn’t matter, and a lot of slate roofs have been installed with no underlayment whatsoever and are still functioning quite well a century later. Most of the roofs shown in the article with the dates on them have no underlayment whatsoever. Why? Because they were barns, and it didn’t matter if it rained into them while the slate was being installed. It’s pretty hard to argue that underlayment is important on a slate roof when you look at one installed 150 years ago, still in good condition, with no underlayment whatsoever.

I don’t know why underlayment would be required for fire-rating purposes. It’s very flammable.

Joseph Jenkins
Joseph Jenkins, Inc.
Grove City, Pa.