Hello Roger –

It is not surprising to find conflicting statements on vapor barriers, and it is not surprising to find that the conflicting opinions may both be correct under the specifics being discussed by each expert.

Before, I address the VB on walls, as Ron Bibler points out, vapor barriers in crawlspaces can INCREASE the moisture in a crawlspace and in other circumstances, a VB can decrease the moisture. The specific situation will dictate the results.

When folks speak of controlling moisture, we often assume that “moisture control” is synonymous with “moisture reduction.” However, the control action we take may increase the moisture and control the moisture to an higher level than before. Vapor barriers are a good example.

Some years ago, I was asked to comment on altering building codes to address the “growing” problem of indoor mould in new construction (no pun intended). The municipality wanted to require the automatic inclusion of VBs and active ventilation in the crawlers; I said it was a bad idea. They ignored my arguments and made the changes, and the net result was that the incident of mould in houses in one area went up – not down - as a direct result of the VBs plus the crawlspace fans.

The problem with VBs is that they restrict water vapor migration. Unfortunately this not only means that they prevent water vapor from entering a building system, they also prevent water vapor from LEAVING. Like anything else, until one understands the specific situation, blindly requiring a “fix” may not fix anything at all.

The assumption was VBs reduce %RH in crawlspaces, and therefore, control (reduce) mould growth. But, for a start, the relationship between mould growth and %RH is very complex and not at all simple. In one house I looked at, a swimming pool size of 20 inches of free standing water had been present in the crawlspace for three years. The %RH in the remaining 12 inches of head space was almost always 80-90%. Yet there was NO mould growth in the crawler – Why? Answer: Why would there be?

There is no correlation between relative humidity and mould growth outside the context of dew point temperatures and especially dew point temperatures on surfaces. Let’s look at a simple example of the temperature and moisture complexities –

Posit: A nice summer day (in Colorado) with a relative humidity of only 35% and an outdoor temperature of 98°F, the outdoor air will contain approximately 16 grams of water per cubic meter of air. Crawlspace vents are closed and %RH in the crawlspace is also 35% relative humidity. However, the temperature in the crawler is 65°F and, therefore, the air in the crawler only contains about 5.6 g/m3.

The minimum temperature needed to precipitate the water from the outdoor air is only 66°F (the dew-point). That means that if the air temperature or surface temperatures in the crawlspace are less than or equal to 66°F, the %RH in the crawlspace will skyrocket to 100% RH and the moisture from the outdoor air drawn into the crawlspace will precipitate onto those surfaces (even though the actual moisture content in the air has not changed). The condensed moisture could provide a water source for mould to grow if that water does not migrate somewhere; such as the soil, provided there is no VB. If the effect is prolonged, and wood may remain damp, and the probability is that moulds will proliferate. In this way, fans and vapor barriers meant to “dehumidify” crawlspaces actually defeating their own purposes by increasing the potential humidity in the crawlspace.

Where the soils are a source of moisture, the vapor barriers may be a good idea, maybe not; where the soils are dry, the installation of a vapor barrier may or may not be a good idea. My point is that there should always be an articulable reason to do something.

Regarding VBs on/in walls – there was an excellent article by Joe Lstiburek which addressed this issue in a very succinct manner. I believe it is copy-righted material, and so I can’t supply it here, but it was in the February 2002 ASHRAE Journal (pp. 36-41) as part of the “Moisture Control Series.” It is one of the best discussions on the matter I have seen.

Dr. Lstiburek breaks down the country into climatological regions, and in the context of those regions explains when a VB is appropriate, and explains where the VB should be installed.

Hope that helps.

Cheers,
Caoimhín P. Connell
Forensic Industrial Hygienist
Forensic Industrial Hygiene

(The opinions expressed here are exclusively my personal opinions and do not necessarily reflect my professional opinion, opinion of my employer, agency, peers, or professional affiliates. The above post is for information only and does not reflect professional advice and is not intended to supercede the professional advice of others.)

AMDG