Question concerning mildew/mold in crwlspc

I’m sure this has been discussed numerous times but I’m looking for fresh input. I inspect in a high humidity area and I notice that crawlspaces that have NO venting seem to have very little to no mildew/mold problems. I know that the pros in this area say moisture moisture moisture is the love of mold and mildew which is definitely in our air here. What my issue is everything tells me to look at proper crawlspace venting but with what I have seen over the years I am leaning toward telling clients to shut down the crawlspace for good. Any input on this would be appreciated. Scott Truslow

Scott. It largly depends on the geographic area. Basically speaking southern locations benifit from vented crawl spaces. Northern location are generally better off with out it.

In northern locations where there is snow during the winters and the crawl spaces are vented, often the vents are blocked by the snow and not allowing the cold dry air to enter the crawlspace and reduce moisture levels. During the summer months the warm humid air enters the crawlspace where it is usually cooler so the moisture in the air condenses in the crawlspace.

When I bought my 136 year old house I was told by the inspector to increase the venting in the crawlspace from what was there. I did just the opposite and sealed it up good. It is now extreamly dry.

A vapour barrier on the ground or slab of the crawl well sealed at the edges is also recommended to prevent ground moisture from comming up.


There is venting and then there is venting!

I have assessed the hypothesis that ventilating outdoor air from the state of Tennessee into your crawlspace produces more moisture issues in the crawlspace then reduces.
This issue can be plotted on a psychometric chart to support this theory. However, adding ventilation which increases moisture from condensation occurring from the water vapor in the outdoor air which is ventilating the space is still **insufficient **ventilation. Adding extra vents does not mean the problem is solved. It can actually make the situation worse (as many home repairs also do).

If you’re going to adequately ventilate then you must increase the temperature of the air and all objects within the crawlspace to a point above the dew point temperature of the ventilation air (outside air). Failure to do so will result in condensation. HVAC is a term to describe heating, ventilation and air-conditioning. The letter “V” is overlooked, misunderstood and improperly applied in most cases. I have even seen service vans with the “V” removed from their vehicle signs!

You may not be looking at the entire scenario when considering completely encapsulating the crawlspace area and preventing ventilation from occurring. Often a device is installed on the HVAC system to introduce outdoor air through the HVAC equipment (which dehumidifies the outdoor air being entrained). This slightly pressurizes the atmosphere inside the house. As you know, plumbing and electrical penetrations through the flooring system allow air passage between the living space of the house in the crawlspace. The pressurized indoor air also pressurizes the crawlspace and forces humidity out of the crawlspace by introducing air which has been conditioned to remove the water vapor so that the dew point temperature of the mixed andair in the crawlspace is higher than the objects within the space (condensation can no longer occur).

As you are a home inspector in the state of Tennessee, the new state inspection law relieves you from performing engineering and mold assessment. However, understanding these principles will help you assess whether adequate ventilation actually exists. You can inform your client that adding a couple of vents may not correct the situation without actually having to do the psychometric evaluation. Simply report on the conditions within the space (damp, musty, condensing water vapor) and the presence of a substance that grows in high moisture environments which may affect the wooden structural components of the flooring system within the crawlspace. :slight_smile:

In conjunction with moisture you need the presence of mould spores. Mould is part of natural plant life & travel in & out of our homes evertime a window or door opens & closes. Maybe your on to something with the closed vent idea. No ,now that I think about it , it would open up other issues. Doug

Scott, check out this web site. It changed my way of thinking on the whole “vent or not to vent”. I used a similar system on my second home and the results were phenomenal. I can vouch first hand that not to vent is the way to go. Hope this helps. Better late than never. Andrew