Ventillated crawl space


Old 10-26-05, 02:19 PM
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Question Ventillated crawl space

In a conversation with a local builder (Chesapeake, VA), this subject came up, but he had only heard of it. The idea, as we understand, is to remove the batts from the crawl (it's my house, and I'm only at +6.75 feet, so it's pretty damp already), put down at least 6-8mil pf plastic, then glue up foam board on my solid 8" foundation all around, covering over the natural vents, but leaving them in place. Follow this with one AC vent, with no return, creating a positive pressure set up that is supposed to dry out the crawl.

So does anyone know about this sort of thing, and is is feasable? What is wrong with it? What is right with it? Are we missing something?

thanks for the time, whoever you are...
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Old 10-27-05, 08:35 AM
Join Date: Nov 2001
Location: USA
Posts: 1,873
High Pressure to Low Pressure

This literally applies to everything, like a higher temperature object will give heat to a lower temperature object and not vise-versa. In other words, equilibrium is obtained by lowering the temperature of the higher temperature object equal to the amount of energy absorbed by the lower temperature object. Thereby increasing the temperature of the lower temperature object, until both objects are equal in temperature.

This same rule applies to moisture. In fact, you will find this throughout nature. This is also the reasoning for the best way to describe nature is balance. A thermal boundary that physically separates a conditioned space from a unconditioned space explicitly implies that it is there to defy nature or to prohibit the two space from equalizing.

There are several processes that occur within a structure. However, these processes are usually explained independently of other processes when dealing with specific applications. In other words, this particular applicatiion you mentioned may address the moisture problem but it will interact with the other processes within the structure that may have an adverse effect. This type of thinking follows Newton's third rule of Motion. "For every action, there is an equal reaction."

If I were to apply this to the situation you decribed, what I would base my opinion on is that the volume of air within a confined space, regardless if it is the house or a crawl space, remains constant. So if you were to take house air and blow it into your crawl space through a supply duct, without a return duct. It would create a higher pressure area that would force the moisture inside the crawl space outwards, away from the crawl space. Which follows the rule High to Low.

However, the same rules that apply to the crawl space applies to inside the house. If house air is used to create this positive pressure inside the crawl space, then an equal amount of negative pressure is created inside the house. If you follow the same rules, High to Low, then the moisture problem is moved from the crawl space to your exterior walls and attic floor. If you blow air into a balloon, it gets bigger. And if you let air out of it, it gets smaller. Any confined space of the structure cannot get bigger or smaller as air is either blown into or taken away. Which creates the high and low pressure areas.

This exemplifies how an application clearly addresses a problem but fails to include how the process to address the problem interacts with other processes within the structure.

Mind you, this application has been around for decades. It is primarily used in skyscrapers or very tall buildings. Altitude, stratification, stack effect and much more may have some effect with residential structures but never at the magnitude of very tall buildings. These systems are quite elaborate and have a direct relationship to the amount of maintenance and the longevity of the tall building itself. Structural components of tall buildings must maintain a certain amount of moisture to remain stable. In other words, too much or too little moisture these structural components have results in considerable more maintenance, which eventually affects the life of the building. Furthermore, these system are multi-functional, example, maintaining a Sustainable Healthy Indoor Environment for the occupants of the building. While Make-up air supply systems can also be applied to residential structures to offset the effects of the negative pressure inside the structure, there are other concerns. Such as Economy of Scale. Furthermore, there are probably other solutions to your situation that are far more cost effective.

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