08:44PM | 05/20/04
Member Since: 05/14/04
6 lifetime posts
Have a 1700 sq ft pier and beam house in east-central Texas. There is about 3 feet of air space between the soil and joists. Have 6-mil plastic over the dirt/sand in the crawl space. Foundation has seven 6x12" screened vents on 3 of the 4 foundation walls. The house is along a shoreline with the bottom of the crawl space about 4 feet above water level.

Durring the high heat and humidity of the summer months, I have had condensation on the top of the plastic. So far this year the top of the plastic has been dry. There is moisture on the bottom of the plastic.

The previous owner added a ventfan to the crawlspace entry cover to increase airflow.

As part of the current remodeling, I've considered removing the ventfan from the entry cover, running a 8" duct from the crawlspace to the roof (sealed through house)and putting a turbine vent on top of the duct. Since we almost always have a breeze, this would provide good air draw and eliminate an unsightly ventfan.

My question is: Do I really want to draw more air through the crawl space?

During the summer months, if I draw warm humid air into my cooler crawl space...won't I get condensation...and perhaps make the situation worse? ?

A second question:

The previous owner put r30 fiberglass insulation between the joists in the crawl space with the paper facing down. I've read that this may be backward...any comments?

Thanks in advance for any thoughts!


09:03AM | 05/21/04
Member Since: 07/01/03
550 lifetime posts
The comment about the direction of your vapor barrier (facing crawlspace) caught my attention. For every rule there is an exception, and you live in one. The conventional rule of vapor barriers facing conditioned space is the absolute rule for cold climates, and even moderate dry climates. You live in a hot to moderate humid area. Relying on common sense, most of the year, you use cooling rather than heating. Warmer humid air in your crawlspace naturally trys to rise and mix with the cooler air in the house. If it was able to do so, it would condense the moisture it contains causing a wet floor or framing condition. This would be bad, but it explains why your vapor barrier faces downward.

The other side of the equation might be during the relatively brief heating season, but heated air contains lower humidity levels to start with, and temperature differentials are exactly like what is found in Minnesota, so, IMO, your vapor barrier is in the right place.

Maintaining ventilation in the crawlspace simply means that the temperature and humidity is maintained closer to equilibrium with the outdoors. During periods of temperature change, when humidity is near dewpoint, you will see condensation on one side of your floor barrier or the other, but overall, it should not build up. Here in California, we have 35 6x12 vents in some 3000 sf of crawlspace. That would be like you having twice as many as are present or 14 vents. I don't know what is right for your area, but here, the objective is to move air through to equalize moisture and temperature. Your vent stack concept sounds interesting, but would rely on convective flow to work. The turbine fan does give a mechanical assist to move air through the stack, but it does not get the thermal lift like when you ventilate an attic with a turbine. It might be easier just to add additional passive vents. Mine are located every other stud opening around the back and sides, but none in the front of the house.

Since our climate regions are so different, you may get better advice from a fellow Texan, but I think most ventilation issues are common sense.


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