Crawl Space Vents
There are numerous people who simply do not know the use of crawl space vents. I was puzzled with these small vents when years ago I was involved in installing them in rooms that were built new. I used to wonder at the science behind the vent that mostly prevented air from entering the crawl spaces. I could hardly feel any draft of air coming through the vent when I was in the crawl spaces even on windy days.
The logic with which crawl space ventilation operates is not so difficult to decipher. Place a clear plastic piece on the dry ground in your yard. Place some boards on the edges of the plastic to keep it on the ground so that air does not get under the plastic sheet. You can see the results immediately if it is a sunny day.
A slight fog begins to appear within minutes underneath the plastic sheet. This fog after some time begins to turn into water droplets. Though the ground seems dry, actually the soil in your yard liberates water vapor constantly. This is the same case with crawl spaces. The moisture is pulled from the ground due to the natural evaporation, wind and sunlight.
The humidity in the soil produces the water vapor. The water vapor that escapes from the ground in the extremely arid parts of Southwest USA for most part of the year is very little. The soil gets wet during the monsoon season anyway. But for those who live in the Midwest, Northeast, Northwest and Southeast, water vapor actually streams out of the soil all the year round.
The water vapor that emerges out of the soil in your yard escapes into the atmosphere. This happens where the yard has open parts. The water vapor collects in the case of crawl space on the under side. This then condenses on the subfloor structure and the foundation walls. If the subfloor were to be wood, then mildew results from the water. Wood rot will ultimately develop.
It was believed that crawl space vents should be installed to make sure that outside air enters the crawl space in order to reduce the relative humidity. From my experience with being in a number of crawl spaces which has a large number of open vents is that I felt that I am in damp cave. I surmise from this that the vents hardly circulate sufficient outside air into the cramped crawl space.
Besides installing the foundation vents that passes the required building code, it will be a good idea to install an efficient vapor retarder in the crawl space over the soil. The vapor retarder is secured on to the wall which has wood that is resistant to rot or a treated lumber and lapped up to the sides of the foundation. Superior vapor retarders have a special tape which permits you to seal all seams permanently where the vapor retarder laps over itself. Or it may be cut around those objects that stick through the ground in the crawl space.
The water vapor is blocked by this vapor retarder. This makes the crawl space vents redundant. But then you need to check out whether what I just said satisfies the building code that is applicable in your area when you are adding another room. May be the building code insists in the installation of the crawl space vents. Whatever may be the case, it is always better to make sure that you completely comply with the applicable building code for your locality.
There are some building departments which permits what I have just suggested. They permit vapor retarder installation in just the way I have described and moreover do not need any foundation vents. The side walls of our crawl space require to be insulated in case vents are not installed. Conditioned air in small amounts has to be piped in by the heating contractor into the space. In addition, the foundation insulation should either be covered with material that is fireproof or it must be fireproof.
You need not bother about vents if the high-performance vapor retarders are installed correctly. It really does not make any difference if you keep them closed or open.
The crawl space ventilation represents minimum specifications of building code. The regulatory function of the building code is great in setting standards. But this in no way means that these standards are the best nor does it allow better way of doing things. There are innumerable instances where the builders could go beyond what the code specifies. They can also use materials that are better or put in that additional work that will make sure that a home could go beyond 40 to 70 years and stretch to a few hundred years.
Residential construction encounters water related major problems. Leaks can create despair and havoc. It may be even a little water in the exterior walls, attic farming, floor joists or the crawl space that could pose major problems in due course. Preventing the water from entering the crawl space completely and keeping them in the soil is the best protection against problems from water vapor. This is also applicable for concrete slabs. Vapor retarder that has a high efficiency should always be kept under the slabs to prevent water vapor seepage through the concrete
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August 1, 2008 | Ventilation
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