Crawlspace Insulation


Old 09-05-03, 08:21 AM
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Crawlspace Insulation

We own an old 1911 house in N.C. We are in the process of installing a HVAC system. The floors have no insulation at all. Were thinking about installing R19 insulation with a vapor barrier. If we do this will we need to put down a plastic barrier on the ground as well? Right now it is dry as a bone. What about the vents? Should they be opened? closed?

Any advice is appreciated.
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Old 09-08-03, 06:37 PM
Join Date: Nov 2001
Location: USA
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This topic is actually very confusing for the average person and I am going to try to explain it. The reason for the confusion is that most people think of ventilation as being open for the summer and closed for the winter. Ventilation is primarily used to control moisture, so the exact opposite is true. This is because when addressing a problem, you must address the source of the problem.

For example, in the winter, the primary source of moisture in a crawl space comes from the house. This is assuming that all other sources are being controlled, like having a moisture barrier over a dirt floor. Insulation does not stop heat flow, it slows it down. Meaning to say there is a constant flow of heat through the insulation into the crawl space. Since all heat has moisture in it, it is the primary source of moisture into the crawl space during the winter. The outside cold air cannot hold much moisture and as it enters the crawl space it absorbs the heat that managed to get into the crawl space from the house. At the same time this gives the cold air from the outside the ability to also absorb the humidity in the heat it absorbed. The result is that opening the vents in the winter avoids moisture problems in the crawl space by addressing the source of the moisture. This example exemplifies how ventilation deals with heat transported moisture.

In the summer we are not dealing with heat transported moisture. Instead the primary source of moisture in a crawl space is air transported moisture. There are two reasons for this. One is warm air condenses on cooler surfaces and the other is the masonry and dirt in crawl spaces are heat sinks. As this term implies it is something that drains heat from the area. This makes the dirt and masonry cool surfaces for warm air to condense on. Now you could insulate the crawl space to avoid the cooler surfaces but that does not address the source of the moisture. Which happens to be the warm humid air outside the crawl space. The real way to address the source of the moisture in a crawl space is to close the vents during the summer.

You lay down the plastic over the dirt because it becomes a source for moisture that is pressure induced. Equilibrium Relative Humidity (ErH%) states that any two objects or more that have different humidity levels will absorb and expel humidity until the humidity levels are equal in the objects. In other words, let's say it rains but the air inside the crawl space has a low humidity level. ErH% states that this will force moisture from the outside into the air in the crawl space through the dirt floor in the crawl space. This is done through a static pressure process known as capilliary action. Meaning to say that the difference in water vapor pressure causes the attraction. Laying the plastic over the dirt floor prohibits that from occuring, hence it addresses a source of moisture inside the crawl space.
Old 09-09-03, 04:42 PM
Ed Imeduc's Avatar
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,389
Lightbulb crawl way

We have always closed them up 6mil poly on the floor over lap 2' tape seams . up the wall 2' also . then 2" polystyene on the walls up to the joist and Insulattion in the joist there just on the sill plate all around the home. When the duct work is in the craw way we cut in 2 outlets and one return in the duct.this works like a heat sink for you . look at and think it over.


Old 09-21-03, 01:08 AM
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Thanks for the info. We are insulate and covering ground with plastic. I think I will install one of those remote humidity sensors in crawl space. The DOE site had lots of useful information.

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