cellar basment vapor shield


Old 04-22-05, 01:08 PM
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cellar basment vapor shield

I just bought a house from 1900. It has a partial cellar basement with stone, dirt and bare earth foundation. My furnace is down there. It is about three years old and works great, so this hasen't been a problem yet, but the home inspector advised me to put a vapor shield down. What is that and can I do it in that kind of basement? The floors are dirt.
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Old 04-23-05, 03:28 AM
Join Date: Nov 2001
Location: USA
Posts: 1,873
Vapor Shield may be a moisture barrier product that I never heard of.

A moisture barrier primary purpose is to prohibit moisture from entering the structure.

A vapor barrier primary purpose is to prohibit condensation in relations to temperature loss (Dew Point).

Unfortunately, the terms "moisture" and "vapor" are used interchangable by everyone.

In your situation a moisture barrier is applicable. This is designed to prohibit moisture in the dirt floor of the basement from evaporating and entering the basement and eventually the entire structure. The "GAS LAW" states that a liquid can only turn into a vapor when this liquid is absorbing heat (evaporation) and a vapor can only turn into a liquid when this vapor is expelling heat (condensation). This is important because heat is energy and energy can neither be destroyed nor created, it can only be transformed.

So what is occurring in your basement during the winter is the moisture in the dirt floor is evaporating into the basement. But according to the "GAS LAW", this liquid requires absorbing heat to evaporate. It obtains this heat from the heating system located in the basement during the winter. And during the summer the warmer weather outside. The evaporation then enters your basement and finds its way into the rest of the structure through a variety of ways but primarily through equilibrium. This evaporation of the moisture in the dirt basement floor increases the humidity level throughout the structure which increases the probability that a moisture problem can occur, example mold and mildew. While this is considered the most important factor for moisture barriers, there are other things to consider.

For example, let's say you apply a plastic sheet over the dirt floor (moisture barrier) or similar functioning product. This prohibits the moisture in the dirt floor from evaporating. In turn, it reduces the probability of a moisture problem occuring inside the structure by reducing the amount of moisture entering the structure. And at the same time, it reduces your heating and cooling costs.

During the winter, the heat required for the evaporation is extracted from the heating system, either directly or from the house itself. In other words, by prohibiting the evaporation in the dirt floor, you lower your heating costs.

During the summer, the heat outside is used to cause the evaporation. This moisture vapor permeates the structure and because of cooling, it condenses. But according to the "GAS LAW" this vapor must give off heat to condense. The amount heat energy outside used to transformed the moisture in the dirt floor into vapor is equal to the amount of heat energy given off when this vapor condenses. In other words, by prohibiting the evaporation in the dirt floor, you lower your cooling costs.
Old 04-23-05, 03:42 AM
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Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,389

Id just cover the floor with a 6 mil poly. Over lap it 2' and tape the seams. could cement over it later.

Old 05-11-05, 12:36 PM
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now i know

Okay, now I know what it is and why I need it. Thanks you guys.

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