Quick crawlspace question

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  #1  
Old 01-25-05, 06:20 PM
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Quick crawlspace question

I have a family room over an unheated crawlspace. There is plastic covering the floor and what appears to be R-25 insulation. The floor above is cold, even with carpeting. Few questions:

1) What R is recommended for a crawlspace? I am in CT and it is 4 degrees right now.

2) Since ceiling is insulated, insulating the walls is out correct?

3) Finally, is it ok to close the vents in the winter to warm the crawlspace a bit? If so, what problems should I be checking for? Moisture, etc. Thanks!

Andrew
 
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  #2  
Old 01-27-05, 04:29 PM
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You are correct, if the ceiling is insulated you should not insulate the walls. Since the ceiling of the crawl space is insulated you have to vent the crawl space to remove the moisture in heat that manages to get pass the insulation.

The floor is cold over the crawl space because of the exposure of surface areas of this room to unconditioned spaces, like the crawl space. This makes this room harder to heat and in fact requires more heat to the room due to the heat loss created by the exposure. Suggests asking question in heating forum or a qualified contractor on the variety of ways to increase and/or supplement heating to this room to compensate for heat load.
 
  #3  
Old 01-29-05, 07:36 PM
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Can the venting be reduced during winter to reduce the amount of cold air? I may be wrong in assuming there is less humidity/moisture during winter.
 
  #4  
Old 01-30-05, 10:17 AM
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Correct, there is less humidity in the air during winter.

The reason why the inside air is drier (lower humidity level) during the winter is the heating of the air inside the house. Relative Humidity (RH%) describes the amount of humidity an air mass can hold at a certain temperature. For example, if an air mass is at 50 degrees Fahrenheit with 100 grains of moisture in it, it may have a RH% of 50%. But if I raise the temperature to 70 degrees Fahrenheit. The amount of moisture in the air does not change but the RH% goes down to 20%. The reverse is also true. By dropping the temperature of the air mass, the RH% goes up but the amount of grains of moisture remains the same. In other words, RH% describes the relationship of certain amount of moisture in something under different temperature conditions. For example, in the above example let's say Dew Point, 100% Saturation or the formation of condensation should occur with this air mass with 100 grains of moisture is 35 degrees Fahrenheit. But let's change the amount of moisture in this air mass to 80 grains. Then at 50 degrees, the RH% is at 40%, at 70 degrees, 10RH% and the Dew Point is at 25 degrees Fahrenheit. This is why it is called RELATIVE. Phychrometrics is the term used to describe this behavior under Physics.

The Water Shedding, Expulsion or Drying Out process of the structure relies of RH% in the process known as Equilibrium Relative Humidity (ErH%). This states that an object of lower humidity will absorb humidity from an object(s) of higher humidity levels, until the humidity levels in the objects are equal. The principle of RH% still apllies but the rule lower absorbs from higher until equal is added.

To answer your question you have to apply the aforementioned. It is more important to me that you understand the reasons why you have to leave your vents open during the winter, rather than me expecting you to accept what I say. In other words, merely stating that you should leave your vents open during the winter because I say so and I am an expert, is not only inappropriate, it does not mean squat. Anyone can call themselves an expert.

Insulation does not stop heat flow, it slows it down. Vapor barriers slow down moisture flow that is inside the heat, but does not stop it. So there is a continuous flow of heat that contains a percentage of moisture in it, into the crawl space. Think of it as a leaky faucet that drips water into a bucket. If the bucket is in a sink, the water that overflows goes down the drain. If the sink drain is blocked or closed, the basement will eventually flood. The dripping water represents the continuous flow of heat with a percentage of moisture in it. The bucket represents the crawl space. The overflow represents the condensation of the heat. The sink drain represents the vents in the crawl space.

Even though the amount of moisture is reduced by the vapor barrier, the moisture flow is continuous. As the heat with this reduced amount of moisture in it enters the crawl space, it begins to lose or lower its temperature. And when you lower the temperature of air, you increase the RH%. At first this should pose a problem, but the continuous flow of this heat with a percentage of moisture into the crawl space, eventually 100% Saturation, Dew Point or Condensation will be achieved.

The outside air during the winter is not only cold but also possesses less grains of moisture per volume of air. The vents in the crawl space allows this outside air to enter the crawl space continuously. As it enters the crawl space it absorbs the heat. The rule low absorbs from high also applies to temperature. When you increase the temperature of air, you decrease its RH%. Now the colder air from outside has a considerable less humidity level than the heat with a percentage of moisture in it that managed to get through the insulation. ErH% takes over and the rule applies. The lower humidity of the cold air from outside absorbs the humidity from the higher humidity heat that managed to get through the insulation. As with the heat flow with a percentage of moisture in it, the vents provides a continuous flow of cold air into the crawl space, thereby creating balance. Which is the best way to describe nature.

Moisture barriers (plastic sheets over dirt floors in crawl spaces) also reduce the amount of moisture introduction into the crawl space but address different functions. Namely capilliary action and evaporation. While all the things mentioned here attempts to prevent moisture problems in crawl spaces, they do it by controlling the amount of moisture into the crawl space. They DO NOT impede the Water Shedding, Expulsion or Drying Out process of the structure(crawl space).
 
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