Vapor Barrier question-Basement


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Old 12-28-07, 12:21 PM
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Vapor Barrier question-Basement

Simple question really. Most of the books I've read suggest framing your exterior wall, installing insulation, THEN putting a plastic vapor barrier between the insulation and the sheetrock.

However, I've seen at least one DIY book that suggest putting the plastic directly over the concrete walls, then frame-insulate-drywall.

I realize this is a Vapor barrier, NOT a Moisture barrier, but I still want it to be as effective as possible. Are there pros/cons to each method, or does is make any difference at all? Also, how does this method compare to using insulation with the vapor barriar built in OR spray foam?


Thanks
David in NC
 
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Old 12-28-07, 12:42 PM
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David. The purpose of the vapor barrier is to keep moisture in the wells of your wall framing from extending to your sheetrock. If it is placed directly on the concrete block, the moisture won't have a place to dissipate as it would in your joist wells. If the walls are under grade level, then insulation would be a moot point since the earth is geothermally correct, and won't vary more than a few degrees year round. Installing insulation with a vapor barrier already installed is easier and will suffice for the vapor barrier requirement.
 
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Old 12-28-07, 02:37 PM
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Larry, I appreciate your answer, and especially the part about moisture being able to escape up through the ceiling (so to speak), BUT

everything else I've read says the vapor barrier is NOT to keep moisture out, but it's to keep the moist warm air inside from condensing against the cold outside walls.

Seems to me that it is a little of both.... and now I'm even more confused, although at least it seems like there are several options, and none are necessarly "wrong"

Again, thanks
 
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Old 12-28-07, 03:24 PM
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David,

You are absolutely correct about the vapor barrier. It's job is not to keep moisture out, it's to keep the vapor generated in your house from getting into your walls. The confusion is caused because the most common product used as a vapor barrier (plastic) could also stop moisture. But the building code is very specific in differentiating between vapor and moisture. In climates where freezing takes place, it is very important to always have the vapor barrier on the warm side of the insulation. Reason? Winters are typically very dry and the without a vapor barrier, the vapor created inside your house (showers, cooking, etc) would be sucked through your walls like a magnet. On the cold side of the insulation, the vapor could condense and freeze onto the nearest surface. This will continue to build until spring comes, it melts, then you have a whole host of problems to deal with. The vapor barriers job is to stop this transfer.
 
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Old 12-28-07, 06:32 PM
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great answer. That makes great sense. Unfortunately, many home improvement books don't differentiate between climatic conditions, and so it has caused my confusion.

Again, thanks!
 
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Old 12-28-07, 09:23 PM
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http://www.eere.energy.gov/consumer/.../mytopic=11810

This site discusses Vapor Diffusion Retarder's (VDR's) or vapor barriers if you prefer. Some States and/or municipalities mandate moisture barriers applied directly to foundation walls. This probably has to do with soil type, climate conditions and/or water tables. You should check with your local city officials if there are any such type of requirements prior to starting your project.
 
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Old 12-29-07, 05:50 AM
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great link, I visited that site, but missed this particular page.

I'm in the "interior" section, just barely, but it certainly help clear up the confusion for me.
 
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Old 12-31-07, 06:15 AM
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I have a quick follow up. Thinking of installing a "sleeper" floor. Everything I've read suggests putting down plastic first, then installing the joists/insulation/subfloor on TOP of the plastic. In cold climates, this seems opposite of the advice we discussed for walls.

In a flooring situation, are we talking about a "vapor" barrier, or in this case is it really more of a moisture barrier.

And if this advice is correct, would I wrap the plastic on the floor directly to the plastic on the front of my insulated walls???

I know I'm being overly particular (I'm an engineer), but I'm just trying to understand the reasoning behind all of this.

Thanks
 
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Old 12-31-07, 07:27 AM
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Dave, I wish I had more time to respond - I could write for an hour on your last question but obviously I can't. Instead I'll ask you a question. Why do you think you need a sleeper floor and even more importantly, why do you think you need plastic draped over the existing concrete floor?

In the little time I have right now I will say that the method of construction you are considering is a recipe of mold. If you have water issues on your basement floor, covering it with plastic is not the answer, trust me it will cause you more headaches that it's worth. I don't know how many times I've had to pull these floor systems out of basements, and I'm usually wearing a sheetrockers dust mask when I pull the plastic off because of the bad odor from the years of water getting trapped below the plastic.

Jay
 
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Old 12-31-07, 08:51 AM
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I agree with MN Jay and would like to add more to it. Besides leakage there are three moisture transfer mechanisms in basements. They are Equilibrium, Capillary Suction and the most overlooked, even by professionals is Gravity. The first factor one has to consider is that all materials must maintain a certain amount of moisture in order to remain stable and the amount of moisture within materials are in constant flux. A moisture problem is then defined as the inability of a material to either absorb or expel the amount of moisture being introduced.

To illustrate a single aspect of this is hanging a wet towel on a clothes line. After a few minutes if you touch the top of the towel it may be moist but not wet. If you move your hand down the towel the more moisture you will sense and at the bottom of the towel it will be wet. The towel represents the structure and the bottom of the towel represents the basement. In other words the basement masonry is a essential part of the drainage plane of the structure.

While there may be other sources for the moisture problems in basements and professionals who are well aware of the need to control the amount of moisture being generated inside the structure, things rarely stay the same. It's called "Murphy's Law".
 
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Old 12-31-07, 11:56 AM
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I may have not been clear enough. This is a new home, poured concrete walls, new concrete floor with plastic underneath the floor. It also has, to the best of my knowledge, propper drainage and waterproofing (rubberized coating, plastic corrugated sheets etc) around the outside basement walls.

Anyway, I don't anticipate having a water problem. I'm looking to totally finishing the basement, and am trying to avoid condensation and moisture entrapment issues.

Every home improvement book I've read, and most of the TV shows, suggest putting plastic under any basement concrete flooring system. In addition, most suggest a sleeper floor system (often with foam insulation panels) to both give the floor a "warmer" feeling, and a "softer" feeling.

Our basement is nearly 2000 sq ft, and will have a home theater, exercise room, spare bedroom, and a very large living area. We want it to be as cozy as the rest of the house, and the sleeper floor seems to be suggest by many sources.

So, back on topic, I'm looking at keeping condensation out of my floors, NOT preventing external water from flooding. In this case, what do you suggest. I appreciate your experience and oppinions.

Dave in NC
 
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Old 12-31-07, 02:42 PM
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http://www.dricore.com/en/eIndex.aspx

This site shows a product that will work better than sleepers. Keep in mind that cement came to your home in a liquid form and it will give off a considerable amount of moisture as part of the curing process the first few years. Because your house is new I strongly recommend that you use a dehumidifier once you finish the basement. In fact I would go as far as recommending an "Air to Air Exchanger".

There is a process known as "Equilibrium Relative Humidity" (ErH%). This states that an object that comes in contact with another object of lower humidity, the object of higher humidity will give moisture to the object of lower humidity until both objects are equal in humidity and not vice-versa. The most common form of this moisture extraction from structures is known as "Air Exchange". Fresh from outside the structure during the winter is usually at a lower humidity level than inside the house. So as this fresh air enters the house it absorbs moisture generated inside the house. As the air exchange continues this higher humidity air inside the house will eventually find its way out of the structure. Thereby extracting the moisture generated inside the house.

Air exchange for residences is measured in volume per hour. A new house today under "Energy Code" will have a one of less air exchange per hour. An "Energy Star" home will have a 1/3rd exchange per hour. What this means is that a new house will exchange all the air within the house every hour and an Energy Star home will take 3 hours to exchange all the air inside the house. Since Air Exchange is a major source for extracting moisture generated inside the structure which house has the higher probability to have a moisture problem?

One of the factors that determine air exchange is known as "Openings in the Envelope" and they happen to be the windows and doors of the house. If we consider the number and size of windows and doors upstairs compared to the basement, we can deduce that the air exchange in the basement will be considerably less than upstairs. Which area of the house has the higher probability to have a moisture problem, upstairs or in the basement?

As an energy conservationist I was not instrumental in determining Energy Code. However, I did influence the methods on measurement and verification for compliance to the Code. So when the problem with "Sick Building Syndrome" came about in residential homes, I was asked to investigate some of them. FYI all the house that got condemned all the problems were started in the basement and all the basements were finished. Oh and let's not forget that all these basements were finished in accordance with energy conservation practices and purpose.
 
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Old 12-31-07, 03:12 PM
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definitely in no hurry to finish the basement, and we will be dehumidifying.

I've seen the dricore, as well as some competitors (they of course claim to be better than each other). I'm looking into that as well as sleeper floor, or just putting carpet directly onto the concrete.

From what I'm hearing, it almost sounds like whatever you put down, you might as well plan on tearing it out every ten years or so to dry things out. Very frustrating to say the least.

Thanks for the info.
 
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Old 03-23-08, 07:16 PM
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replacing a vapor barrier in a crawl space

I have a vaper barrier in a crawl space thats in bad shape. I was wondering if i can put new over the top or do i need to remove the old first?
ranch house in lower michigan, unheated, dirt floor, few air vents.
 
 

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