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Old 05-08-2008, 12:12 PM   #1
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Vapor Barrier for Crawl Space. What mil plastic?


My home was built in 49 and has a crawl space. I need to have a vapor barrier. I have read that you only want to cover the area 80% and leave 20% uncovered. Does this sound correct?

Also what size plastic should I use? I was told 4 mil plastic sheeting was ok to use but 6 mil plastic sheeting was the best. I was wondering what is the best to use. Thanks.

It hasn't had a moisture barrier ever on it but I was told it needs one so my wood won't get a fungus.

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Old 05-08-2008, 01:06 PM   #2
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Vapor Barrier for Crawl Space. What mil plastic?


You are not really putting in a "moisture barrier", but a moisture retarder. The ability to stop moisture depends on the thickness and number of perforations/seams.

For me, 6 mil would be a minimum, with 10 mil better. As you install and do any future work, the 10 mil is superior and much more durable.

It all depends on how important the cost is to you.

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Old 05-08-2008, 01:39 PM   #3
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Vapor Barrier for Crawl Space. What mil plastic?


The 2003 IRC code allows you to use a vapor barrier in a crawl space if the crawl space is supplied with conditioned air (AC and heat via large openings or actual duct outlets) and has insulated exterior walls. As far as thickness, it just calls for "approved" vapor barrier. 6 mil is the requirement for poly under concrete, so I'd probably call for the same thickness.

If your crawlspace has ventilation openings to the exterior, the vapor barrier is a no-no.
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Old 05-08-2008, 03:01 PM   #4
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Vapor Barrier for Crawl Space. What mil plastic?


Code is the worst you can do and still be legal. - It does mean that it is appropriate, practical or econonical in the end.
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Old 05-08-2008, 04:04 PM   #5
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Vapor Barrier for Crawl Space. What mil plastic?


Yup, I'm an expert on minimum standards!

You're always better off doing more than the code says. Most people don't, but I wish they would.
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Old 05-09-2008, 09:26 AM   #6
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Vapor Barrier for Crawl Space. What mil plastic?


We are talking about a vapor barrier on the ground? In the northwest most homes with a crawl space and vents have a vapor barrier on the ground. I cannot imagine having just dirt.
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Old 05-09-2008, 04:14 PM   #7
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Vapor Barrier for Crawl Space. What mil plastic?


Yes I am talking about a crawl space with a dirt floor in the southeast. It does have vents so it can breath. I was told by 3 different companies to lay down plastic barrier. It is very common around here to cover 80% of the ground with a plastic vapor barrier. I just wanted to make sure I got the correct size.
If I covered all of the holes and put a barrier all over my wooden joists would be too dry I think.
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Old 02-22-2011, 02:20 PM   #8
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Vapor Barrier for Crawl Space. What mil plastic?


I too have an older home in a damp area. I am preparing to order 10 mil plastic sheeting to lay in my crawl space. I was confused by the post from thekctermite "If your crawlspace has ventilation openings to the exterior, the vapor barrier is a no-no." I have them but have them blocked off. What is the reason for not putting down a barrier if there are vents. I'm just trying to understand the science behind all of this.

Thanks.
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Old 02-22-2011, 06:25 PM   #9
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Vapor Barrier for Crawl Space. What mil plastic?


I'd like to know more about this 80% coverage. Whats that all about?
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Old 02-23-2011, 11:44 PM   #10
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Vapor Barrier for Crawl Space. What mil plastic?


80% coverage sounds like an old wive's tale. Why let 20% get at your floor? Any poly is, essentially, a vapor barrier. 6 mil has a permeance of 0.06, which is just short of pure vapor barrier. The only reason I see to go heavier is if you are concerned about people walking/crawling, etc, on it. If that is the case, then go heavy. I also find the black visqueen a whole lot tougher than the "clear" stuff. You should run the VB up the walls about 6" and secure them tight; use Tremco acoustical caulk, or equivalent (it will stick to visqueen), and/or some kind of mechanical fastener, ie 1x2 strips locked into the wall.
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Old 02-24-2011, 05:30 PM   #11
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Vapor Barrier for Crawl Space. What mil plastic?


Quote:
Originally Posted by thekctermite View Post
If your crawlspace has ventilation openings to the exterior, the vapor barrier is a no-no.
Aah, not necessarily true. Where I live, crawlspaces are required by code to be ventilated. But I'm also required to have a Radon mitigation system consisting of a layer of gravel, perforated pipe, and at least 6-mil plastic sheeting. And no 80/20 coverage either - has to be 100% and sealed to the walls.
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Old 02-25-2011, 01:49 PM   #12
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Vapor Barrier for Crawl Space. What mil plastic?


I mis-spoke....Failed to mention that if the exterior ventilation openings are reduced to a factor of 1/1500th of the floor space that vapor barrier is still required. That's a heck of a lot less than the prescriptive code in R408.2.

Md2glyk, ventilation of some sort is always a requiremt, but there are at least three good ways to accomplish that ventilation. Some require a poly vapor barrier, some do not.

The vapor barrier is generally pointless if you have the code-prescribed exterior ventilation in my opinion. No point in holding the moisture below the vapor barrier and promoting a damp environment 100% of the time when it can be ventilated to the outdoors. However if the openings are reduced to the 1/1500th of the floor space the ventilation is so choked off the vapor barrier is warranted.

Here's what the code says.

R408.2Openings for under-floor ventilation.
Theminimum
net area of ventilation openings shall not be less than 1 square
foot (0.0929 m
2) for each 150 square feet (100 m2) of under-
floor space area. One such ventilating opening shall bewithin 3
feet (914 mm) of each corner of the building.Ventilation open-
ings shall be covered for their height and width with any of the
following materials provided that the least dimension of the
covering shall not exceed
1/4 inch (6.4 mm):
1. Perforated sheet metal plates not less than 0.070 inch
(1.8 mm) thick.
2. Expanded sheet metal plates not less than 0.047 inch (1.2
mm) thick.
3. Cast iron grills or grating.
4. Extruded load-bearing brick vents.
5. Hardware cloth of 0.035 inch (0.89 mm)wire or heavier.
6. Corrosion-resistant wire mesh, with the least dimension
being
1/8 inch (3.2 mm).

Exceptions:
1. Where warranted by climatic conditions, ven-
tilation openings to the outdoors are not re-
quired if ventilation openings to the interior are
provided.
2. The total area of ventilation openings may be
reduced to
1/1500 of the under-floor area where
the ground surface is treated with an approved
vapor retarder material and the required open-
ings are placed so as to provide cross-ventila-
tion of the space. The installation of operable
louvers shall not be prohibited.
3. Under-floor spaces used as supply plenums for
distribution of heated and cooled air shall com-
ply with the requirements of Section M1601.4
4. Ventilation openings are not required where
continuously operated mechanical ventilation
is provided at a rate of 1.0 cfm (10m
2) for each
50 square feet (1.02 L/s) of underfloor space
floor area and ground surface is coveredwith an
approved vapor retarder material.
5. Ventilation openings are not required when the
ground surface is coveredwith an approved va-
por retardermaterial, the space is supplied with
conditioned air and the perimeter walls are in-
sulated in accordance with Section N1102.1.7.

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Old 03-29-2012, 10:13 PM   #13
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Vapor Barrier for Crawl Space. What mil plastic?


Quote:
Originally Posted by jklingel View Post
80% coverage sounds like an old wive's tale. Why let 20% get at your floor? Any poly is, essentially, a vapor barrier. 6 mil has a permeance of 0.06, which is just short of pure vapor barrier. The only reason I see to go heavier is if you are concerned about people walking/crawling, etc, on it. If that is the case, then go heavy. I also find the black visqueen a whole lot tougher than the "clear" stuff. You should run the VB up the walls about 6" and secure them tight; use Tremco acoustical caulk, or equivalent (it will stick to visqueen), and/or some kind of mechanical fastener, ie 1x2 strips locked into the wall.
I think I found out what they are talking with not covering all the ground. Supposedly it is if you have wood floors, some humidity would not allow them to "dry out". I saw it on ehow.com.

This whole subject seems to be a complex one. I'm trying to research it as well and am finding suggestions from one end of the spectrum to the other, all of which contradict each other.
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Old 03-30-2012, 11:31 AM   #14
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Vapor Barrier for Crawl Space. What mil plastic?


I'd be tempted tp install 1" rigid foam board to the underside of the joists (with unfaced fiberglass between the joists, and then seal the joints between the boards. If the crawl space is isolated from the rest of the basement, I'd leave the vents open year round and forgo the plastic barrier

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