# Vapor Barrier for Crawl Space. What mil plastic?



## twilightcall (Jan 4, 2008)

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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## concretemasonry (Oct 10, 2006)

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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## Termite (Apr 13, 2008)

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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## concretemasonry (Oct 10, 2006)

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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## Termite (Apr 13, 2008)

Yup, I'm an expert on minimum standards! :laughing:

You're always better off doing more than the code says. Most people don't, but I wish they would.


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## mikey48 (Dec 6, 2007)

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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## twilightcall (Jan 4, 2008)

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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## Posty60 (Feb 22, 2011)

*Vapor barrier for crawl space*

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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## mrgins (Jan 19, 2009)

I'd like to know more about this 80% coverage. Whats that all about?


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## jklingel (Dec 21, 2008)

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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## md2lgyk (Jan 6, 2009)

thekctermite said:


> 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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## Termite (Apr 13, 2008)

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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## TxJack (Jan 13, 2012)

jklingel said:


> 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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## mrgins (Jan 19, 2009)

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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