Vapor Retarder Under Slab

Voting section related to Minnesota building codes.

Does the vapor retarder need to be directly under the slab?

Poll ended at Fri Feb 29, 2008 1:36 pm

Yes
16
53%
No
14
47%
 
Total votes : 30

Vapor Retarder Under Slab

Postby forumadmin » Thu Jan 31, 2008 11:00 am

Is the vapor retarder of IBC 1910 or IRC 506.2.3 required to be placed directly under and in contact with the concrete floor, or may a layer of sand be placed on the vapor retarder prior to pouring the concrete be considered an “alternate method”?
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Re: Vapor Retarder Under Slab

Postby RDavidson » Thu Jan 31, 2008 4:43 pm

IRC 506.2.3 requires the vapor retarder "shall be placed between the concrete floor slab and the base course or the prepared subgrade where no base course exists." So, yes, it is required to be placed directly under and in contact with the concrete floor. I would also say that, yes, I would approve as an alternate putting a cushion of sand over the top of the vapor retarder unless the sand is the type they have in Ramsey that continues to weep water for decades. But if it is your normal, inert, water-free sand, why not? Is there any rule that requires a concrete floor in a basement, slab on grade house, or heated garage? Could I use pavers? What about class V? Would I still need the vapor retarder?
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Re: Vapor Retarder Under Slab

Postby Jimmy » Fri Feb 01, 2008 10:25 am

I believe the intent for the vapor barrier is to minimize moisture migration from the ground to the building. The location of the vapor barrier would make little difference for that intent. The concrete experts would require placement of sand over the plastic and in direct contact with the concrete to minimize curling of the slab. The mold experts would not allow sand in the confined area between the slab and the plastic as it is an excellent breeding area for nasties. The placement of sand on top of the plastic would be an acceptable alternate but I personally would use an open graded gravel vs sand.
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Re: Vapor Retarder Under Slab

Postby Joe Ehrlich » Wed Feb 13, 2008 3:31 pm

The most informative discussion that I've found on the subject of the under-slab vapor retarder is located at the following link (highlight the link, right-click, and select "search the web" to follow the link). I encourage you all to read the article and tell us what you think.

http://www.buildingscience.com/document ... main_topic

I’m convinced by available science that best practice and the intent of the code are served only by assuring that the vapor retarder is placed in contact with the slab. Placement of a sand blotter between the slab and the vapor retarder provides a reservoir of moisture to migrate upwards through the slab, to condense on the underside of the floor covering, providing a perfect environment for mold and defeating the purpose of the vapor retarder.
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Re: Vapor Retarder Under Slab

Postby RDavidson » Fri Feb 15, 2008 2:50 pm

The "poll" question is not the same as the "discussion" question. The discussion question asked if the vapor retarder was "required" to be directly under and in contact with the slab. The code is pretty black and white on that. The "poll" question asks if the vapor retarder "need be" directly under the slab. I might say that, yes, the code requires the vapor retarder directly under the slab but, no, it not need be directly under the slab to properly perform its function. The poll question is more a matter of opinion.
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Re: Vapor Retarder Under Slab

Postby inspector senior" » Fri Feb 15, 2008 6:20 pm

yes the vapor barrier is required (see exceptions), no it does not have to be in direct contact with the slab.

I reference the 2006 IBC commentary volume 2 section 1910 " The tendancy for concrete to develop both plastic and drying shrinkage cracking and the slab has a tendancy to curl because the bottom of the slab remains moist longer than the top portion due to the vapor retarder... if none of the exceptions apply; one solution would be to place a 2-3 inch layer of sand on top of the vapor barrier prior to concrete placement. The sand will absorb some of the excess water from the concrete and allow earlier finishing. If a layer of sand is used, it must be compacted by wetting the day before the concrete is placed. However, if the sand is to function effectively as a "blotter", it must be free of drainable water at the time of concrete placement"

This leads me to believe you can place sand between the concrete and the vapor barrier.. That's all folks'
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Re: Vapor Retarder Under Slab

Postby Joe Ehrlich » Tue Feb 19, 2008 10:20 am

The IBC commentary would lead one to believe that a blotter of sand under the slab is acceptable. But are we being lead or mis-lead? How much sand is acceptable? 2 inches? 4 inches? 2 feet? Isn't a moisture blotter exactly what we seek to avoid by requiring the vapor retarder? Again, I refer you to Joe Lstiburek's website which has posted a thorough discussion of this matter.

http://www.buildingscience.com/document ... main_topic

The science is clear. It is detrimental to trap a sand blotter of moisture between the slab and the vapor retarder. The moisture has only one way to dry, through the slab. This is not my opinion. This is basic physics. Curling and shrinkage of the slab can be avoided by using only enough water in the mix to support curing. Quality concrete is the result of curing not drying. A quality concrete mix wouldn't need a blotter to absorb excess moisture. There wouldn't be any excess moisture.
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