Vapor barrier under slab

[theoryshaw]

@kraut34,

Can you please share the flooring specs for the basketball court and turf area?

We will use these to determine vapor barrier approach.

Without seeing the specs and assuming the flooring has no excessive vapor barrier requirements, I would recommend (and i'd like @andyeck1 to verify this approach), that the floor composition be in the following order...

• concrete slab
• 3" sand *(to help concrete curing and prevent puncturing of the polyethylene, as well as provide a 'capillary break')
• 6mil polyethylene vapor barrier.
• 3" crushed gravel "with a maximum particle size of 1 inch containing no more than 5% passing the number 200 sieve (fines) and follow the recommendations of ACI 302.1, Section 4.1." -- from geotechinal report

Thanks, Ryan

(ping @smilliecraig)

[andyeck1]

@theoryshaw The current recommendation from American Concrete Institute (ACI) 302 for all slabs would be to place the concrete directly on top of the 10 mil polyethylene vapor retarder. The floor slab should bear on top of a 6" to 8" thick well compacted gravel subbase (the geotechnical engineer's report recommends 6"). The use of a layer between the slab and vapor retarder is not recommended because it can hold moisture which can cause problems with floor coverings as the vapor escapes to the atmosphere. If the layer between the slab and vapor retard was installed after the roof is in place and the layer can be kept dry, then the layer would be acceptable. Please see the following links for additional information.

http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-construction/vapor-retarders-in-concrete Location of Vapor Retarder

Construction practice and placement of vapor retarders has been the subject of much debate for many years. Some experts believe that concrete placed directly on a vapor retarder will bleed excessively, warp and crack more frequently, and take longer to dry than a slab placed on a compacted granular subbase. Other experts believe that vapor retarders function best to exclude moisture when directly below the concrete with no intervening material that can act as plenum space for the passage of moisture. This debate has resulted in many articles and letters in construction trade journals.

ASTM E1643, Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs, includes an Appendix with a detailed discussion of materials that should or should not be used above and below the vapor retarder, along with arguments in favor and opposed to cushions, blotters, and protective courses.

The 1996 ACI Committee 302 Guide recognized the importance of maintaining the integrity of the vapor retarder and its resistance against moisture transmission by recommending a minimum vapor retarder thickness of 0.25 mm (10 mils). Additionally, a thin layer of fine material was recommended over any crushed stone subbase to protect the underside of the vapor retarder sheet. In April 2001, after much debate, ACI Committee 302 issued an update to its 1996 report defining recommendations for vapor retarders. The Committee now recommends that any floor that will receive a moisture-sensitive finish should have a vapor retarder directly under the concrete slab with no intervening blotter or cushion layer (Figure 2).

See the flowchart (Figure 2) to determine how vapor retarder should be installed. Note it permits omitting vapor retarder for floors without floor coverings where humidity will not be controlled, such as unconditioned warehouse space. However, adaptive reuse and installation of flooring in such spaces often leads to flooring problems due to subslab moisture. Therefore, vapor retarders should be considered for use under all floor slabs (adapted from Figure 3-1 of ACI 302.1R-04).

http://www.nrmca.org/aboutconcrete/cips/28p.pdf

http://www.nrmca.org/aboutconcrete/cips/29p.pdf

[theoryshaw]

Thanks much, Andy, for the breakdown. I too have run across these debates as well.

@kraut34/@smilliecraig,

Unless there's any objections, let's use Andy's recommendations "to place the concrete directly on top of the 10 mil polyethylene vapor retarder. The floor slab should bear on top of a 6" to 8" thick well compacted gravel subbase (the geotechnical engineer's report recommends 6")."

Thanks Gentlemen.

[tfodden]

My experience says no to the sand cushion - that the slab is poured on top of the vapor barrier. I will in fact be specifying a Class A 15 mil product. Also, the concrete used at the slabs should have a maximum W/C ratio of 0.45. That will be in the concrete spec. Thanks!

[theoryshaw]

Gentlemen, this is Tony Fodden of http://www.fodden.net/

He'll be helping us out on the technical/specification review and documentation.

Thanks Much, Ryan

[andyeck1]

@theoryshaw; Is Tony writing specifications? If so, when will they be available for review? We need the drawings to match. @tfodden Please take a look at the general structural notes. I will add the note for the maximum W/C for slabs of 0.45.
What advantages do you see for a 15 mil vs 10 mil vapor retarder?

[theoryshaw]

Hi @andyeck1 Yes, he will.

[theoryshaw]

We're working toward a ARCH permit set, 3 to 4 weeks out -- barring any design changes.

The 'Permission to Start' & 'Footing & Foundation Plans Only' set will most likely come before that. 2 weeks out, perhaps, if all are ducks are in a row.

See related #46 issue.

[tfodden]

HI @andyeck1 Sorry it took me this long to reply. Just getting the hang of GitHub. About 5 years ago all the better manufacturers of below-slab vapor barriers - and there are about 6 of them - just started making 15-mil barriers so the Industry association just decided to make 15 mil the new definition of Class A. So just going with the industry recommendations I suppose. It probably holds up better under all that foot traffic during rebar installation. TF