Vapor retarders, vapor barriers, waterproofing - there are differences between the three materials and why they are used under slabs on grade. This month's article addresses those differences and focuses on the use and proper installation of vapor retarders.
Visit our website at http://www.conspectusinc.com/publications.htm to view or download A1030‑Slab on Grade Vapor Retarders 10.01.01. We invite your comments and questions about the article here.
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Hello Dave, thanks for publishing the Where, When, and Why of Slab on Grade Vapor Retarders. Since we manufacture such a product, I just wanted to add a few thoughts. I believe that a vapor retarder is also inherently waterproof, so it can be used under a slab even in the case mentioned – where soils report indicates there is subgrade water. (However, waterproof materials are not necessarily vapor proof.) In addition, ASTM E 1745 was revised in 2009; the new maximum water-vapor permeance is now 0.1 perms replacing the less stringent 0.3 perms. This specification rules the plastic sheet vapor retarders which I believe are now predominantly composed of polyolefins. The industry is moving away from high density polyethylenes (HDPE) due to long term durability concerns. The standard for bituminous vapor retarders is ASTM E 1993 and requires a maximum permeance of 0.002 perms. I think that ASTM does not distinguish between a vapor “retarder” and a vapor “barrier” the way ACI does. Thank you for noting that ACI 302.1R requires a minimum sheet thickness of 10 mils and that 6 mils is no longer acceptable. Regarding installation, most manufacturers will recommend that the vapor retarder be placed directly under the slab. Otherwise, any sand or fill between the slab and the sheet will only serve as a water reservoir that will prolong drying of the slab. As pointed out in the article, proper curing will help to alleviate problems that can occur if the surface dries too quickly. Lastly, while vapor retarders can prevent soil gases such as radon from entering the building through the slab, these gases could possibly enter below grade through walls that are not protected with a vapor retarder. Thanks!
Justin Henshell commented "Please refer to IBC par 1807.2.3, 1807.1.3, 1807.2 and 1807.3 which will mandate whether you will dampproof or waterproof."
We wish to than Justin for his comment and we offer the following reply.
Section 1807 of the International Building Code (IBC) is titled Dampproofing and Waterproofing. This section of the code dictates the circumstances and locations requiring protection of below grade building spaces from water intrusion.
Our article discussed using waterproofing and vapor retarders under slabs on grade. IBC Section 1807.2 requires floors to be dampproofed when hydrostatic pressure will not occur as determined by a subsurface soil investigation. By code, the dampproofing material installed under the slab must be a not less than 6 mil polyethylene or other approved methods or materials. The materials discussed in the article that are used as vapor retarders will meet the code requirement for dampproofing.
IBC Section 1807.3 requires floors to be waterproofed when the subsurface soil investigation indicates a hydrostatic pressure condition will exist and the design does not include a ground-water control system.
We believe our article is consistent with these code requirements by recognizing the need for a soils report. The code specifies dampproofing and waterproofing prescriptively by material, not by performance. Underslab vapor retarders will act as dampproofing by exceeding the water vapor resistance of 6 mil polyethylene. We indicated that waterproofing is meant to resist liquid water - the cause of hydrostatic pressure - and a vapor retarder is meant to resist water vapor pressure.