Title:
Concrete Q & A: Vapor Retarder Placement
Author(s):
Kevin MacDonald
Publication:
Concrete International
Volume:
36
Issue:
7
Appears on pages(s):
79-80
Keywords:
DOI:
Date:
7/1/2014
Abstract:
Figure 3.1 in the 2004 edition of ACI 302.11 and Fig. 7.1 in ACI 302.2R-062 (refer to Fig. 1) provide a decision flowchart for determining the need for and location of a subslab vapor retarder. It’s very clear that if a project has a vapor-sensitive floor covering or a humidity-controlled area, a vapor barrier is required. If that need is established, the decision chart bifurcates into “Slabs with Vapor- Sensitive Coatings” and “Slabs in Humidity-Controlled Areas.”
What constitutes a humidity-controlled area? Does any space qualify if it has a humidifier or dehumidifier (that would include my house)? Or, is a “humidity-controlled area” a space with more critical needs, such as a museum?
If my project requires a vapor-sensitive flooring in a humidity-controlled area and the slab will be placed after the watertight roofing system is in place, it’s not clear from the figure whether I should place the vapor barrier above or below the granular fill. Based on the presence of a vapor-sensitive covering on the slab, the chart directs designers to Fig. 2, which recommends that the barrier is in direct contact with the slab. But if the slab is to be placed in a humidity-controlled area after the watertight roofing system is in place, the chart seems to direct designers to Fig. 3, which recommends that the barrier is covered in dry granular material. How can these contradictions be resolved? Where do I place the vapor barrier?