Vapor Retarder Placement for Radon Resistant Slab Construction

The question of where to place the vapor retarder has been debated extensively. In general, the vapor retarder is placed in one of two locations: between the concrete slab and a 4" layer of granular fill (either gravel or rounded aggregate) or beneath the granular fill.

There are justifiable problems with both options. Concrete contractors often argue that placing the vapor retarder on top of the granular fill and directly under the slab can lead to curling and cracking of the slab due to uneven drying. They also state that placing the vapor retarder on top of the granular fill leaves it exposed to punctures and tears from equipment and workers while the concrete slab is poured.

On the other hand, placing the vapor retarder below the granular fill can trap water in the gravel or sand. (Note: The use of sand as a drainage layer beneath slabs is discouraged because of its strong capillary action.) This is especially true when the slab is poured prior to construction of the roof, leaving the fill exposed to rain and snow. It may also occur when workers spray water on the sand to tamp and even the surface. This scenario has led to severe damage in moisture-sensitive flooring. In addition to this concern, placement of the vapor retarder beneath the granular fill does not satisfy current radon resistant construction techniques.

If concerns about radon are negligible, moisture-sensitive flooring will never be installed, and indoor air quality and humidity levels are not a concern, then no sub-slab vapor retarder is required. If the concrete floor slab is poured with a watertight roof and envelope in place and radon resistant techniques are not utilized, then the vapor retarder may be placed below the granular fill layer. If radon resistant construction techniques are specified and the slab is poured without a watertight envelope in place, then the vapor retarder should be installed between the granular fill and concrete layers. Minnesota code requires radon resistant construction in all new single-family homes. Radon resistant construction is strongly recommended for ground-connected multi-family buildings as well.

Placing the vapor retarder directly beneath the concrete slab addresses both water vapor intrusion and radon concerns. However, it also places the vapor retarder at significant risk for tears and punctures and increases the potential for curling and cracking as the concrete slab dries.

chart from Craig, 2004

There are several measures to address the increased risk of curling and cracking in the concrete slab. Using low-slump concrete (0.5 or less water/cement ratio) will help reduce curling and reduce drying time. Type III cement, with high early strength, may also help prevent curling. Decreased spacing between contraction joints and additional rebar reinforcement in the slab may also be necessary.

In general, the vapor retarder installation should include lapping and sealing of joints. The retarder should also be sealed to foundation walls or wrapped under shallow footings. Penetrations in the vapor retarder for utilities should be carefully sealed. Taking measures to protect the vapor retarder during installation of concrete reinforcement and during concrete pouring is also recommended. If the vapor retarder is damaged, it should be repaired by lapping another appropriately sized sheet over the damaged area and taping the seams.

Other Resources

Publications

1 Craig, Peter. Committee member ACI 302, "Vapor retarders: Nuisance or Necessity?", 2004.

2 . Lstiburek, Joe. "Foundations - Moisture Resistant Construction" Research Report 0206, Building Science Corporation, 2002.