Recently constructed buildings are more energy efficient than their predecessors because of better insulation and tighter building envelopes that don't leak much air, in or out. The goal is to save on heating and cooling costs and buildings constructed with structural concrete can attain the best energy efficiency over all other building materials because of concrete's impermeable characteristic.
But no good deed goes unpunished and there are significant problems to overcome with tighter structures. Relative humidity (RH) increases when moisture or water vapor can't escape—above 70% supports mold and algae growth. Tighter structures also increase the buildup of radon gas inside buildings, especially residential units because they typically lack the air exchange systems that commercial structures have in place. The Environmental Protection Agency (EPA) lists radon as the second leading cause of lung cancer, killing 21,000 people in the U.S. each year, just behind smoking.
Monitoring and controlling moisture levels in floors is important—during the construction phase and the operation of a building afterward. Water vapor from the ground moves through concrete floors adding to the RH of the air inside a building and when low permeable floor coverings are installed on top of concrete floors, the moisture content of the concrete must be low enough to permit the installation.
Moisture from the ground
You might think that moisture from the ground is only an issue in parts of the country with larger rainfall totals. But desert sand with a low RH located under new concrete floors doesn't have a low RH level for very long. It's almost always 100% shortly after placement, raising the RH inside the building.
Installing more dense concrete mixes for floors can help to reduce the rate of water vapor transmission into rooms but it doesn't stop it. The only way to reduce infiltration to an acceptable level is to install vapor retarders with low permeance ratings between the subgrade and floor slabs, properly sealed against foundation walls.
Managing radon gas
Radon is a naturally occurring radioactive gas produced by the breakdown of uranium in soil, rock, and water. Some regions of the country have more radon gas than others, depending on the rock formations in those areas. See www. epa.gov/radon/zonemay.html for more information. Radon concentrations can be controlled by both active and passive systems. Commercial buildings usually feature good air exchange systems so the greater concern for radon gas buildup is in residences.
Active and passive systems
Good systems for managing both water and radon gas incorporates both active and passive protection. Passive systems involve the placement of barriers that limit the passage of water and radon.
An active system for removing water in residential settings typically involves placing drain pipes on one or both sides of the footings. The drains either move water away from the building by gravity or it flows to a sump pit where it is pumped outside. Drainage mat products also can be placed vertically against the outside of foundation walls to drain soil and reduce water pressure against the walls. Water from the mats is collected by the drainage pipes alongside the footings. Some permanent footing forms incorporate drains, providing both the form and the drainage system. Warehouse and big box facilities typically don't use active systems to remove water, only doing so when water tables near the surface. Then drainage pipe is installed in grid patterns to drain the soil under large floor slabs.
Active systems to remove radon gas from the subgrade under floor slabs are more complicated. In commercial buildings, air circulation is greater, ceilings are higher, and more fresh air is exchanged, thus radon gas buildup is less a concern. Residential buildings, however, don't feature such systems. For retroactive installations, the EPA suggests installing one or more suction pipes through a floor slab and into the crushed rock or soil below. These pipes vent through the roof and usually incorporate a fan to suck radon gas from the subgrade and through the pipe—creating a negative pressure or partial vacuum beneath the slab. For new construction, the same system is used but contractors add vent layers, such as crushed rock under floor slabs. See ASTM E1465-08 “Standard Practice for Radon Control Options for the Design and Construction of New Low-Rise Residential Buildings” and ASTM E2121-09 “Standard Practice for Installing Radon Mitigation Systems in Existing Low-Rise Residential Buildings.”
Passive systems for managing both moisture and radon gas are accomplished by placing vapor retarders between concrete and subbase or subgrade soils. They can be as simple as polyethylene plastic sheeting used for general construction purposes or as sophisticated as puncture-resistant, low-permeability products made for use below concrete slabs and in contact with soil. Bret Houck, the national marketing director for Stego Industries, San Clemente, Calif., says ASTM E 1745 defines three classes of plastic vapor retarders by strength characteristics: A, B, and C, with Class A being the strongest. Products also are measured by their permeance—the amount of water vapor that can pass through them. ASTM E1745 permits a maximum of 0.1 perms for all three classes of vapor retarder, but many experts and industry professionals suggest specifying permeance levels below 0.01 perms.
Aside from choosing good products, the effectiveness of a vapor retarder depends on how well they are installed. Each manufacturer specifies how much overlap is required at joints and how pieces should be seamed together. Proper attachment to foundation walls or footings also is important as well as detailing at penetrations.
Houck says a third equally important consideration about which vapor retarder system to use is longevity—how they hold up to wetting and drying, as well as soil organism attacks over time. Manufacturers, which claim that their products comply with ASTM E 1745, must conduct studies to determine how these products hold up under such conditions. Generic poly sheeting can degrade under these sub-slab conditions.