Issue link: http://cp.revolio.com/i/281113
Most building designers are familiar with the basic tenants of balancing air leakage, vapor transmission, water leakage, thermal insulation, and ventilation. As more insulation is added to buildings and increased focus is placed on reducing thermal bridging by using continuous exterior insulation such as rigid foam plastics, traditional "rules of thumb" for vapor retarders are evolving. By changing the thermodynamics of the wall and moving the condensation potential outward, designers are finding they have more options for placement of vapor retarders in a wall. So what are the basics? Walls are expected to be designed to shed bulk water from the exterior. Air barriers such as house-wrap (which are usually not vapor retardant despite continued misperception) are expected to reduce entrained moisture and energy movement through the wall that comes with air leakage. And walls are expected to be insulated enough to meet codes and assure comfort to the occupants. If all of these systems are in place and working with no failures, the remaining concern is slow diffusion of water vapor through the wall depending on the vapor transmission properties of the materials, and potential for the vapor to condense to liquid and cause problems. Walls represent boundaries between warm air (which holds water vapor) and cold air (which encourages water vapor to condense). The basic premise that nothing is "perfect" leads to the practice of keeping the wall "vapor open" to either the interior or the exterior, so it can dry should wetting occur. And the basic rule has traditionally been to leave the wall vapor open to the exterior in very cold climates, and vapor open to the interior in very warm climates. So while we use the term "retarder", we are really talking about the walls ability to preferentially "dry" towards the side without a retarder, not to hope the "retarder" is actually stopping all vapor transfer through the wall. What does this have to do with rigid foam insulation? Since foam insulation is used on the exterior of the wall (referred to as "continuous insulation", or "ci"), it moves the condensation potential of any wet air in the wall cavity outward. Condensation requires a surface on which to form, so if the temperature for condensation is not reached until the middle of a solid sheet of foam, it does not occur. The model codes now recognize this phenomenon, allowing walls in cold climates to be built with a more vapor open design to interior provided enough exterior insulation is used. The amount of ci required varies by climate and ratio Tech Talk – Vapor Retarders & Vapor Barriers 12