The challenge with every design is making sure that it will work in a specific environment. Through understanding the principles of a "perfect wall" - one which contains a water-shedding layer, an air control layer, a vapor control layer, and a thermal control layer - we can generate a wall solution that will work in every environment.
The control layers are listed in order of importance. All are important, but not equally important. The ranking comes from historic experience and the underlying physics. Controlling water in the liquid form (rain and ground water) has been the focus of architects for generations.
Controlling air is a much more recent focus - less than a century. The corollary, however, is too often true for many in the industry. There should be no doubt, the water control layer is much more important than the air control layer.
Controlling vapor is even more recent - only a generation or two. Air movement transports significantly more water in vapor form than does vapor diffusion and therefore air control is more important than the control of molecular water vapor. "Air barriers" are more important than "vapor barriers".
Thermal control dates back millennia - but getting it wrong has rarely led to durability failures. The thermal control layer failures have been typically limited to comfort issues and operating cost issues. Hence, thermal control layers are listed last on the control layer "priority" list.
In the last decade we have been successful at combining the water control layer, air control layer, and vapor control layer into a single layer that can be a film, coating, membrane, or sheet goods. We have also had good success with wrapping the exterior of a building with all of these control layers and then enclosing those control layers with the fourth control layer - the thermal control layer.
This configuration, with the thermal control layer outboard of the water, air, and vapor control layers, allows the assembly to be constructed in all climate zones: cold, mixed, hot and humid, or dry. Even better, this configuration allows this assembly to enclose virtually all interior environments in all climate zones: offices, commercial, residential, institutional, pools, museums, art galleries, and data processing centers. The sole exception being refrigerated buildings and cold storage facilities. In such assemblies the location of the thermal control layer is "flipped" with the other control layers - the thermal control layer now becomes located on the interior of the other three control layers.
Utilizing spray foam technology, you can create the "perfect wall" with spray polyurethane (SPF) which meets;
Water Control Layer - SPF is inherently moisture resistant.
Air Control Layer - SPF has an Air Impermeability of <0.02 (L/s/m2) @ 1 inch of mercury.
Vapor Control Layer - SPF has a water vapor permeability of 1.4 perm @ 1 inch of mercury.
Thermal Control Layer - SPF has an R-Value per inch of 6.9. This also allows for thinner walls and continuous insulation without thermal breaks.
Visit the Carlisle Spray Foam Insulation website at carlislesfi.com for more information on how your next project could utilize spray foam insulation as a "perfect wall" solution.
Contact Brian Emert at Brian.Emert@CarlisleCCM.com with further questions.