Building Envelope and Quality
By: Matt Muller
A building envelope is defined as the exterior shell of a building that repels the elements. It is a complete system that includes moisture, temperature, and air control and is the physical barrier between the conditioned interior and unconditioned exterior
Why Is the Building Envelope Important?
Comfort: Our perception of comfort is based on our immediate environment and is different for each person - Cold drafts, humid air, hot environment, etc. The exterior environment is dynamic, and we want the interior environment to be constant and comfortable.
Safety: If air and moisture are allowed through the envelope this can result in decay or damage to interior finishes or structural elements and cause mold growth.
Efficiency: HVAC systems are typically designed with some assumption, like the amount of insulation in walls or the roof, the amount of solar heat gain through windows or walls, and humidity control. The building envelope, if properly designed and installed, will control air flow (think leakage) which has the largest impact on energy consumption.
Liability: In design and construction, if we do not understand and manage the complexity of the envelope systems, we can encounter one or all of these which could lead to rework or litigation.
The building envelope includes the roof, weather barrier, storefront / curtainwall, doors, waterproofing, and under-slab vapor barrier.
The weather barrier can be a moisture barrier, air barrier, or vapor barrier. The selection of the appropriate barrier is critical to the proper function of the building envelope. Envelope continuity is critical and any void in the envelope can lead to numerous issues.
One method to check the continuity of the envelope during the design phase is the Pencil Test. We should be able to trace the weather barrier from the base of wall section all the way through the roof membrane without lifting the pencil off the “paper”. If you pick up the pencil at any point, you’ve discovered a flaw in the control layer (weather barrier). The point at which the pencil is lifted is a continuity problem or break in the layer. This Pencil Test is now in the Appendix of the EPA’s IAQ document “Moisture Control Guidance for Building Design, Construction, and Maintenance”
Why Do Envelopes Fail?
Design: In some cases, the drawings and specifications can include two materials that are not compatible or provide details that are not complete, coordinated, or are not constructible.
Compatibility: Compatibility, or lack thereof, causes more envelope failures than any other single issue. What if the details show a bituminous sheet material (think ice and water shield) lapping onto a thermal plastic roof membrane (TPO or PVC)? These materials are not compatible causing the asphaltic material to react with the roof membrane and causing the joint to fail.
Material Failure: A simple example is sealant installed with no backer rod. Sealant functions to its tested limit only when properly installed. A backer rod must be set at a proper depth to create a sealant cross-section shaped like an hourglass. Anything short of this has a high potential to fail when the building is exposed to movement (expansion or contraction). Sheet membrane air barriers have exposure limits and if not covered with a finish material, the barrier can become damaged by UV degradation.
Installation: Not following the manufacturer’s written installation instructions for any component of the envelope can lead to issues or failure.
A properly designed and installed building envelope can provide an efficient and comfortable interior environment for the building occupants. With all the challenges we face to make this happen, it is critical that we utilize a robust quality program that is resilient to a single point of failure and has multiple steps that allow the team to flush out issues prior to installation.