The Building Envelope: A Home's Main Defense

TIM, Why do some homes encoun- ter moisture related issues such as rot, leaks, mold etc. while other homes never seem to succumb to nasty moisture ailments? There is more to the exterior design of a home than just a pretty façade. In resilient, comfortable, high-per- formance residential projects, design of the building envelope is the homes — and its occupants- primary defense against unnecessary moisture and comfort related failures. Given the issues of building resil- ience, durability, design, and client comfort, some builders are leaning on new performance standards to create high-performance home exteriors. It all starts with understanding a few important principles. ADDRESSING CONDENSA- TION ISSUES Many folks attribute moisture issues with rainwater. But from attic rain (water droplets that form on rafters inside the attic) up north to wet wall insulation in humid climates , builders are becoming more aware of another kind of water intrusion: condensa- A Home's Main Defense The Building Envelope:

tion. Opposing exterior and interior temperatures create condensation where cold, warmth, and humidity meet, which turns vapor into droplets. Depending on the outside temperature, temperature difference, and relative humidity, these droplets may form on the siding, on the water-resistive barrier, on back of the plywood, on the studs, and even on the drywall. Two primary defenses against con- densation are: 1) Limit the amount of humid air inside the building assembly. This is typically done with good air sealing; air leaks carry far more humid air into the building assembly than vapor diffusion. 2) Raise the temperature of the building assembly to keep materials above the dew point temperature. This is increasingly done using continu- ous exterior insulation, which greatly improves overall thermal performance and relocates the dew point condensa- tion away from the wall cavity. COMBINING CONDENSATION CONTROL AND BULK WATER CONTROL Condensation in walls can be dev- astating, but by far the most common failure mechanism is exterior water seeping into walls through the exterior cladding such as siding or brick (yes, brick leaks — a lot-). The best way to keep rainwater from soaking the home interior is by creating dimensional sep- aration. This necessary disconnect can be achieved in a variety of ways. Adding .5 inch to 1.5 inches of foam to the exterior of the wall system, or using a foam-sheathing combination (sometimes referred to as "structural insulation") is a great way to build in a rainscreen adding a robust, high-per- formance, thermally broken building enclosure, solving for both condensa- tion control and water control. When I began building homes 30+ years ago, most had never heard of building science. Fast forward to today, and universities are offering degrees specializing in building science. The pace of improvement in the building industry is increasing on a seemingly exponential trajectory. Hiring a well-trained building professional has never been more important that in today's fast paced en- vironment. A builder with a thorough understanding of building science can offer high-performance, ultra-com- fortable homes that keep the elements outside where they belong and durabil- ity that serves you and your family for many years to come. BG