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Building Envelope

The building envelope is the physical separator between the conditioned and unconditioned environment of a building including the resistance to water, air, heat, light, and noise transfer.

Often, people think the building envelope only refers to the cladding—the exterior of the building—including the roof line. While in some cases that is true, sometimes the building envelope goes below grade or it can follow the inside of the attic or walls. It can be part of the crawlspace if there is one, or maybe of an elevated space.

For proper performance, we need to ensure that we control everything from the weather on the outside to anything that infiltrates the wall systems to the heat that hits those systems, all of which are related to moisture management.

The goals for a building’s design are for it to be durable and long-lasting; energy-efficient; and comfortable, healthy and safe to work and live in. Moisture, it should be noted, represents the greatest threat to sustained achievement of these goals. Moisture management, therefore, is key.

Moisture flow mechanisms

Bulk water intrusion is typically a source of weather and site conditions. It can enter through cladding deficiencies, and then it’s absorbed by materials through capillary action.

Also, both air-transported and diffused moisture can contribute to moisture-related issues as well. We’ve previously learned that moisture moves from wet to dry, but how? Bulk or liquid water (which is the most severe) can move by various forces, including gravity, wind-driven rain, and capillary action, which is aided by pressure differentials. Water vapor moves via air currents, infiltration, exfiltration, and diffusion.

In terms of priorities, protect against bulk water first (water control layer, flashing, drainage, capillary breaks); protect against air entrained moisture second (continuous air barrier, internal air seals); finally, protect against diffused moisture (vapor permeability of materials, location).

Preventing Problems Starts with Good Design

Good designs include water-resistive barriers (WRBs), flashing, drainage, redundancy and compatible materials. Moisture issues can start at ~15 percent moisture content and that can cause corrosion of metal fasteners. At 16 percent, it may lead to fungal growth.

The issues found here could have been avoided by simply having a properly designed water management and drainage system capable of managing the typical moisture that penetrates behind stucco cladding systems. Stucco, along with other absorptive claddings like brick and fiber cement, require enhanced drainage protection to avoid moisture related issues.

Moisture Management: The 4 Ds in Design

In Moisture Management, we organize our thinking around the four Ds: deflection, drainage, drying, and durable materials.

• Deflection. This is your exterior cladding and roofing systems, which deflects ~ 80 percent of the water away from the building
• Drainage. Claddings allow ~20 percent of moisture to penetrate, it’s important to find a way to make it drain out as quickly as possible
• Drying. It’s important to always include provisions for proper drying of components so moisture doesn’t sit on the wall for long. You should have a ventilated wall system in at least one direction, but having it in two directions is actually much better and recommended
• Durable materials. It’s crucial we always include materials that are compatible with one another and that are resistant to UV rays in order to have a robust system

Causes of Moisture Related Building Failures

We know that wall assemblies dry slowly, so it is critical that everything is done to avoid complications. This is accomplished through proper design, material specifications, construction, and maintenance.

Moisture control and management should be a consideration for the lifecycle of a building, and in order to reduce risks:

• You should avoid designs that trap or direct water into the building.
• You should detail a well-thought-out moisture management plan, specifying drainage and compatible materials.
• Ensure that proper workmanship and standard construction practices are being used.
• Plan for the moisture generated from the occupants.

Moisture Management – Types of Wall Systems

There are three main types of wall systems in moisture management:

1. Barrier walls – Considered least effective, relies on face sealing for moisture protection – biggest weakness being the joints.
2. Drainable walls – Use water sensitive barrier (WRB) to protect the wall assembly from moisture intrusion. Depending on what cladding is being used, will have drainage limitations.
3. Enhanced Drainable Walls – Newer to the market, incorporates a spacer that promote enhanced drainage and drying. Key things to look for are non-compressive material, non-linear drainage pattern, and system that will give you 90% drainage efficiency or better.  
4. Rainscreen walls - Climates with severe weather exposure or behind absorptive claddings call for a physical airspace between WRB and cladding. Creating an air space anywhere from ¼” to 1” is key when selecting your rainscreen option. 

Design

When you are designing a moisture management system, there are a number of key details to focus on, including the following:

• At roof-to-wall intersections, you need to have a step out and seamless kick-out flashing.
• At a minimum, you should use a well-designed, continuous water-resistive barrier (WRB) and air barrier system. Include enhanced drainage and drying by using drainable membranes or rainscreens when applicable.
• Care must also be taken to seal all penetrations and openings.
• Carefully select materials that are compatible with each other and all wall system components such as windows, doors, cladding, etc.
• Through-wall flashings should be used at material transitions and at the base of wall assemblies.
• A continuous exterior air barrier and interior air seal are also appropriate for all climates.
• Develop clean, detailed and executable design drawings.

Enhanced Drainable Walls

Drainable Wraps provide a cost-effective enhanced drainage by creating a drainage channel behind the cladding system. For best results, use a non-compressive gap material that is non-directional to ensure that a drainage gap is present, and that moisture will be able to drain out efficiently. Enhanced drainage is required behind stucco, fiber cement, and other absorptive claddings but can also enhance the sustainability and resilience of other cladding systems. As wall systems get tighter and tighter the need for a consistent draining plane channel becomes crucial to the success of the wall. This is where having WRB with these capabilities comes in. By creating non-compressive gap of at least 1mm it allows 90% drainage efficiency or better in the wall system.

Author: Milton Lozano
Milton Lozano is a Product Specialist at Tamlyn. He is based out of Houston, TX and serves on the Specifications Team with an emphasis in Moisture Management and Building Envelope. He holds a B.S. in Mathematics from the University of Houston and brings 13 years of experience in Education. During his many roles he has acquired skills in leadership, planning, teaching, active problem solving, presentation, team management, business development, and many more that translate into his current role. Milton’s job emphasizes architect/builder presentations, job site walkthroughs, interactive mock installs, and product solutions.