What is a vapor permeable air barrier?

A vapor permeable air barrier is a plastic film or thin membrane that allows water vapor to pass but blocks liquid water. Air barriers are commonly used in exterior wall construction to allow structures to breathe, while blocking external weather. These are called weather resistive barriers or WRBs.

defendair 200 Archives - Atlas Supply, Inc.

Architect's Rendering of Renovated Medallion Apartments, Southeast View — Figure 1: Architect’s Rendering of Renovated Medallion Apartments, Southeast View (Source: Holst Architecture)

DOWSIL DefendAir 200 Weather Resistive Barrier (WRB) underlies the Medallion Apartments’ exterior renovations. The apartment building is 6-stories high with 90-unit affordable residential homes, located in Portland’s "Alphabet" Historic District. The Historic Landmarks Commission designated the Medallion Apartments as a non-contributing residential development, because it was built in 1965, well after the historic period. Nonetheless, the building renovation project required Landmarks Commission approval, which concluded:

The proposed exterior alterations and site changes to this existing multi-dwelling development will increase the building’s compatibility with its surrounding historic district and improve and activate the surrounding pedestrian network. The proposed materials are of high quality and the careful details will contribute to the durability of the building.

— City of Portland, Oregon – Historic Landmarks Commission

Increase the Medallion Apartments’ Compatibility with its Surrounding Historic District

Prior to its renovation, a failed, stucco Exterior Insulation and Finish Systems (EIFS) dominated the building’s architecture. The stucco EIFS and uninsulated concrete masonry unit (CMU) around each apartment’s entrance contrasted with surrounding Victorian and Federal-style homes.

After renovation, the building’s exterior will be a blend of sand colored, thin brick tile cladding, earth-colored EQUITONE facade panels on the stairwells and cedar plank-inspired fiber cement panels around each apartment’s entrance. The new variations in texture and color will help to soften the Medallion Apartments’ appearance, thus bringing it into harmony with its neighbors.

High Quality Materials Contribute to the Medallion Apartments’ Durability

The use of CMU to construct the Medallion Apartments significantly influenced its renovations.

First, CMU lacks the strength of modern, post-tensioned concrete construction able to support a heavy curtain wall. This limited the choice of brick cladding to lighter thin brick instead of heavier face brick.

Second, CMU is quite porous and requires a WRB to protect against water intrusion. The weather resistive barrier acts like a one-way street for moisture to travel. Water vapor may escape from the building’s interior via tiny holes in the barrier membrane. These same holes are too small for liquid water to penetrate the interior from the building’s exterior. The result is a drier, more comfortable residence, less prone to mold and mildew.

DOWSIL DefendAir 200 Benefits

The key benefits of applying DefendAir 200 include a water-based formula, ultraviolet (UV) stability, primerless adhesion:

Water-Based formula enabled easy cleanup of rollers and sprayers using tap water, without gray water retention and disposal, ideal on a space-constrained worksite
UV Stable composition allowed the barrier membrane to remain exposed to sunlight after its application, while construction proceeded elsewhere, thus eliminating weatherproofing as a scheduling bottleneck
Silicone formula ensured compatibility with DOWSIL 791 Silicone Weatherproofing Sealant
Liquid Applied having milkshake consistency using rollers or an airless paint sprayer (Graco® Sprayer or similar);
Primerless adhesion to clean CMU or CMU covered with thin-set mortar
Quick Turn-Around Time meaning the liquid dries in 2-4 hours and rainwater will run off, if it were to rain later in the day.

Architects are hungry to learn more about the benefits of the next generation of liquid applied air barriers. These benefits include easy water cleanup, indefinite UV stability, and high moisture vapor permeability. DOWSIL DefendAir 200 delivers all of these and more.

— Lorne Andrusiak, DowSil-Specialist, Atlas Supply and AIA Continuing Education Provider

Project Challenge: Residual Cementitious Adhesive

Removal of the failed EIFS cladding revealed a unique challenge: residual cementitious adhesive used to glue polyisocyanurate (polyiso) insulation directly to the CMU walls. Mechanical removal of the adhesive proved unworkable, without damaging the CMU substrate. A layer of thin-set mortar proved to be the best solution to bring the CMU to plane and provide a primerless surface to receive the DefendAir 200 Air and Weather Barrier. Figure 2 shows a section of CMU with residual adhesive on the left, a strip of exposed thin-set mortar in the center and roller-applied DefendAir 200 on the right.

Figure 2: North Wall Showing CMU with Residual Adhesive, Thin-Set Mortar and DefendAir 200

Thin Brick Veneer Curtain Wall Details

The thin brick veneer curtain walls are the dominant visible feature of the renovated Medallion Apartments building and include a hidden, careful detail: a moisture drainage plane missing from the failed EIFS. This drainage plane allows intermittent moisture to escape from behind the curtain wall and drain away via a horizontal opening at the bottom of the wall assembly. Figure 3 shows a cross section schematic of the complete wall assembly comprising the following components:

CMU Wall
Thin-Set Mortar to smooth up to plane (when required)
DOWSIL DefendAir 200 Air and Weather Barrier (WRB)
RockWool™ insulation mounted on posts and secured with pins
Metal furring strips or hat channel used to create an open-air space (drainage plane)
WonderBoard® Lite Backerboard with integrated waterproofing
Thin-Set Mortar
Mutual Materials Slimbrick® Thin Brick Tiles
Masonry Pointing Mortar
Horizontal Metal Flashing.

Thin Brick Veneer Assembly, Moisture Drainage Plane, RockWool Insulation, WRB and CMU Wall — Figure 3: Cross Section of Thin Brick Veneer Assembly, Moisture Drainage Plane, RockWool Insulation, Weather Resistive Barrier and CMU Wall (Source: Holst Architecture)

The curtain wall, moisture drainage plane and weather resistive barrier work together to ensure the interior CMU wall stays dry.

DefendAir was the right choice for this project because it was able to meet a number of different performance criteria. First, there was the fact that on several elevations, Dow AllGuard had been applied. The DefendAir will both adhere to, and is compatible with AllGuard. The resistance to both air and water infiltration are excellent, and the vapor permeability is right in the sweet spot for our environment. In addition to that, DefendAir offers long term UV exposure, adding flexibility to our schedule, while the water based clean up means we can spend more time installing material instead of cleaning equipment at the end of each day. Finally, we place great value on the partnership and support we receive from both Dowsil and Atlas – they have been long term partners with Walsh in achieving high performance projects for our clients and we knew we could trust them to contribute positively to the team.

— Martin Houston, Quality Director, Walsh Construction Company

DOWSIL DefendAir 200 Air and Weather Barrier For Your Next Project

Dow High Performance Building solutions include proven and innovative materials for structural and protective glazing, weatherproofing, insulating glass, high-efficiency insulation, and window and door fabrication. Experience the benefits of DOWSIL DefendAir 200 Air and Weather Barrier on your next weatherproofing project. Call Atlas Supply at 1-800-347-5767 to speak with your team of local weatherproofing experts. Refer to the DefendAir 200 Technical Data Sheet before using this product.