Drainage and Drying in Low- and Mid-Rise External Walls

Low profile drainable housewrap removes moisture vapor and bulk water from wall assemblies
This course is no longer active
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Sponsored by Benjamin Obdyke and Tamlyn
Karin Tetlow

Learning Objectives:

  1. Identify the health and safety issues relating to managing moisture infiltration in buildings.
  2. List the characteristics and effectiveness of flat and drainable housewraps in addressing moisture management in wall assemblies.
  3. Summarize the criteria for selecting high performance drainable housewraps.
  4. Discuss the future direction of building codes in regard to water resistive barriers (WRBs).



Ever since people created shelter, a perennial challenge has been dealing with the intrusion of moisture. Even today, with smart tight cladding systems and expert installation methods, building scientists agree that moisture will always find its way into a building enclosure and cause a multitude of problems. These may range from undermining the structural integrity of the building to shortening the life of paints and causing exterior surfaces to deteriorate. The effects of moisture can also lead to the growth of mold, health hazards and costly legal disputes that follow when these problems develop.

For several years building codes have mandated the use of water-resistive barriers (WRB), but recently Oregon has added prescriptive requirements that presage more stringent exterior wall assembly building codes elsewhere in the U.S.

There are many ways water and moisture infiltrate a building. High humidity and extreme temperatures can cause vapor diffusion, with moisture flowing from warm to cold (transported by air movement through leaks/penetrations in the assembly) and condensing on the colder surface. Wind-driven rain can be forced into small openings in the exterior cladding at joints, laps, utility cut-outs, electrical outlets, nail holes, etc. Wind blowing around the building can create a negative pressure within the wall assembly, which siphons water into the wall. Some “reservoir” claddings, such as brick, stone and stucco, can absorb and store moisture, which the sun then drives into the wall assembly (solar drive).

High performance drainable housewrap has spacers that create a drainage space between sheathing and cladding.

High performance drainable housewrap has spacers that create a drainage space between sheathing and cladding.

Image courtesy of Tamlyn


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