Research at the University of Waterloo in Ontario by Dr. John Straube and his team of scientists has shown the benefits of using a 3/8-inch to 3/4-inch venting rainscreen. The data show a significant increase in drying potential versus a conventional wall with simple building paper and no ventilation space. Based on this and other research we can conclude that a ventilated rainscreen accelerates evaporation of undrained moisture behind cladding materials nearly three times faster than without ventilation. It is important to note that a majority of the moisture will drain from the wall system when an effective drainage plane is used. This is in combination with the accelerated drying times with vented wall systems. Other studies by Mark F. Williams with Williams Building Diagnostics show that vented rainscreens are the best method we can use to ensure long-term performance and durability.
Credit: Courtesy Construction Instruction
Battens installed over housewrap.
The best practice in rainfall regions with greater than 20 inches per year is creating an intentional airspace between the cladding and sheathing by installing furring strips or drainage mats over the weather resistive barrier (behind the cladding) that create at least 3/8 inch to 3/4 inch of airspace. This requires some planning to detail the type of furring strips, cavity depth, possible use of exterior rigid foam or mineral fiber insulation, window installation sequence, penetrations, and trim details. This might sound complex, but planning ahead will make the experience work well.
Figure 1 (page 18) shows a typical wood-framed wall with a draining housewrap installed over the sheathing. The window is installed and flashed at the sheathing interface, and the air barrier is the housewrap. Ventilation behind the cladding is enhanced by the batten system installed over the housewrap. In Figure 2 you can see a well-installed and flashed weather barrier with foam and 3/8-inch wall battens spaced 16 inches on center behind the siding. (See page 18 for information on how to download these animated construction details from Construction Instruction’s new mobile app for iPhones and iPads.)
These techniques are effective methods for draining and drying excess water. The best drying performance is achieved by installing ventilation openings at both the bottom and top of the wall. There also needs to be an insect screen at the base and at the top to protect the space. The battens shown in Figure 2 are called Eldorado battens, and they allow airflow both vertically and horizontally. They are shown at 1/2 inch deep and 2 inches wide. Other methods include Benjamin Obdyke’s Home Slicker and Cosella-Dörken’s Delta Dry, among others.
Credit: Courtesy Construction Instruction
Battens installed over foam.
It’s a great idea to build a mock-up of the method you will be using to work out the cladding and trim details before you put it on the house. Once you create the best approach, you’ll find it works very well, extends the paint film life, and allows all cladding systems like stucco, manufactured stone, cement siding, and wood to perform as they should and experience fewer callbacks.
We know that reductions in energy use in buildings have been targeted by both codes and rating systems, citing a net-zero goal for new homes by 2030 (California hopes to achieve this by 2020). The new requirements for Energy Star Version 3 are taking a step-down approach improving the requirements as codes catch up. This will help builders reach these national goals in a strategic approach.
Regulations will always lag behind a best-practice approach. In January 2010, Oregon, with the assistance of the Oregon Home Builders Association and using information from the Waterloo research and Williams study, passed a code requiring a 1/8-inch minimum draining space unless an effective draining housewrap was used. Most of the building science community hopes it doesn’t take laws to inspire good building techniques. Detailing buildings with proper flashing, rainscreens, and improved thermal performance is where our industry needs to head now.
Justin Wilson with Construction Instruction contributed to this article.