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Drain the Rain, On the Plane. The Drainage Plane.

By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

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©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 8, August 2019.

About the Author
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford, Mass. Visit

Ten years ago (“Taming the Terminology,” ASHRAE Journal, October 2009), I issued a challenge regarding water control layers asking folks for ideas regarding a performance metric for water control layers. I suggested that we might need a material metric or an assembly metric or perhaps both. At the time no rational performance metric or metrics existed for a water control layer.

There were irrational ones. For example, how about requiring a water control layer (a “material metric”) to resist 25 inches of standing water (6,200 Pa—the force exerted by a 225 mph wind)? This is obviously madness (yes, some products are rated this way) as house wrap products that are rated this way and pass the test subsequently leak when they are installed on a wall with holes created by staples and cap nails that are used to install them and through nail holes penetrating them when cladding is attached.

What is the point of having a metric that assumes no holes? How would you install it? Yup, put holes in it. Or if you can get it installed without holes how do you attach stuff to it or through it? Yup, you put holes in it. Couldn’t you seal the holes after you put them in? Sometimes, but mostly take “good luck” for an answer.

But house wrap products—and water control layers in general—with holes in them do not always leak. Huh? Yes, you read that right. They don’t always leak even though they have holes—lots of holes. They can in fact have thousands of holes and not leak. You also need a driving force—hydrostatic pressure is the big one—air pressure not so much—and yes we were here before (“Hockey Pucks and Hydrostatic Pressure,” ASHRAE Journal, January 2012).

Photo 1

Check out Photo 1: cedar shingles nailed through a house wrap water control layer resulting in thousands of holes on a large house on the ocean in New England. But the wall assembly does not leak because the hydrostatic pressure driving force is controlled by the drainage mat installed between the cedar shingles and the mechanically attached water control layer resulting in, wait for it, “drainage.”

It was clear that if you drained the water penetrating the cladding you would control the hydrostatic pressure and small holes would not matter. This is not new news. I think Vitruvius got there two millennia ago.

The obvious “ah-ha” for more than two millennia is that you can’t drain water on a drainage plane (aka, “water control layer” or “water resistant barrier [WRB]) without a drainage space or gap. The issue for quite some time has been twofold: how big of a space or gap do you need and when do you need one.

So where am I going with this? Well, I think we now have a reasonable metric on drainage (an “assembly” metric), and that gives us lots and lots of options because with good drainage pretty much everything works. I still don’t think we have a reasonable metric for water control layer materials (the “material” metric), more on that later. The good news is that assembly performance trumps material performance. Always.

Time for some heresy: with a good drainage metric I don’t think we need much of a material metric for water control layers. The goal should be to get rid of the crazy metrics that restrict materials and innovative approaches—if you have good drainage.

Although we now have a reasonable metric on drainage (in my opinion) and the necessary associated drainage space or gap (in my opinion) we do not have much agreement on when we need the drainage and the gap. Some background is necessary here.

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