Drainage planes for walls an important subject even in dry Colorado

"Rain is the single most important factor to control in order to construct a durable structure." 
- Joe Lstiburek 

"You must learn from the mistakes of others. You can't possibly live long enough to make them all yourself" 
- Sam Levenson

Sometime during the 1990s, forensic engineer and moisture guru Joe Lstiburek (Building Science Coil) Westford, Mass. found himself examining numerous multi-million-dollar building failures in Vancouver, British Columbia. Those failures were caused by rain intrusion into wall assemblies that couldn't easily dry out. During testimony before a commission investigating the problem, he eventually induced the members to chant with him: "drain the rain on the plane!"

No, Colorado isn't nearly as wet as Vancouver. Or North Carolina, with all its building failures caused by rain. Yet based on the emerging evidence, Joe's advice applies in Colorado anyway.

Here's the rub. Over time, all exterior claddings leak. Over time, most windows and doors exposed to the elements leak. And today's leaks tend to cause more problems than yesterday's leaks, as explained below.

Enter the drainage plane - a set of products that work together to drain rain from wall assemblies. A drainage plane sheds water. Unfortunately, OSB isn't a drainage plane.

Callbacks Context

The 2-10 Home Buyer's Warranty program reports that of 3,218 written complaints filed with them last year. Half tied back to water problems. On their prioritized list, leaking windows ranked No. d, roof leaks came in No. 5. followed by pipe leaks, foundation leaks and im­proper surface drainage.

Clearly. builders should hate water. Too often water is just a callback problem coiled to strike.

Codes Context

Section R703 in both the 2000 and 2003 IRC requires a "weather-resistant exterior wall envelope." Further it specifies that, for many applications, walls need to be covered with a "weather-resistive barrier." But those versions of the code allow lots of exceptions; the in-progress 2(x)6 version won't be so generous.

Unfortunately, current codes don't provide a clear definition for these barriers. Read between the lines and you'll find that qualifying materials include asphalt-saturated felt and other approved weather-resistant materials. Typically the approved products include housewrap materials (e.g.. Tyvck) plus foam sheathing. House wraps and foam sheathing both need all joints sealed with approved tapes. All products require an overlap dwing installation; the code minimums call for 2-inch laps for horizontal seams and 6-inch laps at vertical joints.

Last January the town of Parker started requiring weather resistive barriers over all exterior wall systems. It may take several years before this requirement moves through Colorado. But since exterior claddings and windows and doors can and often do leak, building scientists highly recommend getting ahead of the code curve here.

Climate Context

When it comes to climate and moisture. we live in one of the most forgiving climates in the country. Who can't deal with problems caused by a measly 15 inches of moisture a year? Historically, we bene­fited tremendously from that reality.

Yet several years ago folks in dry Pueblo received a 9-inch rain dump over the course of one day. So it isn't just a matter of how much water we get. During design and materials selection, you have to consider wind strength, wind direction and rainfall intensity. There is a new climate map laying out these vari­ables for the building community. Apparently it shows the Colorado climate with enough climate intensity that we need to install drainage planes.

 Building Science

You probably wonder why in hell we need a drainage plane over today's walls when lots of homes built decades ago did fine without them. Walls got wet, then they dried, end of story. Well, things change.

For starters, when our older walls got wet, they could dry either to the interior or the outdoors, and they could dry faster. Recent wall assemblies include more insulation: that slows down the drying cycle. Today's walls are tighter, again slowing the drying process.

If your wall includes a polyethylene vapor retarder attached to the interior face of your studs, it can't really dry in that direction at all. If your wall includes a foil-faced product on the exterior, it can't readily dry in that direction either. Even OSB sheathing doesn't allow drying as fast as plywood and some of the other sheathing materials it replaced.

The bottom line here: once wet, today's walls will stay wetter longer. So part of the battle comes down to working harder to keep hulk water out. That struggle revolves around two products - drainage planes and lashings - properly integrated and installed by people rated building paper or special house wrap plus another layer.

After those experiments, Lstiburek made some particularly important points in several technical papers www.buildingscience.com about weather resistive barrier choices behind stucco and brick.

"A drainage space between stucco and building papers or housewraps is essential to control liquid phase water penetration. Stucco can `bond' or adhere to the housewrap surface altering its surface energy thereby allowing housewrap pores to become `wetted' and subsequently establish capillary flow....

"Bonding typically does not happen between stucco and building papers. However, with most stucco applications over building papers, insufficient drainage results. It is recommended to use at least two layers of building paper under stucco in order to allow some drainage between the two layers. Even better is to provide a spacer between the two layers of building paper by using a textured building paper or a building paper with granules or cork adhered to its surface thereby creating a space."

Both stucco and brick veneers are what Lstiburek calls reservoir cladding systems. "When they get wet from rain they act like a reservoir or sponge. Wet brick can then get heated above ambient tempera­ture by solar radiation creating a powerful inward-acting concentration gradient and thermal gradient. These gradients are ex­acerbated by air conditioning (cooling and dehumidifying the interior)."

Behind reservoir claddings, you want to install components that can resist the movement of solar-driven water vapor into the wall assembly. A good method is use of taped foam sheathing.

When it comes to designing and installing a drainage plane, your job in the product selection realm would be easier if we had more results to report about the full range of applications available to you. Suffice it to say that building papers work, and house wraps and foam sheathings work very well when combined with drainage matt material.

Stucco, Brick, and Building Papers

Back in 2001. Building Science Cor­poration built small test wall sections, covered them with a variety of building papers and house wraps, and finished them off with various cladding systems (mostly stucco). Then, in the best tradition of backyard but real-world building science, they sprayed them with a garden house. The highest-performing assemblies always included two drainage layers: either double type-D, asphalt-satu-permeability) with OSB (low permeability). Using a house wrap with asphalt­impregnated sheathing or gypsum board - all highly permeable products - be­hind reservoir claddings is a bad idea.

Integrating Flashings

"Flashings are the most under-rated building enclosure component and ar­guably the most important," according to Lstiburek in EEBA's Water Management Guide. The second edition of this guide will be available in June. It's loaded with illustrations showing a number of ways to properly install flashings. Just as im­portant, it shows how to properly inte­grate flashings with drainage planes. (A word of warning: without integration with a drainage material, installing proper flashings around doors and windows may not substantially reduce water-related damage.)

Even the best windows can leak over time. To prevent water entry from a leak­ing window, start by sloping the bottom of the window's rough opening with a piece of beveled siding, then cover it with a flexible flashing product - typically backed with bitumen. From here, one of several methods involves installing the window, then applying bottom, jamb and head flashings, and tying in the weather resistive barrier. (See diagram). Remember that you'll need the bottom pieces of flashing to drain down and out over the weather resistive barrier: dumping water between the barrier and sheathing is completely counter-productive.

So you can't follow the directions in the last paragraph? I couldn't either. It's too complicated to write it up without a lot of photos. That's why you need to buy a copy of The Water Management Guide www.eeba.org.

 Inspection

Designate someone on your staff - site super, project manager, whoever-as your water management guru. As building scientist and trainer Mark LaLiberte likes to put it, that person needs to be fitted with a pair of "water management glasses" before you apply exterior finishes.

"Put those glasses on, walk around the home, and pretend you're spraying the roof and walls with a garden hose," said LaLiberte. "If all the water drains off, you're ready for exterior cladding. Remember, all claddings leak rain, so it is essential to have a back-up system to allow the rain to drain out and let the wall dry."

LaLiberte insists this is "a good business decision. A good business decision is one that leads to higher profits and more satisfied customers."

[Endnote: LaLiberte will be speaking in Colorado July 7 (Colorado Springs), 8 (Denver) and 9 (Durango) during "Houses That Work" sessions cospon­sored by EEBA and E-Star Colorado. He's very good and extremely entertain­ing. If you haven't heard him, put one of those sessions on your calendar.]

Steve Andrews consults with builders for E-Star Colorado and writes on energy issues (sbandrews@att.net). E-Star (www.e-star.com), is a nonprofit home energy rating system that works with both new and existing homes statewide.

2008 Built Green Colorado

Home Builders Association of Metro Denver, 9033 E. Easter Place, Suite 200, Centennial, CO 80112
(303) 778-1400 fax: (303) 733-9440  info@builtgreen.org

Last Updated: 10/05/2007