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Building Science

Can Your Water-Resistive Barrier Take UV Exposure?

A recent report shows alarming degradation of water resistance in some WRBs after exposure to ultraviolet radiation

Water-resistive barriers like the plastic housewrap on the walls of this home can lose their water resistance if they are exposed to sunlight for too long.
Image Credit: Image #1: Energy Vanguard - All other images: Marcus Jablonka

A water-resistive barrier (WRB) provides protection against water damage for water that gets behind the cladding of a building. But what if it doesn’t really resist water? I’ve written a lot about installation problems that can lead to compromised water resistance. (See the article I wrote last week, for example.) But other factors can make them leaky, too. Too much exposure to ultraviolet (UV) light is one of them.

At the Air Barrier Association of America conference in Baltimore last month, I attended a presentation by Marcus Jablonka of Cosella-Dörken. [Editor’s note: Cosella-Dörken is a manufacturer of water-resistive barriers.] The manufacturer has done a study of UV exposure to different WRBs, and the results should be of great concern to all builders, architects, and product specifiers. This is a preliminary study and there are still a lot of questions to be answered, but I’d be concerned if I were a builder.

A short primer on water-resistive barriers

As mentioned above, the WRB’s purpose is to allow water that gets behind the cladding to drain down and be removed from the building. They come in different flavors and have a variety of properties. First, you can get your WRB as a material that is:

Some properties are more important than others. Building enclosure control freaks understand that you need to control the flows of heat, air, and moisture to have a building that functions well. Controlling heat is typically not done by WRBs (although it can be), so let’s focus on liquid water, air, and water vapor.

No matter which WRB type you choose, you want it to have a high water resistance. Yes, I really said that. As obvious as it seems, sometimes it’s good to state — and question…

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  1. Dillon Vautrin | | #1

    Brittle WRBs
    Is this the first study done on the affects of UV on WRBs? I thought this was a pretty well known property of WRBs. I don't know how many times I have removed siding to find Tyvek that had no tensile or tear strength left. It was very brittle and while I didn't do any water or air permeability tests I doubt they would be flattering. This was over ten years ago when I was in the siding trade. That would mean the Tyvek was installed long before that. I have no Idea if the manufacturing formula, or process has changed over the years. And off course it is impossible to tell if the WRB was exposed to the sun long before it was sided over. I live in Oswego County NY which is not the wealthiest of communitites. My friends at work who are not from around hear call Tyvek Oswego County siding. I imaging some of these homes my friends joke of will someday be sided right over the Tyvek that has been exposed for years. Then someone might strip that siding off and ponder of the products qualities. Of course I just built my garage using OSB covered with tyvek, then a rainscreen and LP smart products. I covered most of the WRB withing two months since I decided to start siding by myself in December. Then there is the cupola which hasn't been sided for four months now since I would rather risk water intrusion than breaking my leg from falling on the steel roof that still has a little snow on it today. Not to mention it feels as though the sun hasn't been around in five months so I will probably be alright.

  2. Derek Roff | | #2

    What about UV through the siding?
    Some siding lets some light through. I don't know if any UV gets through or not. If 90% of the UV is filtered out, after the siding is in place, then the WRB could get the equivalent of 500 hours of UV lamp exposure in about three years, for a site in Florida or Arizona (calculating from your charts). Is this of any concern, or does all siding block all the UV? Has any testing been done on heat exposure, without UV? I would think that a WRB in Phoenix behind dark vinyl siding might experience very high temperatures.

    I notice that the chart labeled "Self-Adhering WRBs/Air Barriers - Change in Water Resistance" appears to omit a line on the graph for product B. Looking at the link to Jablonka's slides, I can see that product B and product E have identical graphs. That wasn't visible to me on the lower resolution image on this page.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Derek Roff
    The only type of siding that I know of that lets a significant amount of UV light penetrate to reach the WRB is so-called open-joint cladding (see photo below). I don't think that this type of cladding makes any sense, but there is no accounting for taste.

    Cosella-Dörken, the WRB manufacturer that employs Marcus Jablonka, promotes a WRB designed for use behind open-joint cladding. The WRB is called Fassade S, and you can read more about it here: New Green Building Products — September 2010.

    The fact that Cosella-Dörken paid for this research is either (a) admirable, because it furthers human knowledge about WRBs, or (b) indicative that we should be skeptical of the results, because the sponsors of the research have a financial interest in the study's results. Depending on your level of cynicism, you can choose either (a) or (b).


  4. Derek Roff | | #4

    Thank you, Martin.
    I appreciate your taking the time to answer my question.

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