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Green Basics

Water-Resistive Barriers

A water-resistive barrier (WRB) is a material installed between the sheathing (or studs if there is no sheathing) and the siding. It is designed to prevent water from reaching building components that could be damaged by moisture. Builders should assume that siding intallations aren’t truly waterproof, and that some water will find its way through or around the siding (at least once in a while). Without a WRB, sheathing and other parts of the wall assembly would be much more susceptible to damage.

A WRB can be one of a variety of materials sold under dozens of trade names. There are two types of building paper, asphalt felt and Grade D building paper; plastic housewraps such as Tyvek and Typar; rigid foam insulation; liquid-applied sealers; Zip System sheathing; and a unique product called Delta-Dry.

Most WRBs are vapor-permeable. Whatever type is used, it’s essential that wall assemblies have the ability to dry out.

The International Residential Code requires builders to install a layer of number 15 asphalt or paperbacked stucco lath over the wall sheathing or studs. However, builders can still meet this requirement with some “other approved water-resistive barrier.” A number of materials have met the requirements of the International Code Council Evaluation Service (ICC-ES) for use as a substitute, including plastic housewraps, liquid-applied WRBs, Grade D building paper, and certain wall assemblies incorporating rigid foam insulation.

Common WRBs, including most housewraps, asphalt felt, and Grade D paper, offer only so much protection against water. Eventually, water will get through. George Tsongas, a former professor of mechanical engineering at Portland State University, says, “In fact, they are not moisture barriers. If you get any significant amount of water behind the siding, the building paper will not hold back water — not even 15-pound felt. All the papers will…

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8 Comments

  1. User avater
    Albert Rooks | | #1

    Treating cut edges on Zip?
    Since, as I understand it, the Huber Zip product is a lamination of a woven WRB onto a specialized OSB panel. As that makes it a "topical treatment" in my mind, what is recommended treatment for cut edges of panels? Such as a lap joint at the building corner, window & door RO's, service penetrations. Is the standard practice to tape and cut edge to seal it?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Albert Rooks
    Albert,
    According to the installation instructions provided by Huber, there is no special treatment required for panel edges that have been cut. Here is a link to the web page with installation instructions:

  3. User avater
    Ken Levenson | | #3

    Some updated information is needed
    SOLITEX Mento 1000 and Mento Plus WRBs can be fairly described as "waterproof". Please see spec sheet here:
    With taped seams they can also provide Passive House level airtightness - while being quite vapor open. See a description of the membrane here:

    The SOLITEX Mento membranes' waterproof performance are also not degraded by building site contaminants or wood siding tannins.

    The SOlTEX Mento membranes offer an affordable high-performance approach. And they are available across the US from .

    I hope you can update the information provided in this WRB posting. Let us know if you have any further questions.
    Sincerely,
    Ken

  4. User avater
    Albert Rooks | | #4

    Thanks for the link Martin
    I read the short instructions. Since this is a whole wall WRB, and Huber requires an 1/8" gap where 4' edges meet, the WRB system is heavily dependent on the tape joint, and the tapes bond in order to stop the weather. For every panel there is a 12' tape joint that needs to last the life of the building. These joints don't even have the benefit of gravity lapping (is that the right term?).

    It does make me appreciate a good lapped membrane since they can be laid in a way that even if a tape joint failed, proper lapping would keep the water out.

    Not that I'd even admit that tape fails :) (Just joking... Everything fails at some point.)

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Ken Levenson
    Ken,
    The link to the spec sheet doesn't really tell me how Solitex Mento behaves in a wall without a rainscreen gap. If the membrane is pinched between lap siding and sheathing -- or perhaps panel siding and sheathing -- without a drainage gap, and if I subjected the wall to several hours of exposure to a water hose, would the sheathing get wet?

    I don't know the answer. But I know that if the test is performed with a plastic housewrap or asphalt felt, the sheathing will get wet.

  6. User avater
    Ken Levenson | | #6

    Response to Martin Holladay
    Hi Martin,
    I'll admit to not having done that particular test - however, my own anecdotal test was driving on the highway for 4 hours at 70mph in a driving hurricane like rainstorm (the whole way) with a large package wrapped in SOLITEX Mento 1000 WRB on the roof of the car. Upon arriving in Washington DC, we nervously removed the WRB, finding much to our relief, a bone dry back to the WRB and bone dry contents within. The membrane is waterproof. (And tannins don't degrade it.)

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Ken Levenson
    Ken,
    I think that some plastic housewraps (and 30-pound asphalt felt) would probably pass your "Christmas present on the roof of a car" test.

    The issue seems to be fasteners and sandwiching. Once you staple or fasten a membrane to a wall, and pinch it between the siding and the sheathing, most membranes leak. The solution is to include an air gap between the WRB and the back of the siding.

    It's a little like an old-fashioned single-wall tent. The tent won't leak until you touch it from the inside with your finger. Then your finger gets wet.

  8. User avater
    Ken Levenson | | #8

    Response to Martin Holladay
    There is no "tent effect" with the SOLITEX. It is a different animal. With the typical housewraps the surface tension of the water is broken when you touch it with finger or other contaminents and it tents through the micro-tears. The SOLITEX is a solid membrane - the vapor permeability is at the molecular level.
    Agreed that penetrations need to be self-sealing/under positive pressure to maintain waterproofness - if you have an exposed tear it is going to leak.

    Please take a look at this brochure:
    I'll also try to track down further documentation and will follow-up.

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