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    OSB Sheathing

    OSB Sheathing

    New postby aaronm on Thu Mar 31, 2011 8:41 am

    It is a common practice here to use Exposure 1 OSB for sheathing behind brick veneer without a WRB such as felt of foam sheathing. It is my understanding that Exposure 1-rated OSB is designed to be exposed to water only during construction delays an not permanently as it is when it is the drainage plane behind brick veneer. It is also my understanding that only Exterior-rated OSB can be used without a WRB. IRC 703.7.4.2 says:
    R703.7.4.2 Air space. The veneer shall be separated from the sheathing by an air space of a minimum of 1 inch (25.4 mm) but not more than 4.5 inches (114mm). The weather-resistant membrane or asphalt-saturated felt required by Section R703.2 is not required over water-repellent sheathing materials.

    I say that Exposure 1 OSB is not water-repellent. Builder says it is. What do you say?
    "What the plainspoken man lacks in subtlety he makes up in clarity." - A.D. Miller
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    Re: OSB Sheathing

    New postby Jerry Peck - Codeman on Thu Mar 31, 2011 7:53 pm

    Hi Aaron,

    Unless the builder is using one of the water-resistive coated OSB panels (such as those by - note, I am not recommending this product as I feel that a water-resistive barrier is needed even with this product to raise the coating to a comfort level I would be comfortable with, but even this product requires taping of all the seams and exposed areas where the coating has been damaged), the OSB needs to be protected by a proper water-resistive barrier (WRB) as OSB is water resistant for temporary exposure to the elements during construction (Exposure 1 rated panels).

    This link from APA ( ... N=69253323 ) shows a WRB is needed over the OSB when the OSB is used as a drainage plane, in this case the OSB is being used as the drainage plane behind stucco, and the WRB over the OSB makes the WRB the drainage plane.

    This link from the The Brick Industry Association ( ... otes_1.pdf ) shows the WRB covering the wall sheathing behind the minimum 1 inch air space.

    Here is another link from The Brick Industry Association ( ... otes_2.pdf ) showing the installation of a WRB.

    Then the 2006 IRC states: (underlining and bold are mine)
    - R703.1 General. Exterior walls shall provide the building with a weather-resistant exterior wall envelope. The exterior wall envelope shall include flashing as described in Section R703.8. The exterior wall envelope shall be designed and constructed in a manner that prevents the accumulation of water within the wall assembly by providing a water-resistant barrier behind the exterior veneer as required by Section R703.2. and a means of draining water that enters the assembly to the exterior. Protection against condensation in the exterior wall assembly shall be provided in accordance with Chapter 11 of this code.
    - - Exceptions:
    - - - 1. A weather-resistant exterior wall envelope shall not be required over concrete or masonry walls designed
    in accordance with Chapter 6 and flashed according to Section R703.7 or R703.8.
    - - - 2. Compliance with the requirements for a means of drainage, and the requirements of Section R703.2 and Section R703.8, shall not be required for an exterior wall envelope that has been demonstrated to resist wind-driven rain through testing of the exterior wall envelope, including joints, penetrations and intersections with dissimilar materials, in accordance with ASTM E 331 under the following conditions:
    - - - - 2.1. Exterior wall envelope test assemblies shall include at least one opening, one control joint, one wall/eave interface and one wall sill. All tested openings and penetrations shall be representative of the intended end-use configuration.
    - - - - 2.2. Exterior wall envelope test assemblies shall be at least 4 feet (1219 mm) by 8 feet (2438 mm) in size.
    - - - - 2.3. Exterior wall assemblies shall be tested at a minimum differential pressure of 6.24 pounds per square foot (299 Pa.
    - - - - 2.4. Exterior wall envelope assemblies shall be subjected to a minimum test exposure duration of 2 hours.
    - - - - The exterior wall envelope design shall be considered to resist wind-driven rain where the results of testing indicate that water did not penetrate: control joints in the exterior wall envelope; joints at the perimeter of openings penetration; or intersections of terminations with dissimilar materials.
    - R703.2 Water-resistive barrier. One layer of No. 15 asphalt felt, free from holes and breaks, complying with ASTM D 226 for Type 1 felt or other approved water-resistive barrier shall be applied over studs or sheathing of all exterior walls. Such felt or material shall be applied horizontally, with the upper layer lapped over the lower layer not less than 2 inches (51 mm). Where joints occur, felt shall be lapped not less than 6 inches (152 mm). The felt or other approved material shall be continuous to the top of walls and terminated at penetrations and building appendages in a manner to meet the requirements of the exterior wall envelope as described in Section R703.1.
    - - Exception: Omission of the water-resistive barrier is permitted in the following situations:
    - - - 1. In detached accessory buildings.
    - - - 2. Under exterior wall finish materials as permitted in Table R703.4.
    - - - 3. Under paperbacked stucco lath when the paper backing is an approved weather-resistive sheathing paper.

    If the above does not convince the builder, all I can to is repeat the information above which states (in various wording) that OSB (along with plywood and other structural panels) in their natural manufactured stated (not coated as the Zip System is) is water-resistive for short term exposure, after which it needs to be covered with an approved WRB.
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    Re: OSB Sheathing

    New postby aaronm on Fri Apr 01, 2011 9:37 am


    Thanks! You saved me cutting a handle in a 2X4 and going after the builder with gusto . . .=-)

    "What the plainspoken man lacks in subtlety he makes up in clarity." - A.D. Miller
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