Waterproof Breathable Membrane, WRB & Housewrap FAQ

A waterproof breathable membrane is a water-resistive sheet used in a wall, roof or facade assembly. It is intended to resist exterior liquid water while allowing water vapor to diffuse through the selected material. It is one moisture-management layer, not a complete building-envelope system by itself.

A water-resistive barrier manages liquid water behind exterior cladding. An air barrier limits uncontrolled air movement when its material and all joints are designed as one continuous system. A vapor retarder slows water-vapor diffusion. One material can contribute to more than one control layer, but each function must be verified for the selected product and installed assembly.

ASTM E2556/E2556M is commonly referenced for vapor-permeable flexible sheet water-resistive barriers intended for mechanical attachment behind exterior cladding. It addresses material evaluation; it does not by itself confirm the performance of every installed wall, flashing detail, fastening schedule or interface.

Compare the reported water-vapor transmission or permeance together with the test method and test conditions. ASTM E96/E96M is a common test method for water-vapor transmission. Results from different conditions, units or product constructions should not be treated as directly equivalent without a technical review.

No. Vapor diffusion and air leakage are different transport mechanisms. ASTM E2178 is commonly used to measure air permeance of a building material, but material testing alone does not prove that a completed wall is airtight. Joints, penetrations, transitions and adjacent materials must be detailed as part of the system.

The 2024 International Building Code Chapter 14 addresses exterior-wall weather protection, including a water-resistive barrier behind exterior veneer and a means to direct water to the exterior. The adopted code edition, local jurisdiction and project assembly govern the actual requirement.

Request the product data sheet and the relevant test documentation for water resistance, vapor transmission or permeance, air permeance when required, tensile and tear strength, UV exposure limit, dimensions, roll mass and compatibility with tapes and flashings. Fire, facade or roofing requirements should be reviewed separately for the project and market.

Basis weight can help identify a product construction, but it does not establish water resistance, vapor control, mechanical durability, UV performance or compatibility with the intended assembly. Compare the actual specification and test data for the required application rather than selecting only by GSM or color.

Not automatically. Wall WRB, roofing underlayment and facade membranes can have different water, vapor, UV, strength, fire and installation requirements. Select the product for the actual application and confirm its use against the project specification instead of substituting one generic grade.

Only when the selected product is specifically designed and documented for the roof assembly. Roof pitch, roof covering, fixing method, drainage, construction exposure and local requirements can differ from wall conditions, so a wall housewrap should not be substituted without a specification review.

The membrane, flashing and drainage plane must form a continuous water-managed path to the exterior. Detail the opening so upper layers lap over lower layers where applicable, and use compatible materials at corners, sills, jambs, heads and penetrations. Follow the project detail and the selected flashing-system instructions.

Use a compatible seam tape for membrane overlaps and repairs where specified; use flashing tape for openings and transitions; and use butyl tape where a closed, conformable sealing layer is required by the detail. Do not assume every tape is vapor open, UV resistant or compatible with every membrane without the specific product data.

There is no single lap width that applies to every membrane and assembly. Follow the selected product data sheet, the cladding or roofing-system detail, the applicable code path and local project requirements. Laps, end laps, fastening and tape placement must work together with the drainage direction.

No. Reliable performance depends on continuous membrane installation, correct laps, compatible tapes, window and door flashing, sealed penetrations, drainage paths and appropriate fixing details. A membrane is a component of the assembly, not a replacement for installation detailing.

The permitted exposure period must be taken from the selected product data sheet. UV exposure, wind, rain, temperature and site handling can affect performance, so install the final cladding or roofing within the stated limit and repair damaged areas before covering.

Open-joint facade systems require a membrane selected for the expected UV exposure, color, mechanical strength, water resistance and any applicable fire-performance requirement. A standard wall WRB may not be suitable; KOSITE should review the facade design and project specification before recommending a grade.

Keep rolls dry, covered and off the ground on a stable pallet or rack. Protect them from prolonged sunlight, standing water, sharp edges and crushing. Inspect the membrane before installation and repair damaged areas with a compatible product before the assembly is closed.

EN 13859 is a European reference standard series for flexible sheets used as underlays in discontinuous roofing and for walls. The relevant part, declared performance and CE or DoP documentation must match the actual product, intended use and destination-market requirements; a standard reference alone is not a product approval.

No. A test report records results for the tested specimen and method. A certificate or declaration may cover a defined product and scope. Project approval depends on the current product specification, applicable code route, assembly design and authority or customer requirements. Review the documents together before selection.

KOSITE can provide samples, product specifications, SDS where applicable and available technical or test documentation for the selected grade. Confirm requirements against the destination market and project specification before purchase or installation.

Yes. KOSITE can discuss custom basis weight, roll format, color, printing, labels and export packaging after the product structure, application, requested performance and sample approval are confirmed.

Widths, lengths and basis weights are selected by product type and project requirement. Common membrane programs include 1.0 m, 1.2 m or 1.5 m roll widths and 50 m or 100 m roll lengths; the final format must be confirmed against the selected specification.

MOQ and lead time depend on product structure, roll size, printing, packaging, material availability and destination requirements. Share the expected volume, specification and delivery location early so KOSITE can confirm a realistic production and shipment plan.

Please provide the application, wall or roof system, required performance, target roll size, expected order volume, destination market, packaging needs and any OEM printing or certification requirements. This allows KOSITE to recommend an appropriate specification and document set.