Rain-screen wall (C) Wiley Sons S Bliss Water Resistive Barriers on Building Exterior Walls

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This article explains the need for water resistant barriers on building exterior walls and explains the concept of a rain screen.

This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.

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Water Resistive Barriers on Building Exterior Walls

Leaks through building siding (C) Wiley Sons S Bliss

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction. Steven Bliss.

Water leakage through building exteriors has been the source of numerous callbacks and lawsuits across the United States. In nearly every case, the problems have been traced back to missing or poorly designed flashings or to weather barriers that inadvertently directed large amounts of water into building cavities or interiors.

Most of these leaks occur at window and door openings or at intersections between building components. In some cases, caulks and sealants forestalled leakage at these poorly designed joints for the first few years. But eventually most caulk joints fail, allowing water to enter.

All residential cladding systems are more or less porous to water, particularly during wind-driven rain when high air pressures on the windward side of a building force water to flow toward lower-pressure areas behind the siding.

Under pressure, the water exploits butt joints, lap joints, nail holes, and other openings to flow inside (Figure 1-1 at left ). Even without wind, some water will migrate through tiny gaps to the back of siding through capillary action, the way water is siphoned up a stalk of celery. This is true of brick, wood, and stucco, as well as the newest composite materials.

Water penetration of an older home siding (C) Daniel Friedman

In older construction, water that penetrated the outer cladding had ample opportunity to dry both to the interior and to the exterior as wind washed through the wall cavities, which were kept warm by heat leaking from the building’s interior.

In modern construction, however, with high levels of insulation, continuous air and vapor barriers, and low-perm sheathing panels, when water gets in, it is much slower to dry and more likely to cause damage.

Our photo (left) shows water stains on the interior of a clapboard-sided building wall on an 1860's vintage home restored by the website editor (DF) in Wappingers Falls, NY.

This structure relied on diagonal bracing for stiffness rather than an exterior sheathing board. Later an insulation improvement included blowing cellulose into the building walls - which was fine.

But now, water that used to leak into the wall cavity during windy rainy weather soaked the wall interior and was more of a problem.

Luckily cellulose insulation, probably because of the chemistry of its fire-retardant treatment, is rather mold-resistant. But that doesn't necessarily prevent an attack by termites or carpenter ants.


In freezing climates ice can also show where water leaks or moisture problems are occurring 

Leaks behind vinyl siding form ice (C) Daniel Friedman

While the exterior finish should be detailed to repel and shed water, a backup system is needed for the times when the primary system fails. The backup system needs to catch any water that penetrates the cladding and to drain it safely to daylight at the bottom of the wall.

Our photograph (left) of ice hanging from drainage openings in the building wall siding demonstrates how freezing weather can sometimes prove that a lot of water is running behind the building siding. The source of this leakage needs to be found and cured to avoid costly problems such as structural rot, insect damage, and even a wall cavity mold contamination issue.

The backup layer in an exterior wall, called a water-resistive barrier by the International Residential Code (IRC), typically consists of properly lapped building paper or plastic housewrap integrated with all flashings to safely drain water away.

It is also called the drainage layer or drainage plane. In this approach, the outer cladding functions as a decorative “rain screen,” slowing down wind and water, but it is not expected to be 100% waterproof.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Foundation Waterproofing Coatings: Exterior Use

Details about foundation waterproofing are at FOUNDATION WATERPROOFING and at GEOTEXTILES & DRAINAGE MATS. Excerpts are below.

Basement foundation sealer (C) Carson Dunlop Associates

As we report at WET BASEMENT PREVENTION, true foundation waterproofing, such as heavy textured plastic or rubber membranes placed against the foundation wall form a drainage layer to conduct roof spillage or ground water down the exterior foundation wall and into a drain system to carry water safely away from the building.

Carson Dunlop Associates' sketch (left) shows the use of a plastic membrane, protected by a geotextile to combine good water drainage down the foundation wall (and into the footing drains) with gravel backfill to nearly the top of grade (photo above left).

This basement waterproofing system was installed on a home that had suffered recurrent basement flooding due to a combination of in-slope grade at the rear and right side of the home combined with improperly installed and non-working footing drains, aggravated by wet soils in the area.

See BASEMENT HEAT LOSS for a discussion of foundation and basement insulation methods.

See POLYSTYRENE FOAM INSULATION for a guide to using this material in below-grade applications.

See TERMITE SHIELDS vs TERMITICIDE for a discussion of avoiding insect damage when foam insulating board is used below or at ground level.

Details about water barrier coverings for foundations are at WATER BARRIERS, EXTERIOR BUILDING and FOUNDATION WATERPROOFING and also GEOTEXTILES & DRAINAGE MATS.

Rain-Screen Principles Keep Water Out of buildings

As detailed in Best Practices Guide to Residential Construction:

Rain-screen wall (C) Wiley Sons S Bliss

The optimal way to protect the structure, siding, and exterior finishes from moisture damage is to design the outer layer of the house as a decorative “rain screen” that is solid enough to shed rain, block wind, and protect the sheathing wrap, but porous enough to dry to the exterior when wet.

This is accomplished by separating the outer cladding from the building’s water-resistive barrier by using an air space. This system takes advantage of the fact that no siding system is entirely waterproof and relies, instead, on the drainage layer for waterproofing (see Figure 1-2at left).

The rain-screen system has four components: an exterior cladding, an air space, a drainage plane, and weep holes.

  1. Wall Cladding as a rain screen. While the main function of the exterior finish material in a rain-screen wall is aesthetic, its durability can have a big impact on the costs of home ownership. Frequent repair, repainting, or replacement can be very costly.

    The cladding also protects the sheathing wrap from wind and ultraviolet (UV) radiation, and sheds most of the water that strikes the side of the building. While some exterior claddings are more porous to water than others—for example, brick, vinyl, and vertical-wood sidings are particularly leak prone— all can function well with a proper drainage plane.
  2. Air space behind wall cladding. The air space serves several functions.
    1. First, it provides a space for any water that has penetrated the cladding to drain safely away.
    2. Second, it provides a capillary break between the cladding and the building paper. Wet wood siding or stucco has been shown to degrade both building paper and plastic housewrap if it is in direct contact with the wet cladding. Cedar and redwood sidings can leach out tannins that are particularly corrosive to building papers.
    3. Third, the air space helps promote drying from the back of wood siding or from the framing and sheathing in the event of a leak.

      With stained or painted wood sidings, the air space will significantly extend the life of the finish. Some siding materials, such as vinyl, aluminum, and wood shakes and shingles, are self-ventilating. For others, an air space can be created by installing vertical furring strips over the building paper.

      Although furring out the siding provides optimal protection for the siding and structure beneath, it also adds significant cost and complication to the job, so it is not commonly done. However, manufacturers are responding to this need with a variety of thin drainage materials that either install over the sheathing wrap or replace it (see “Draining Housewraps,” page 5).
  3. Drainage plane behind wall cladding. The drainage plane typically consists of asphalt-impregnated building felt or a plastic housewrap that is fully integrated with all door, window, and wall flashings.

    The system must provide a clear drainage path out the bottom of the building. In general, the housewrap must be cut to lap over window and door cap flashings and under window and door sill flashings.

    In addition, the housewrap should lap over step flashings, the upper leg of abutting roof flashings, and deck ledger flashings. Upper courses of sheathing wrap should lap lower courses by at least 6 inches and vertical seams should lap 6 to 12 inches.
  4. Weep holes in building walls. Any trapped water must freely drain to daylight at the bottom of the wall either through weep holes, as in brick veneer, through a weep screed in stucco, or out the bottom of vertical furring strips installed beneath wood siding.

    If furring strips are used, the openings at the bottom should be screened against insects. Short sections of corrugated plastic ridge vent material placed between furring strips work well to provide solid backing for the bottom course of siding.

    Vinyl and aluminum siding products include weep holes in the bottom edge of each siding course.

    Watch out: Siding vents such as the round spot vent shown in our photo (left) are in our experience (DF) ineffective. Some painting contractors who are repainting older painted wood clapboard-sided buildings drive wedges between overlapping clapboards to try to break the paint seal to improve wall venting and to reduce moisture problems in the wall.

    We prefer to reduce indoor moisture to proper levels (see HUMIDITY LEVEL TARGET), and to provide a vapor barrier at the proper location in the wall structure.

See WET BASEMENT PREVENTION for more details about use of foundation sealers. Also see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION

Siding vents (C) Daniel Friedman

Rigid Foam Sheathing Behind Wood Siding

Although a rain-screen wall design will improve the longevity of any siding and finish, it is particularly critical when installing wood siding over foam sheathing. Research has shown that wood sidings installed directly over foam sheathings are more prone to cupping, cracking, and paint problems than when installed over wood sheathings.

Wood sheathing acts as a reservoir for moisture that penetrates the siding.With foam, the moisture tends to build up on the back of the siding and cause problems. An air space, even a shallow space of 1/4 to 1/2 inch, between the siding and foam sheathing has been shown to reduce these problems.

For details about wood siding failures when installed over foam board insulation, see SIDING WOOD, FAILURES OVER FOAM BOARD

-- Adapted with permission from Best Practices Guide to Residential Construction.

Our home page for diagnosing and curing foundation leaks and wet basements or crawl spaces is WATER ENTRY in BUILDINGS.

Readers needing more extensive guidance on preventing or fixing basement leaks and moisture should see BASEMENT LEAKS, INSPECT FOR, or if your building includes areas over crawl spaces, see CRAWL SPACE DRYOUT - home.

If your building has been flooded, see FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP.


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