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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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.
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. (see Cellulose loose fill insulation and also INSULATION MOLD TEST.)
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
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.
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.
As detailed in Best Practices Guide to Residential Construction:
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.
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 should also see BASEMENT LEAKS, INSPECT FOR and BASEMENT WATERPROOFING. 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. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Also see WALL CONSTRUCTION BARRIER vs CAVITY .
Readers can compare air and water barrier concepts at WATER BARRIERS, EXTERIOR BUILDING and AIR SEALING STRATEGIES. In addition to VAPOR BARRIERS & HOUSEWRAP, readers should see HOUSEWRAP INSTALLATION DETAILS and HOUSEWRAP at SILLS, SOLES, TOP PLATES for details.
Also see VAPOR BARRIERS & CONDENSATION in buildings. Additional remarks about the requirement for weather barriers behind vinyl siding specifically, are found at SIDING VINYL.Also see FLASHING MEMBRANES PEEL & STICK and FLASHING WALL DETAILS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs) about water barrier requirements in building exterior walls: house wrap, sheathing wrap, vapor barriers, moisture barriers, water barriers
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