Keeping Water Out
Provide proper flashing on all windows and doors: All components should be layered so that water is shed down and outward. There are numerous ways to flash windows—flashing can be installed before or after housewrap or building felt “drainage plane,” specialized formable flashing can be used at the sill, and special measures are required with masonry walls and when rigid foam provides the drainage plane (see illustration, as well as the EEBA Water Management Guide review in book review department). Provide a rain screen behind siding: To facilitate drying of siding and to provide a capillary break between the siding and the sheathing, most experts now recommend rain screen detailing in wetter climates (see map). This can be provided with vertical strapping (minimum 3⁄ 8”, 9.5 mm thick); a specialized rain screen product, such as Benjamin Obdyke’s Home Slicker™; or, with brick siding, a bottom-draining air space behind the brick facing. Seal wood and fiber-cement siding: Porous siding materials should be sealed on all sides. Pre-priming is recommended prior to installation, with multiple coats as needed, especially on end-grain. Sealed siding will not absorb moisture, thereby reducing moisture migration driven by solar heat. Provide a capillary break above footing: Paint the top of the footing with a dampproofing coating or install an impermeable layer before installing the foundation wall to block the upward migration of soil moisture. Provide drainage layer and poly vapor retarder under concrete slab: Before pouring a concrete floor slab, install a minimum 4” (100 mm) layer of crushed stone (no fines) and a poly vapor retarder. The concrete should be poured right on top of the poly; without a layer of sand between the poly and slab. Provide perimeter drainage at footing: Install crushed stone (no fines) and perforated drain pipe around the footing. The drainage pipe should be slightly pitched but extend neither below the bottom of the footing nor above the top of the footing. Sections of plastic pipe should extend through the footing every 6 to 12 feet (2–4 m) to drain the space under the slab. Wrap the layer of crushed stone with geotextile filter fabric to keep fines out. Paint outside of foundation wall with dampproofing layer: A durable dampproofing should be painted on the outside of the foundation wall. Several layers are suggested, with the minimum thickness depending on the material (follow manufacturer’s recommendations). Install free-draining layer next to foundation wall: Install a specialized drainage layer (free-draining insulation, kinked nylon mesh, corrugated plastic, etc.) against the foundation wall, or backfill against the wall with crushed stone, or do both. When using a layer of crushed stone, protect it with a geotextile filter fabric to keep fines out. Slope ground away from building and provide impermeable cap: The ground should slope away from a building at a minimum pitch of 5% (6” per 10’, 5 cm per m). Provide a low-permeability (high-clay-content) soil cap extending 6 feet (1.8 m) from the building to reduce infiltration and direct surface runoff away from the structure. Provide a roof overhang to keep rainwater away from building: The longer the roof overhang, the greater the protection of the house. A minimum 24” (600 mm) overhang is recommended in most climates; 36” (900 mm) is preferable. Porch roofs and awnings also drain water away. Provide self-sealing ice and water barrier on roof: To protect against water penetration if ice dams occur in cold climates, install a self-sealing protective layer (e.g., Grace Ice & Water Shield®) under roofing. Install gutters and downspouts: Install durable gutters and downspouts to direct rainwater away from the building. Use screening to keep leaves and other debris out of downspouts, and instruct building owners to keep gutters and downspouts clean. At the bottom of downspouts, water should be channeled as far from the building as possible. Do not connect downspouts to footing drains.
Designing Building Assemblies to Dry Out
In most climates, provide drying potential to both interior and exterior: Building Science Corporation now recommends that in all but the coldest climates, above-ground walls should be designed to dry to both the exterior and interior. This means avoiding a poly air/vapor retarder and using permeable or semipermeable layers in the wall system. In the coldest climates, install poly vapor retarder on interior: BSC recommends that polyethylene vapor retarders should be used only in the coldest climates. In such climates (over 8,000°F [3,400°C] heating degree days), install a poly vapor retarder on the interior wall, under the drywall. The vapor retarder should be carefully sealed at all overlaps, edges, and penetrations. This arrangement will allow the wall cavity to dry to the exterior only. Design basements to dry to interior: In all climates, basement walls should be designed to dry to the interior. Insulation should be permeable or semipermeable, such as expanded polystyrene (EPS) or fiberglass with a moisture-resistant gypsum board product, such as Fiberock® or HumiTech™, and vapor-permeable latex paint on the interior. Provide vented rain screen on exterior walls: In climates with more than 40” (1 m) of rain per year, BSC recommends an exterior wall detail with a minimum 3⁄8” (9.5 mm) air space behind the siding, with screened vents at top and bottom. This rain screen detail both provides a capillary break and allows drying to the exterior. In climates with fewer than 40” (1 m) of rain per year, this vented rain screen may not be necessary. See map, page 13. Use plywood rather than OSB to aid in drying: Recent research shows that while both plywood and OSB have low permeability when dry, plywood becomes significantly more permeable than OSB when moisture content rises; this will aid drying to the exterior. With a fully vented rain screen detail, using OSB should be satisfactory. Consider perforating sheathing: The June 2003 issue of Energy Design Update reported on Canadian research demonstrating the effectiveness of drilling 3” (75 mm) holes through the exterior sheathing at the top and bottom of stud bays. Some builders drill lots of smaller holes through the sheathing to aid in drying. Provide a vented roof assembly: In all climates, a vented roof assembly will assist in drying and is generally recommended. In cold climates, a vented roof also helps to prevent ice dams by keeping the roof surface cold. Full-length soffit and ridge vents are the preferred venting strategy. Keep roof geometry simple to aid in venting. Unvented (hot) roofs can be successful, but only with great care in the construction; avoid this approach unless working with an expert.
