Forest Products Before You Install Exterior Wood-Based Siding Laboratory Moisture accumulation and extreme fluctuations in moisture During the 1980s, foam sheathing became popular because of its levels can adversely affect the service life of components, such superior insulating properties. Foil-faced sheathing and extruded as wood siding and windows. Adverse moisture conditions can polystyrene sheathing retard movement of water vapor; their induce checking, warping, paint failure, and in severe cases, rot- vapor permeability is much lower than sheathing systems such as ting of the wood. asphalt felt applied over lumber, wood fiberboard, or plywood. Thus, water that wets the back of horizontal wood siding installed Proper building design and construction can help prevent over foam sheathing is expected to stay in the siding for longer moisture accumulation or excessive moisture fluctuation within periods, particularly if the finish on the face of the siding retards building components. The following are among the well-known evaporation. Although not conclusive, some experimental practices to prevent moisture accumulation within exterior evidence shows that in warm, humid climates wood-based siding siding: installed over foam sheathing stays at a higher moisture level and • Use dry materials during construction. undergoes greater moisture fluctuation than similar wood-based • Provide adequate clearance to grade and drainage at grade. siding installed over wood fiberboard sheathing. We expect that • Design with adequate roof overhang. installation of wood-based siding applied over furring strips ac- • Install appropriate flashings. celerates drying of rain-wetted siding, particularly when installed over foam sheathing. • Install an interior vapor retarder in cold climates. During the 1950s, some U.S. builders An additional less-used technique began installing wood siding over that can improve performance of wood furring strips (usually plaster- wood-base horizontal lap sidings ing lath). These builders reported is to install vertical furring strips improved paint retention when between the sheathing and siding. siding was installed in this manner. Studies performed at the USDA The practice of installing wood siding Forest Service, Forest Products over furring strips is a tradition in Laboratory (FPL), during the Scandinavian countries, where the 1930s and early 1950s indicated climate is damp and relatively non- how rain can wet the back of Wood siding (leaky) decay-resistant woods (e.g., spruce horizontal wood lap siding. Rain Air space formed by furring and pine) are used for siding. strips (open top and bottom) water was shown to reach the Building paper/wind barrier Limited experimental data indicate space between the siding and the (housewrap) sheathing (tight) that ventilating the siding results in sheathing, which confirmed that Batt insulation lower moisture levels in the siding. it was appropriate to use a water- (faced or unfaced) The Moisture Control Handbook, pub- resistant barrier, such as asphalt felt, lished by Oak Ridge National Labora- over sheathing to prevent further tory (Oak Ridge, TN), describes the penetration of rain water into the rain screen design (Fig. 1) as a way wall. Studies in the 1970s and 1980s to reduce the amount of rainwater suggested that siding could be dried entering into the walls. In addi- by solar heating, with some water tion to furring strips, a rain screen moving from the siding into the design with wood siding consists of wall. This may raise moisture levels Air space open at bottom a relatively airtight sheathing and an within the wall, but limited data (insect screen may be desirable at this location) airspace between the sheathing and suggest that this vapor migration the siding that is open at the bottom does not usually cause serious and allows unrestricted air exchange moisture problems. Figure 1—Wood frame wall designed as a rain screen.

USDA Forest Service • Forest Products Laboratory • One Gifford Pinchot Drive • Madison, WI 53726–2398 Phone: 608.231.9200 • Fax: 608.231.9592 • [email protected] • www.fpl.fs.fed.us Revised July 2013 with a ventilated soffit or overhang. Adequate airtightness of the Although this article primarily focuses on moisture issues, prob- sheathing can be attained by a variety of methods. The Moisture ably the most important factor regarding paint life (not penetrat- Control Handbook indicates that careful installation of asphalt felt ing finishes like semitransparent stains) is wood exposure to over plywood or oriented strandboard should provide adequate sunlight. Tests of wood painted, at least primed, before any expo- airtightness. Other sealing methods include using foam sheath- sure to sunlight showed that repainting was not necessary for at ing and tape or synthetic sheet air barriers, commonly called least 30 years. Duplicates exposed to sunlight facing south for one “housewrap.” week prior to priming required repainting at 13 years. Wood ex- posed to two weeks of sun prior to priming required repainting at Some architects and one lumber trade association advise priming 10 years. Light sanding can remove the thin layer of UV-damaged the backside of solid-wood siding with paint or a water-repellent. wood, and it is advised to at least prime immediately. Research performed at the FPL during the 1950s conclusively indicated that back priming the siding with a water-repellent Although instructions on primer paint frequently indicate top- improved paint retention and overall performance of horizontal coats should be applied soon, we found with a limited number of wood lap siding. However, the benefits of back priming the siding primers that 6 months of primer alone over the winter had no ef- with paint have not been experimentally verified. If the lower fect on topcoat life. A second primer can always be applied before portion of the siding back is primed, it is likely that the siding will top coating if it is suspected that the primer had been exposed for absorb less moisture. Note that if the entire siding back is primed too long. with paint, water that has been absorbed by the siding will be In summary, ventilating horizontal lap siding may improve its retarded from evaporating. service life. If the siding is installed using a rain screen design, the If your siding has already been installed, you can still reduce capil- rain water will probably not penetrate past the siding. Installing lary rise of water between lap siding boards by inserting spaced horizontal lap siding using the rain screen method is also reason- wedges or shims under each course of siding. This increases the ably inexpensive and easy to execute. width of the opening, thereby reducing capillary rise and facilitat- ing drying of the back of the siding after rain showers. However, Mark Knaebe wedges also provide larger openings for wind-driven rain and USDA Forest Service may not be appropriate for some locations. Forest Products Laboratory One Gifford Pinchot Drive It is easiest to place them halfway between nails; however, the Madison WI 53726–2398 siding will then be wavy. Partially pulling the nails and inserting wedges near each nail (and resetting the nail) will provide more For more information, consult our website: www.fpl.fs.fed.us opening and the siding is more likely to be flat.

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