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Wood siding failure diagnosis for siding installed over foam insulation:
This article discusses the causes and cure of wood siding failures including when siding is installed over rigid foam insulation - how to prevent siding cupping, siding nail pops, siding splitting, & siding separation or misalignment at butt joints and other wood clapboard and hardboard siding failures. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Wood Siding Failures when Siding is Installed over Rigid Foam Building Insulation
Nails A Poppin: how to keep wood siding from wriggling over foam sheathing - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article.
This article explains the causes of wood siding failures and continues with advice on how to prevent siding cupping, siding nail pops, siding splitting, & siding separation or misalignment at butt joints when wood siding is installed over rigid foam building insulation.
As early as in 1979 failures of wood siding were reported when that material was installed over foam sheathing. By 1982 the problem was under study by the National Forest Products Laboratory and the Society of the Plastics Industry (representing insulating foam building sheathing manufacturers) and a 1983 report by that group made suggestions to avoid siding failures but did not identify the underlying causes.
In addition to the wood (clapboard-type) siding failures listed just above, wood product siding failures in the hardboard siding family were reported: siding waviness, hardboard siding dimpling (caused by over-nailing), or hardboard siding buckling lengthwise when hardboard siding gets wet on its back or building-side face.
Other hardboard siding failures are discussed at SIDING HARDBOARD, and still more serious building damage including rot and mold where leaks occur between synthetic stucco are discussed at SIDING EIFS & STUCCO.
Text below excerpts from and paraphrases the original wood siding failure over foam insulation article shown in the links above.
Causes of Wood Clapboard Siding Failures
Improper nailing of wood siding: Among wood clapboard siding failures when that material is installed over insulating foam building sheathing, the articles above report that improper nailing is the most frequently cited cause.
To avoid these problems it is important that the installer use nails of sufficient length to penetrate both the foam sheathing and the wood framing beneath, and also the siding should not be nailed so hard that it is cupped (lengthwise) by having compressed the foam sheathing. Contractors who installed siding using 7d or 8d siding nails were using fasteners that were too short - the nails barely made it through the foam and siding easily came loose later.
Moisture damage to wood siding: Cupped wood clapboards (across the width of the board) are caused primarily by water or moisture penetrating the wall and becoming absorbed into the clapboard through its back surface. Flat-sawn clapboard stock will cup more than vertical-sawn. The moisture problem in a clapboard wall may come from the building exterior (such as due to wind-driven rain, installation and flashing errors), from the building interior (indoor moisture problems, missing or defective vapor barriers), or from the wall cavity if ice dam leaks, roof leaks, or plumbing leaks wet the wall cavity.
Siding quality affects siding failures: poor wood siding stock is more prone to failures. Lower quality wood clapboard siding qualities that affect its failure rate include:
Siding with excessive moisture content at time of installation, due to improper storage, siding has become wet, siding was not primed or not back and end primed
Siding with too much large, flat, or irregular grain
Siding with too much sapwood (increases twisting and cupping)
Siding stored improperly before installation (got wet)
Siding not properly prepared or not prepared using the best building practices (not primed on all sides)
Siding that is too thin (1/2") and siding that is too wide (more than 8" in width)
However the impact of these characteristics on siding failures varies by wood species. For example, redwood siding is reported to perform well even at thinner thicknesses and when not primed or coated.
Foam sheathing contribution to siding failures: Contributing to moisture damage in wood clapboards nailed over foam is the observation that because the foam provides no support for the nails passing through it, any movement in the wood (such as due to thermal or moisture changes) can cause the wood to loosen and drop out of alignment.
Hardboard siding installed over insulating foam sheathing may also suffer from lateral waviness or "cupping" due to over-nailing that compresses the foam sheathing behind the nail. Observing hardboard siding under proper lighting conditions can make this defect easier to see if you sight along the siding rather than observing it head-on.
Prevention & Cure of Wood Siding Failures
Nailing Advice for Wood Siding
Use nails long enough to give 1 1/2" penetration into solid wood of the building framing. But where foam building sheathing is installed, finding 3 1/2" long siding nails may be difficult - but not impossible. Maze Nail Co. and Independent Nail Co. both provide suitable fasteners for wood siding when installed over insulating foam building sheathing.
Nail siding 16" on center for wood siding - this is the preferred interval for wood clapboards and it is mandatory for hardboard siding wood clapboards.
Hardboard siding nails: good results are obtained using 12d galvanized box nails. Don't over-nail the siding - that can cause both cupping, and where the nails penetrate the skin of the hardboard siding surface future water damage is likely to occur. See SIDING HARDBOARD.
Head lap for beveled siding when it is installed over foam should be at least one inch, or 1 1/4" by come experts.
Nail location: when nailing siding over foam, locate each nail at least 1/8" above the top of the board below - this allows room for expansion (and keeps the nail covered during shrinkage).
In rabbeted patterns, the 1/8" space should be left in the rabbet.
Butt joint nails should be installed through pre-drilled holes to avoid siding splits (leaks and deterioration), and make sure that the nails actually enter the wall stud. Ring shank or spiral nails are recommended.
Double nailing of bevel-sidings (nails that extend through the board being nailed and also through the thin top of the board underneath) is not recommended by the redwood and cedar siding companies, out of concern that the thin siding board tops will split when the siding shrinks and expands. But double nailing was recommended by some experts when installing siding over foam.
Finishing Advice for Wood Siding
Siding should be finished before or shortly after installation, using a water-repellant preservative
Light colored stains and preservatives offer the advantage of keeping siding cooler
We recommend pre-finishing or at least priming siding on all surfaces before it is installed. When making end-cuts to fit pre-finished siding, remember to also apply sealer to the newly-cut siding ends as well. Purchasing pre-primed siding or commercially-dipped siding can reduce the labor cost of this approach.
At Slab Log Cabin Siding we show a building whose siding was hand-stained on all sides during construction, providing a durable wall covering that has proven resistant to moisture-uptake and its related problems. Pre-finished siding won't show light-colored lines of missing stain when the siding shrinks after installation.
Using Strapping or Furring Strips to Install Exterior Wood Siding
Using strapping or furring strips applied on top of insulating foam building sheathing before installing the siding will provide a cold-side air space that helps avoid siding moisture problems in the future. In the original wood siding article provided here, Mr. Bliss points out that furring strips are the standard wood siding mounting procedure in the Canadian Maritime Provinces where otherwise wood rot is a significant problem. The reason you may not see this high quality siding installation method recommended in more manufacturer's literature may be contractors' resistance to the added labor cost of using this method.
Wood Siding Moisture Leaks, Traps, & Dryout
Earlier in this article we cited several possible sources of moisture penetrating wood siding materials, especially from the siding back, leading to a variety of types of siding damage. Best practice to avoid moisture damage to wood siding will include proper siding installation - that we have discussed just above. But in addition to properly selected, installed, nailed, and finished wood siding, here are some other steps to avoid moisture or water damage to wood building siding products:
Siding wedges used to prevent paint failure and siding damage
Wood siding wedges (small wooden wedges, or small plastic wedges) are designed to create a gap between the bottom edge of clapboard siding and the surface of the board below, allowing wall moisture to escape. Especially on older buildings sheathed with wood clapboards that have been painted several times, the paint can seal the bottom edge of each clapboard against the board below, making it more difficult for moisture to escape the wall. Wedges sold for this purpose can be driven up below the clapboards to create a vent opening as a building retrofit. We're uncertain about the effectiveness of this solution to paint and siding failures:
will wind-blown rain now enter the wall cavity more than previously?
have we identified and corrected the original moisture problem source?
will wedges create lontitudinal cupping in the siding?
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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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