The work of FPL’s Durability and Wood Protection Research Unit is broad in scope and includes studies into damage and contamination by decay fungi, mold, and termites. All these household pests are attracted to excess moisture, which can result from inadequate surface drying of condensation, leaks in pipes and foundations, poor ventilation, or flooding.
Homeowners are increasingly concerned about moisture management and indoor air quality. However, chronic moisture problems in a home can lead to more than poor indoor air quality—persistent high moisture can lead to a cascading biological succession from mold to decay to termite damage.
Blue-black color on walls shows evidence of mold growth. (Photo used with permission from A&J Specialty Services, Inc.)
Contamination with mold can render a home unlivable, and cleanup may require gutting the entire structure. In some cases, cleanup costs for toxic molds can equal the value of the home!
- Mold occurs on the surface of wood exposed to excessive humidity or wet/dry cycling.
- Visible mold growth is a good indicator of damp conditions or excess moisture.
- Water vapor in humid air will not wet wood sufficiently to support decay fungi, but it will permit mold growth.
- Mold, though unsightly, causes insignificant strength loss to structural wood components.
- Common mold fungi can cause allergic symptoms; however, some molds (Stachybotrys sp.) produce mycotoxins, which cause illness and make homes uninhabitable.
- New York City Department of Health and the U.S. Environmental Protection Agency have established guidelines for the assessment and remediation of mold fungi in indoor environments.
Photo from Flickr Creative Commons
How best to get a firm seal on your substrate? The Ins and Outs of Caulking states that nothing adheres well to a dirty surface. In addition, new and apparently clean metal components may have oils in their surfaces left from manufacturing processes. Likewise, the extrusions of vinyl, vinyl-clad wood, and wood–plastic composite windows may have residual extrusion die lubricants on their surfaces.
Satisfactory sealant adhesion requires removal of such contaminants. Wiping with a clean rag moistened with mineral spirits is an effective method of removing surface oils and die lubricants. This can, however, pose health and fire risks if done carelessly, and residual mineral spirits that do not fully evaporate before caulk is applied may compromise adhesion. Substituting denatured alcohol for mineral spirits generally poses fewer health risks, and alcohol’s relatively rapid evaporation rate is more likely to leave a clean dry surface. Organic solvents, while generally effective at removing organic surface contaminants such as oils, may not be compatible with all substrates; this is an additional reason that justifies caution in their use. Surfaces contaminated with dirt, airborne dust, and mud usually are most effectively cleaned with a well-rinsed water-wetted rag. Rinsing the rag in a detergent solution can aid in surface cleaning, but if this is done, residual detergent left on surfaces will interfere with caulk adhesion.
If you decide that using detergent solution is necessary to obtain adequate surface cleaning, you must follow the cleaning with a thorough rinsing with a water-wetted rag. It can be difficult to ascertain if the rinsing was adequate; therefore try water and a non-abrasive nylon cleaning pad before resorting to use of detergent solution. Because porous surfaces are generally absorptive and thus difficult to adequately rinse, use of detergent solution on them as in preparation for caulking is not recommended. At application, surfaces must also be free of ice or frost. At below-freezing temperatures, frost may accumulate on surfaces from an applicator’s breath; this is among the reasons that manufacturers commonly restrict application temperature.
Compatibility of the finish and the substrate is critical.
The Ins and Outs of Caulking states that compatibility between substrates and sealants involves two issues. The first is sealant adhesion to substrates (which may be dissimilar); the second is the potential for uncured sealant to damage the substrates (by chemical action of a component in the uncured sealant).
Silicone sealants are recognized as adhering well to most non-porous substrates. Their effective adhesion to porous substrates like wood or masonry may, however, require the use of primers. Polyurethane and latex acrylic sealants are generally recognized as adhering relatively well to porous substrates like wood and wood-based products without the use of primers. Sealant primers are rarely if ever available at hardware stores and home centers. A quality primer paint, suitable for the (porous) substrate, usually improves sealant adhesion. If end and edge cuts on siding or trim surfaces are primed before the siding or trim is installed, the priming is significantly more effective.
The chemical components of sealants with the potential to damage substrates are most commonly either organic solvents or acidic constituents. The organic solvents used in most sealants are not sufficiently aggressive to harm most substrates. The smell of vinegar in uncured silicone sealant indicates the presence of an acidic constituent. Most substrates are unaffected by such acidic-cure silicone sealants, but a few are. Installation instructions for components such as fenestration units may indicate if sealant use is recommended, and if so, what type of sealant to use. Use of a different type of sealant than recommended by the manufacturer may result in chemically induced damage to the component. Some silicone sealants are unsuitable for certain porous substrates because they can leach oily materials, resulting in staining.
Severely weathered wood surface.
When reapplying solid-color stain, wash the surface to remove dirt and mold. If areas have peeled exposing the wood surface, the weathered surface must be removed prior to re-priming the peeled area. The photo shows an extreme case of surface degradation following the failure of the coating. Solid-color stains form films, and like all other film-forming finishes, do not bond properly to weathered wood. It has been well established through several studies that cedar exposed to sunlight for as little as 2 to 3 weeks will not hold film-forming finishes as well as an unweathered surface. It may be possible to remove the damaged wood from small areas by scuff sanding, but this is difficult with shakes and shingles because the surfaces are saw-textured or split. The sanded surface won’t match the unsanded surface. If the previous finish contained lead, do not sand.
The situation shown in the photo is an extremely difficult problem to reconcile. A penetrating finish, such as an oil-based semitransparent stain, cannot be used over the existing finish, and a film-forming finish will not adhere to the weathered surface. It is probably necessary to remove all the finish and the weathered surface by power washing prior to application of the primer and solid-color stain. This may be one of the few instances where careful power washing by a skilled operator is necessary. As with using a garden hose to wash siding, keep the nozzle pointed down to avoid forcing water behind the siding. Paints refinish in the same way as solid-color stains. Lap marks should not be a problem with paint systems.
Careful and judicious work will yield good results for the homeowner.
What about do-it yourselfers who want to work with cedar siding or shingles that have already been finished? The FPL report, Installation, care, and maintenance of wood shake and shingle siding, can help with that.
Finished surfaces (paints, semitransparent stains, and solid-color stains) often develop mold and algae. As with wood surfaces, these microorganisms feed off airborne contaminates, but can also metabolize oils in some finishes. Surfaces of paint and solid-color stain can easily be cleaned with commercial cleaners or a dilute solution of household bleach and as with wood, the gentler the better. Abrasive methods will damage the finish surface. It is more difficult to remove these microorganisms from saw-textured wood; the microorganisms may be inter-grown through and under the finish.
Wood Repellant Preservatives (WRPs) and Semitransparent Stains
Other than mold removal and gentle washing, WRP and semitransparent stains should not require additional surface preparation. They can be reapplied as soon as shakes or shingles are dry following cleaning. The reapplication of semitransparent stains and tinted WRPs requires special care. Usually residual finish is left in some areas, whereas the finish may be completely weathered away in other areas. The weathered and unweathered surfaces accept penetrating stains differently; the weathered areas absorb stain easily, whereas stain forms a second coat on areas having residual stain. During application, feather the stain into areas having old finish to avoid forming this second coat. If a second coat is formed, it will give a shiny appearance and will fail by flaking. When refinishing a structure, it is often necessary to have the north, east, west, and south sides on different cycles. Semitransparent stains usually last 4 to 5 years on the south side and more than 10 years on the north side. These estimates can vary considerably depending on the design of the structure, exposure conditions, and the quality of the shake or shingle. As with the application for new construction, take care to avoid lap marks. Changing colors or switching to a paint or solid-color stain is not an option with semitransparent stains unless the old finish and weathered surface is removed. The surface must be scuff-sanded, primed, and finished (at least two coats may be required to hide residual stain).