How sun at altitude and freezing wear a log finish

A log finish in Jackson Hole wears out from two directions. Sunlight breaks down the wood surface and the finish on top of it, and the elevation and snow make that sunlight stronger. Then a long freezing season adds meltwater and ice. Knowing which force is doing what helps you read your own walls and judge how much work they need.

Sun at about 6,200 feet

Jackson sits at about 6,200 feet, roughly 1,900 meters, based on a USGS elevation query at the town center. The World Health Organization's rule of thumb is that "with every 1000 m in altitude, UV levels increase by approximately 10 per cent." Applied to Jackson, that works out to roughly 19% more UV than at sea level. Treat that number as an estimate from a rule of thumb.

Snow adds to it. WHO says snow "can reflect as much as 80% of UV radiation," and the EPA says that in winter, reflection off snow "can nearly double UV strength." The Jackson NOAA station averages 78.2 inches of snow a year, so for months of the year the lower courses of a log wall can sit above snow.

What UV does to wood

The Wood Handbook, published by the USDA Forest Products Laboratory, explains the mechanism. Lignin is the natural glue that holds wood cells together, and it makes up about 20% to 30% of wood's mass. UV light breaks the lignin molecule. At the surface, lignin starts to degrade after a few hours of exposure.

Once the lignin goes, the surface changes:

• Color. The wood turns gray. The Handbook describes wood that looks weathered and silver or gray as "severely sunburned."
• Checks. As weathering continues, wood develops checks on its faces, and checks and cracks near the ends.
• Erosion. Loose fibers fray off the surface. The Handbook puts the loss at about 6 millimeters (1/4 inch) of wood per century, with lower-density wood eroding faster.

The erosion is slow. The bigger problem for a homeowner is what sunburned wood does to the next coat of finish. The Handbook warns that a film-forming finish applied over sun-degraded wood is prone to peeling, because the damaged surface cells hold onto the finish but not onto the sound wood beneath. Penetrating finishes don't peel this way. On a gray log wall, then, the gray layer has to come off before a film-forming product goes on.

A long freezing season

NOAA's 1991–2020 normals for the Jackson station show a low at or below freezing on about 236 nights a year, a high that stays at or below freezing on about 61 days, and a low at or below 0°F on about 35 nights. Teton County, Wyoming's adopted building code design criteria set the outdoor design temperature at -30°F and rate weathering "Severe," a rating that sets requirements for concrete and masonry.

Freezing matters most where water sits. The Wood Handbook lists temperature and wetting and drying cycles among the factors that affect how fast wood erodes. On a log wall, the places that hold water are the ones to watch:

• Upward-facing checks, where meltwater collects.
• Log ends. The Handbook says liquid water enters end grain 10 to 100 times faster than through other faces, and it calls sealing end grain "probably the single most valuable improvement" for wood exposed to water.
• Lower courses, where snow piles against the wall and splashback hits in spring.
• Chinking joints that have pulled away from a log, letting water run behind the sealant.

Little termite risk

The same county design criteria rate termite risk "none to slight."

Why walls wear unevenly

Exposure varies around a single house. The Wood Handbook notes that an area under the eaves, even on the south side, may be relatively unweathered compared with the lower part of the wall. On a log home, the upper logs on a wall can keep sound finish while the lowest courses go gray. A contractor may treat those parts of the same wall differently, and the bid should say so.

When you walk the house, compare:

1. South and west walls against north and east.
2. Logs under the eaves against the lowest courses.
3. Log ends at corners against the faces of the same logs.

Drafts and energy

Gaps in chinking let air through as well as water. Lower Valley Energy, the electric co-op with offices in Afton and Jackson, offers residential energy audits, with the fee based on square footage and extra charges for blower-door and thermal-imaging tests. Its insulation rebates for existing homes are available to members with primary installed electric heat, and its rebate chart lists insulation, air sealing, windows, and doors. The chart doesn't list chinking or log-wall sealing, so don't count on a rebate for re-chinking.

What to do with this

• Look at the walls each year after the snow melts, and note where the finish has thinned.
• Restain worn areas before they go gray. Sunburned wood needs more prep to take a new finish.
• Seal log ends and fix chinking gaps as you find them.
• Keep the product name and color from the last job so touch-ups match.

The staining season guide covers when that work can happen. For help judging what your walls need, send a request.