Q: Our vertical cedar board siding became uniformly cupped with outside edges pushing outward along with knots pushing outward. Does the direction of cupping and knot displacement suggest where the moisture came from? Thank You — Dan
A: Wood expands and contracts as its moisture content rises and falls. Increases in wood moisture content can result from exposure to liquid water — for example, rain or storing the wood on wet grass. Drying can be hastened by exposure to airflow and sunshine.
Wood moisture content also rises and falls with changes in the air’s relative humidity. That’s why gaps sometimes appear in wood flooring in cold climates (especially in leaky houses with dry air) and expands again in summer. Wood moisture content always tries to reach a state of equilibrium with the RH of the surrounding air.
These changes in lumber dimension are almost entirely across the grain, not along the length. Wood cups when one face becomes wetter and therefore wider than the other face, due to uneven wetting or drying. The convex face — curving outward at the center of the board — is the wetter face. All these effects are reduced if the wood is well sealed and finished on both sides and all edges, slowing moisture migration into and out of the wood.
Where the outside edges of the siding are pushing out away from the sheathing, as on your home, that indicates that the inside face of the wood is wetter than the outside face. This could be due to excessive wetting of the inside face or excessive drying of the outside face, or some combination of the two.
If the cedar siding was wet when installed (higher than the wood’s equilibrium moisture content for your area) it may be that the outside dried more rapidly than the inside, leading to cupping. The outside would tend to dry faster due to exposure to sun and wind.
Or it may be that excess moisture from the building is wetting the backside of the siding.
If you can describe your wall system from inside to out, as well as your climate and the use of the building, I might have a better idea of the cause. Also, is the siding cupping on just part of building, such as the south-facing wall, or on all sides? And when was the siding installed, how is it finished on each side, and when did you first notice the problem? It would be helpful if you could send a photo — may provide additional clues.
Do you know if the wood is flat-sawn vs quarter-sawn? Quarter-sawn wood is a lot more stable lot more expensive. Some people say that the rings tend to straighten out in flat-sawn wood as it dries. While this may be true to a small degree, the effects of uneven wetting and drying are much greater.
If uneven moisture is the culprit, it’s possible that this is a temporary problem that will resolve when wood moisture levels even out. – Steve Bliss, BuildingAdvisor.com