Healthy Indoors Magazine
Issue link: https://hi.healthyindoors.com/i/1216998
44 | February 2020 In one installation of this kind of siding, kick-out flashings were missing, so water from roof edges streamed down the cladding. As a result, the factory-applied paint failed under the roof drip edge in patterns following the water flow. The cement board was also damaged. Another problem with this particular installation was that the angle-cut ends of the planks at the dormers on the gable roof were almost in contact with the roof shingles. These cut ends absorbed water, and all the paint peeled from the cement boards at the bottoms of the dormer walls. (The in- stallation manuals now require a two-inch rather than a one- inch clearance to roof shingles.) Hyphae from decay fungi on sheathing due to leakage behind cladding May Indoor Air Investigations LLC Cement-board siding swollen from moisture May Indoor Air Investigations LLC Similar forces can arise if different paint types in different layers within the paint film swell with moisture to varying extents. (This type of bending is similar to what occurs in bimetallic strips when one metal expands more with heat than the other bonded metal.) The "curling/cupping" forces of the paint film may exceed the bonding strength of the film to the wood, and the film will fail. Water running down the face of painted clapboards causes this type of failure. A moisture-saturated paint film exposed to the sun will dry at the surface more quickly than at the deeper planes. The resulting differences in moisture content can also give rise to cupping as well as forces that eventually tend to pull the paint film away from the substrate. Exposure of a thick, dry paint film to the sun can also give rise to cupping forces, due to temperature differentials between surfaces, causing the heated surface to expand faster than the unheated one. Preventing Paint Failures • All wood intended for exterior use should be painted on all sides. End-grain should be primed (sealed) before in- stallation. • Window- and door-cap flashings should shed water to the front and not to the sides, where water is certain to enter wall gaps. • Cap flashings should be single-pieces whenever possible, but caulked at joints if pieced. • Small dabs of caulk can be applied to cap flashings as "end dams." • As we all know, roof and gutter water should never be allowed to run down the side of a building. • A gutter system should direct water away from the founda- tion wall. If the water empties next to the foundation wall, there is a good chance that you will find signs of water intrusion below-grade. • Weather-resistant species rather than ordinary pine should be used (though borate treatment of pine can re- tard decay). Cement-Board Siding One of my pet peeves has to do with the installation of ce- ment-board siding: a composite of concrete and cellulose (wood fibers), built in several layers. This siding is heavy but porous. I once put a piece of cement-board siding in water, and in ten minutes, the piece had absorbed 30% of its weight in water.