Home Talk USA: Benchmark Mortgage

Building materials have performance characteristics such as strength, permeability, R value, and expected longevity. Some of these characteristics are rated and listed on product labels. A material’s rated characteristics are based upon a given set of conditions. When conditions change, characteristics may change, too.

Here are some examples of characteristics that change with conditions; controlling conditions will help materials achieve their rated performance:

R value, the measure of a materials resistance to the passage of heat (the higher the R value, the more insulating), is reduced if an insulation material is compressed, wetted or has air currents flowing through it. Some insulating foams cure over time, losing R value until they stabilize. Loose-fill insulations can lose R value as they settle, especially if improperly fluffed.
Permeability can change in hygroscopic materials- things that can absorb and hold water- in relation to relative humidity. This can have nice strategic advantages.

The kraft paper fiberglass insulation is a vapor retarder when humidity is low (in winter), but becomes permeable when humidity is high (in summer). The same happens with latex paint. This, in effect, puts a vapor barrier where and when it’s needed (on the side that’s warm in winter), but effectively removes the vapor barrier from the wrong side in the summer. Building material deterioration is largely a function of exposure to water, heat and ultraviolet light. Water is by far the biggest factor.

Here are some other things to consider about the performance of building materials:

Never assume that siding, roofing, “sealed” windows and other outer building parts (known as cladding) are waterproof. Virtually all cladding leaks sooner or later.
Materials expand and contract at different rates, causing movement and separation.
Sealants and adhesives don’t last forever.
Nothings perfect, especially installations by humans in the real world

Moisture-related damage occurs when the rate of wetting exceeds the rate of drying and the water storage capacity of a material. For most structural materials- masonry, solid wood, metal- repeated wetting isn’t a problem if they are allowed to dry quickly enough. Engineered woods, gypsums and other fabricated structural materials have varying abilities to withstand any wetting; refer to manufacturers specifications and grades.

Deterioration begins after accumulated moisture in a material exceeds its water-storage capacity. That capacity provides a kind of buffer to allow drying time.

The higher the water-storage capacity of a material, the more forgiving it is to water intrusion. Choosing high-buffer capacity materials can be a prudent hedge against moisture damage in a high-rainfall climate.

Wood decay starts at 28 percent moisture content (by weight). Decay stops when the moisture goes below 20 percent. Some molds can grow on the wood at or above 16 percent moisture content. If there is no excess moisture source, wood tends to stabilize at 6 percent to 7 percent moisture content. So that leaves about a 10 percent water storage or buffer capacity.

Concrete’s water-storage capacity is about 10 times that of wood. Masonry is a reservoir material; it can hold a lot of water before problems begin.

Steel had no appreciable moisture storage capacity and can be more susceptible to corrosion than wood is to decay, depending upon treatments.

A drainage plane (flashings and layering of water-resistant components to drain water to the exterior) can control rainwater entry. In rainy climates, a drainage plane is needed in building systems that are not made of reservoir material (concrete construction).