Building Material Insights

William Robinson  |  1/1/2007 11:59:52 PM

Building materials have performance characteristics (strength, permeability, R-value, expected longevity, etc.). Some of these characteristics are “rated”. The rating can be read on product labels.

A material’s rated characteristics are based upon a given set of conditions. When conditions change, characteristics may change, too. This is sometimes good, sometimes bad.

Here are some examples of characteristics that change with changing conditions; you should try to control the conditions to achieve the rated performance:
  • R-value 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 - Materials that can absorb and hold water (hygroscopic materials) can change permeability in relation to relative humidity. This can have nice strategic advantages.
    • The kraft paper on fiberglass insulation is a vapor retarder when humidity is low (in winter), but becomes permeable when RH 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 summer.
  • Building material deterioration is largely a function of exposure to water, heat and ultraviolet light. Water is by far the biggest factor.
  • Never assume that siding, roofing, “sealed” windows and other outer building parts (known as cladding) are waterproof. Virtually all claddings leak sooner or later; some leak a lot, some a little.
    • Materials expand and contract at different rates, causing movement and separation.
    • Sealants and adhesives don’t last forever.
    • Nothing’s 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 is not a problem if they 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 water storage capacity provides a kind of buffer or cushion of safety 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% moisture content (by weight). Decay stops when the moisture drops below 20%. Some molds can grow on wood at or above 16% moisture content. If there is no excess moisture source, wood tends to stabilize at 6% to 7% moisture content. So that leaves about a 10% 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 has 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 a reservoir material (concrete construction).

Buffer Capacity (Water Storage Cushion of Safety)
of a Typical 2,000 Sq. Ft. House

Type of Structural Material Approx.  Buffer Capacity
Concrete construction 500 gallons of water
Wood frame 50 gallons of water
Steel frame with gypsum sheathing 3 gallons of water

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