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.
||500 gallons of water |
||50 gallons of water |
|Steel frame with gypsum sheathing
||3 gallons of water |