Building materials’ rated characteristics (strength, permeability, R-value, expected longevity, etc.) are based upon a given set of conditions. When conditions change, characteristics may change, too. This is sometimes good, sometimes bad.
- 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 but can also have disadvantages:
The kraft paper on fiber glass 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.
Sheathing that is normally not very permeable can become damp and vapor permeable from high humidity and heat.
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 real people 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 near a 10% 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|
||500 gallons of water|
||50 gallons of water|
|Steel frame with gypsum sheathing
||3 gallons of water|