All materials are impermeable to some things and forces while they are simultaneously permeable to others. Permeability and impermeability are relative terms depending upon the context. The permeability of any material is dependent upon both its own unique set of physical characteristics and upon the characteristics of the things and forces with which it comes in contact. Think of permeability as the ability for a thing or force to penetrate a material. Impermeability is the resistance to that potential penetration. Impermeable materials are important in our lives and homes because they contribute to keeping us comfortable, warm, safe and dry.
Impermeability, Penetrating Substances and Forces
Permeability and impermeability depend upon the material in question and upon the things and forces acting upon it. The materials we use in our lives and in our homes are affected by the things and forces with which they come in contact. Some of these penetrating (or non-penetrating) things and forces include water, heat, cold, sound and light. Light includes visible light as well as infrared and ultraviolet radiation. Microwaves, radio waves and radioactivity as well as cosmic rays and particles constantly bombard our homes and people.
Windows, Window Glass,
The two main functions of windows are their impermeability to rainwater and their permeability to light. Window glass also has a degree of impermeability to heat and cold transfer, which can be further reduced by installing double panes of gas-filled (or vacuum "filled") window glass and which are reflected numerically in the various R-Values assigned to home window products.
The drywall gypsum board that we use in our homes has some degree of impermeability to fire but it is not totally fireproof. Underwriters Laboratories rates various home materials for fire safety in terms of "Burn-Through" time for each material. Wood and plastics have faster burn-through times than materials made from rock such as gypsum board or the former but now-banned fireproof material of choice, asbestos. Drywall gypsum board is not impermeable to water and in its unpainted state is ruined very quickly by any amount of water. Similar-appearing but heavier cement board materials developed recently have greater water-impermeability and fire-impermeability than gypsum board and are now favored for use in bathrooms and near fireplaces respectively.
Roofs and Rainwater
The main purposes of the roof over your head is to keep water out and to prevent the exchange of heat and cold depending upon the season and your comfort level. Roofing materials must be impermeable to rain water. Asphalt shingles, clay tiles, and steel roofs all perform that function quite well. Wooden shingles and asphalt shingles offer little or no resistance to fire; while tile and steel roofs are quite fire resistant.
Rock-based materials (concrete, brick, marble, stone and masonry) are used for their strength, insulative properties and sometimes for their attractiveness. The porosity of these materials confers excellent impermeability to sound and to heat transfer while also allowing for seepage of water. Water moves by capillary action through stone and masonry materials and can slowly transfer dampness to any wood surfaces in contact with them. These materials are, however, highly impermeable to heat and fire and they offer the greatest degree of fire resistance of all the building and home construction materials.
Wood, Lumber, Plastics and Insulation Materials
Wood, timber and lumber serve as strong structural and decorative elements for our homes. Wood has great strength and insulative properties but it offers little resistance to water and fire. Certain woods such as hemlock and redwood offer greater degrees of impermeability to water. Treated lumber uses chemicals to prevent water damage by decomposition of the wood but it offers no resistance to the water itself. Modern plastics and composites of wood and plastic offer the greatest impermeability to water and are often used these days for outdoor decks and docks.
We insulate our homes because the insulation materials we use offer some degree of impermeability to heat transfer. Fiberglass insulation is somewhat fireproof. Keeping our homes cool in summer and warm in winter depend upon the thermal properties of the materials used and how well they prevent heat transfer to and from our homes. In general, the thicker the insulation, the greater the impermeability to heat entering or leaving our homes.
- Underwriters Laboratories (UL)
- American Society for Testing Materials (ASTM)
- University of Missouri Principles of Building Construction: Non-Combustible Materials
- ASTM E 84-05 Standard Test Method for Surface Burning Characteristics of Building Materials
- Insulation Contractors Association of America (ICAA)
- Photo Credit rain image by tomash from Fotolia.com
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