Absolute Humidity
The ratio of the mass of water vapor present to the volume occupied by the mixture. It represents the density of water vapor in the air.

Absorption
Absorption, in chemistry, is a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This is a different process from adsorption, since the molecules are taken up by the volume, not by surface.

Accreditation
The process of certifying a Home Energy Rating System (HERS) as being compliant with the national industry standard operating procedures for Home Energy Rating System.

Active Solar
Active solar uses mechanical devices such as photovoltaic panels to transfer collected heat from the sun to storage mediums and/or end use. Thus, solar radiation is used by special equipment to provide our homes with space heating, hot water, and/or electricity.

Adsorption
Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (the adsorbate).

AFUE Annual Fuel Utilization Efficiency (AFUE)
Measures the amount of fuel converted to space heat in proportion to the amount of fuel entering the furnace. This is commonly expressed as a percentage. A furnace with an AFUE of 90 could be said to be 90% efficient. AFUE includes any input energy required by the pilot light but does not include any electrical energy for fans or pumps.

Air Barrier
The primary function of an air barrier is to control airflow between a conditioned space and an unconditioned space. Air barrier systems are a component of building envelope systems that control the movement of air in and out of buildings. Many performance problems of homes can be traced to air leakage through the building envelope. These problems include everything from high heating/cooling costs to poor temperature control in occupied spaces to moisture.

Air Flow Retarder
Sealants used to keep outside air and inside air out of the building envelope. Four common approaches to retarding air flow include careful sealing using the following building components: drywall and framing, plastic sheets (should not to be used in hot and humid climates) between drywall and framing, exterior sheathing, and building paper. Air flow retarders define the pressure boundary in a house that separates indoor and outdoor air.

Air Handler
Fan used by a heat pump. central air conditioning system, or furnace uses to distribute heated or cooled air throughout a home or building

Air-Impermeable
A material or assembly having an air permeance equal to or less than 0.02 l/(s-m²) @ 75 Pa tested according to ASTM E 2178 or E 283.

Biodegradable
Capable of being decomposed by living organisms.

Biomass Energy
Energy produced by the conversion of biomass directly to heat or to a liquid or gas that can be converted to energy. It comes from materials that were once living, such as plants of some types of waste. A form of renewable energy.

Building Envelope or Enclosure
The outer shell, or the elements of a building, such as walls, floors, and ceilings, that enclose conditioned space.

Btu (British Thermal Unit)
A standard unit for measuring energy. One Btu is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit from 59 to 60. An Inches-Pounds unit.

CABO (Council of American Building Officials)
A national organization of building code officials and interested parties, which, through a national consensus process, developed, adopted and promulgated the national Model Energy Code (MEC). CABO has recently become CABO International and has taken on the administrative responsibility for the development of a uniform international building code through an International Code Council (ICC).

Capacity
The rate at which a piece of equipment works. Cooling capacity is the amount of heat a cooling system can remove from the air. For air conditioners total capacity is the sum of latent capacity, the ability to remove moisture from the air, and the sensible capacity, the ability to reduce dry-bulb temperature. Heating system capacity indicates how much heat a system can provide. Heating and cooling capacities are rated in Btu per hour

Casement Window
A window with a hinged sash that swings like a door, often with fixed panels, too. When windows crank out, casement windows can be used in green building to catch and funnel prevailing breezes into home.

Channel Air Flow
Channel air flow is air passing through openings in the building envelope typically found at the interface of dissimilar materials where the air entry and exit points are not in a linear pathway.

Chase
An enclosure designed to hold ducts, plumbing, electric, telephone, cable, or other linear components. A chase designed for ducts should be in conditioned space and include air flow retarders and thermal barriers between it and unconditioned spaces such as attics.

Clerestory Window
A clerestory, pronounced clear story, is a high wall with a band of narrow windows along the very top. The clerestory wall often rises above adjoining roofs. In green home building, clerestory windows add natural light to potentially dark rooms or areas, can act as loft windows, and, when they open, can provide a thermal chimney.

Concentration
The relative amount of a substance mixed with another substance. An example is five parts per million of carbon monoxide in air or 1 milligram/liter of iron in water.

Condensation
Condensation is the process in the hydrologic cycle by which a vapor becomes a liquid; the opposite of evaporation.

Conditioned Space
The part of the building that is designed to be thermally conditioned, normally for the comfort of occupants but at times for other reasons.

Conduction
Conduction is the transfer of heat through a material. Heat is transferred directly in and through the substance. Conduction heat loss or gain results from the transfer of heat directly through the materials of the building envelope. If the outside temperature is greater than the inside temperature, there is heat gain from outside the building.

Conservation
Preserving and renewing natural resources to assure their highest economic or social benefit over the longest period of time. Green building, by definition, conserves resources both for the present and for the use of future generations.

Construction Documents
The drawings (plans) and written specifications that describe construction requirements for a building.

Convection
Convection is the transfer of heat from particle to particle. It is the transfer of heat by the movement of fluids such as air or water. For example, the heated air around a stove will tend to rise to the ceiling. We all know hot air rises and cool air sinks. That is natural convection. Forced convection refers to the use of fans or pumps to move a fluid/air and the heat contained in it. We generally have forced-air furnaces in our homes.

COP (Coefficient of Performance)
A measure of efficiency typically applied to heat pumps. The COP for heat pumps is the ratio, at a given point in time, of net heat output to total energy input expressed in consistent units and under designated conditions. Heat pumps result in a COP greater than 1 because the system delivers or removes more heat energy than it consumes. Other specific definitions of COP exist for refrigeration equipment. See HSPF for a description of a unit for seasonal efficiency.

Dew Point
Dew point is the temperature to which air with a given quantity of water vapor must be cooled to cause condensation of the vapor in the air. The temperature of which air must be cooled for it to be saturated.

Diffuse Air Flow
The passage of air through porous building materials such as fiberglass or uncoated concrete block is diffuse air flow.

Diffusion
The movement of a substance from an area of high concentration to an area of low concentration.

Diurnal Temperature Range The temperature difference between the minimum at night (low) and the maximum during the day (high).

Drainage Plane
Drainage planes are water repellent materials such as building paper, housewrap, and foam insulation which drain water. They provide water drainage to the exterior of the building. The materials that form the drainage plane overlap each other shingle fashion or are sealed so that water flow is downward and outward.

Drought
An extended period with little or no precipitation.

Dry-Bulb Temperature
The temperature of air indicated on an ordinary thermometer, it does not account for the affects of humidity.

ECM (Energy Conservation Measure
An individual building component or product that directly impacts energy use in a building.

EEM (Energy Efficient Mortgage)
Specifically, a home mortgage for which the borrower’s qualifying debt-to-income and housing expense-to-income ratios have been increased ("stretched") by 2% because the home meets or exceeds CABO’s 1992 version of the Model Energy Code (MEC). This so-called "stretch" mortgage is nationally underwritten by Fannie Mae, Freddie Mac and the Federal Housing Administration (FHA). This term is often used generically to refer to any home mortgage for which the underwriting guidelines have been relaxed specifically for energy efficiency features, or for which any form of financing incentive is given for energy efficiency.

EER (Energy Efficiency Ratio)
A measurement of the instantaneous energy efficiency of cooling equipment, normally used only for electric air conditioning. EER is the ratio of net cooling capacity in Btu per hour to the total rate of electric input in watts, under designated conditions. The resulting EER value has units of Btu per watt-hour.

EF (Energy Factor)
A standardized measurement of the annual energy efficiency of water heating systems. It is the annual hot water energy delivered to a standard hot water load divided by the total annual purchased hot water energy input in consistent units. The resultant EF value is a percentage. EF is determined by a standardized U.S. Department of Energy (DOE) procedure.

Emissivity
The ability of a material to hold or release heat. Don't confuse with conductivity.

Energy (use)
The quantity of onsite electricity, gas or other fuel required by the building equipment to satisfy the building heating, cooling, hot water, or other loads or any other service requirements (lighting, refrigeration, cooking, etc.).

Energy Audit
A site inventory and descriptive record of features impacting the energy use in a building. This includes, but is not limited to, all building component descriptions, all energy using equipment and appliance descriptions and all energy features.

Energy Star ® Home
A home, certified by the U.S. Environmental Protection Agency (EPA), that is at least 30% more energy efficient than the minimum national standard for home energy efficiency as specified by the 1992 MEC, or as defined for specific states or regions. ENERGY STAR is a registered trademark of the EPA.

Equivalent Leakage Area of a building (EqLA or ELA)
Quantitative expression of the airtightness of a building enclosure. EqLA is the method set by the Canadian General Standards Board in which a blower door depressurizes the building enclosure to 10 Pascals and the leakiness of the enclosure is expressed as a summary hole in square inches. ELA is set by the ASTM equivalent procedure at a pressure differential of 4 Pascals.

Erosion:
The wearing down or washing away of the soil from the action of water, wind, or ice.

Evaporation:
The conversion of a liquid into a vapor usually through the application of heat energy during the hydrologic cycle; the opposite of condensation.

Fenestration
The arrangement or placement of windows or other openings in a building.

Filtering:
The soil's ability to retain substances on the soil particle surface.

Fresh Water:
Water with less than 0.5 parts per thousand dissolved salts.

Geothermal Energy
Energy from within the earth: Natural heat contained in the rocks, hot water and steam of earth's subsurface. A form of renewable energy.

Glazing
Refers to both the type of glass used in a window and the actual process of installing glass in the window frame or sash.

Grade Beam
A foundation wall that is poured at or just below the grade of the earth, most often associated with the deepened perimeter concrete section in slab-on-grade foundations.

Gray Water:
Domestic wastewater composed of wash water from household sinks, tubs, and washers.

Greenhouse Effect
The greenhouse effect is the process in which the emission of infrared radiation by the atmosphere warms a planet's surface. The sun radiates energy to the earth. The earth reflects back about 30% of this solar radiation. The remaining 70% is absorbed and warms the earth's atmosphere, land, and water. The Earth's average surface temperature of 59°F is about 59°) warmer than it would be without the greenhouse effect.

Groundwater
Groundwater is a natural resource composed of water found in the spaces between soil particles and cracks in rocks underground.

Habitable and/or Occupiable Space
Habitable space is an area in a structure for living, sleeping, eating or cooking. It is building space intended for continual human occupancy. Bathrooms,closets, hallways, storage or utility space are not considered habitable space.

Haze Factor
The percentage of light through a glazing material that is not diffused. The ratio between the luminescence of an object and the luminescence of the scattering medium through which it is being viewed.

HERS (Home Energy Rating System)
A standardized system for rating the energy-efficiency of residential buildings.

HERS Energy-Efficient Reference Home (EERH)
The EERH is a geometric "twin" to a home being evaluated for a HERS rating and according to a newly-revised system, is configured to be minimally compliant with the 2004 International Energy Conservation Code.

HERS Provider
An individual or organization responsible for the operation and management of a Home Energy Rating System (HERS).

HERS Rater
An individual certified to perform residential building energy efficiency ratings in the class for which the rater is certified.

HERS Score
A value between 0 and 100 indicating the relative energy efficiency of a given home as compared with the HERS Energy-Efficient Reference Home as specified by the HERS Council Guidelines. The greater the score, the more efficient the home. A home with zero energy use for the rated energy uses (heating, cooling and hot water only) scores 100 and the HERS Reference Home scores 80. Every one point increase in the HERS score amounts to a 5% increase in energy efficiency.

Housing Expense-to-Income Ratio
The ratio, expressed as a percentage, which results when a borrower’s total monthly housing expenses (P.I.T.I.) are divided by their gross monthly income. This is one of two ratios (debt-to-income ratio being the other) used by the mortgage industry to determine if a prospective borrower qualifies (meets the underwriting guidelines) for a specific home mortgage. Fannie Mae, Freddie Mac and FHA underwriting guidelines set an upper limit of 28% on this value for conventional loans but increase ("stretch") the ratio by 2% for qualifying Energy Efficient Mortgages (EEM).

Housewrap
Any of several spun-fiber polyolefin rolled sheet goods for wrapping the exterior of the building envelope.

HSPF (Heating Season Performance Factor)
A measurement of the seasonal efficiency of an electric heat pump using a standard heating load and outdoor climate profile over a standard heating season. It represents the total seasonal heating output in Btu divided by the total seasonal electric power input in watt-hours ( Wh). Thus, the resultant value for HSPF has units of Btu/ Wh.

Humidity
Humidity is the amount of water vapor in the air. It is measured in three ways: absolute humidity, relative humidity, and specific humidity.

Hydroelectric Energy
Electricity generated from the force of moving or falling water. A form of renewable energy.

Hydrophilic
Having a strong attraction to water. Hydrophilic molecules are soluble in water and travel with the groundwater.

Hydrophobic
Having a strong aversion for water. Hydrophobic molecules are relatively insoluble in water and tend to sorb to soil particles.

Indoor Air
Air in a conditioned space.

Indoor Air Quality
Indoor air quality refers to the presence or absence of air pollutants in buildings.

Infrared Emittance
Infrared emittance is a decimal number less than one that represents the fraction of heat that is re-radiated from a material to it’s surroundings.

Insulated Glazing
Insulated Glazing Unit or Insulating Glass Unit (commonly referred to as IGU) is a set of two or more layers of glass spaced apart and hermetically sealed to form a single glazed unit with an air space between each layer. It is used to improve the thermal performance of glass in a green-built home. Since glass has no insulative properties, it is actually the air space between the glass layers that provides the insulation.

The most commonly found IGUs are double glazed, i.e. made with two layers of glass and are therefore also referred to as "double glazing units." Affordable argon is commonly used in insulated glazing. Another name often used in North America is Sealed Insulating Glass (abbreviated SIG).

Irrigation
Irrigation is the controlled application of water to home gardens and yards.

Jump Duct
A flexible, short, U-shaped duct (typically 10-inch diameter) that connects a room to a common space as a pressure balancing mechanism. Jump ducts serve the same function as transfer grilles.

Kiln-dried Lumber
Any lumber placed in a heated chamber to reduce its moisture content to a specified average under controlled conditions.

Leaching
Leaching is the process by which soluble materials in the soil, such as salts, nutrients, pesticide chemicals, or contaminants, are dissolved and carried away by water.

Load
The quantity of heat that must be added to or removed from the building (or the hot water tank) to satisfy specific levels of service, such as maintaining space temperature or hot water temperature at a specified thermostat setting (see also the definitions of energy and thermostat).

Low-E
Refers to a coating for high-performance windows, the "E" stands for emissivity or re-radiated heat flow. The thin metallic oxide coating increases the U-value of the window by reducing heat flow from a warm( er) air space to a cold( er) glazing surface. Low-E coatings allow short-wavelength solar radiation through windows, but reflect back longer wavelengths of heat.

Low-E Windows
Most often used in reference to a coating for high-performance windows, the "e" stands for emissivity. Low-e windows have microscopically thin layers of metallic oxide bonded to the surface of a window's glass and are essentially invisible to the naked eye, yet able to reduce the U-factor by reducing heat flow from a warm( er) air space to a cold( er)glazing surface. A low-e coating works like an invisible mirror to reflect selected portions of the light spectrum back out or back in through windows. The best location for the coating is based on whether the primary heat flow you want to control is from the inside out (heating climates) or the outside in (cooling climates). There is no reason to have a low-E window in the cooling climate of the south if the sun does not hit that window.

Low VOC
A substance that evaporates slowly. Often used to refer to material such as paint or adhesives used in homes and buildings. There is confusion about low VOC. See Semi-volatile Organic Compound.

Mechanical Ventilation
The active process of supplying or removing air to or from an indoor space by powered equipment such as motor-driven fans and blowers, but not by devices such as wind-driven turbine ventilators and mechanically operated windows.

Model Energy Code (MEC)
A "model" national standard for residential energy efficiency. The MEC was developed through a national consensus process by the Council of American Building Officials (CABO) and is the accepted national minimum efficiency standard for residential construction. Since MEC is a model code, it does not have the "force of law" until it is adopted by a local code authority. The MEC is used as the national standard for determining Energy Efficient Mortgage (EEM) qualification, and it serves as the national "reference point" used by Home Energy Rating Systems (HERS) in the determination of energy ratings for homes.

Net Zero Energy House
Any house that over time, averages out to net zero energy consumption. A net zero energy home may supply more energy than it needs during peak demand, typically using one or more solar energy strategies, energy storage and/or net metering. A net zero energy house produces as much energy as it uses.

Occupiable Space
Occupiable Spaces is any enclosed space intended for human activities, including but not limited to, all habitable spaces, toilets, closets, halls, storage and utility areas, and laundry areas.

Outdoor Air
Air outside the building.

Orifice Air Flow
Air flow where the air entry point and exit point are in a linear pathway; such as a crack between a window frame and the window opening is orifice air flow.

Ozone
Ozone is a pollutant at ground level but it is a solar shield in the upper atmosphere.

Passive Solar
Passive solar design uses the sun's radiant energy with natural mechanisms such as conduction and air convection currents. Passive solar utilizes the sun's energy for heating and natural processes for cooling, with only negligibly small requirements for nonrenewable energy. In North Central Texas, passive solar design is more concerned with cooling than heating.

Performance Test
An on-site measurement of the energy performance of a building energy feature or an energy using device conducted in accordance with pre-defined testing and measurement protocols and analysis and computation methods. Such protocols and methods may be defined by national consensus standards like those of the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) and the American Society for Test and Measurement (ASTM).

Permeable Paving
Permeable paving is a term used to describe a paving method that allows water and air to move through the paving material.

Permeance
Permeance is the ease at which water molecules diffuse through a material. It is to vapor diffusion as conductance is to heat transfer. The unit of measurement is typically the "perm."

Pervious Paving
Pervious paving is a term used to describe a paving method that allows water and air to move through the paving material.

Phase change
Phase change happens at constant temperatures with the movement of energy or latent heat.

Porous Paving
Porous paving is a term used to describe a paving method that allows water and air to move through the paving material.

Potable Water:
Water of suitable quality for drinking.

Precipitation:
The part of the hydrologic cycle when water falls, in a liquid or solid state, from the atmosphere to Earth as in rain, sleet, or snow.

Pressure Boundary
The point in a building at which inside air and outside air are separated. If a building were a balloon, the rubber skin would form the pressure boundary. Where inside and outside air freely mingle there is no pressure boundary.

Pressurization Test
A procedure in which a fan is used to place a house, duct system, or other container, under positive or negative air pressure in order to calculate air leakage.

R-Value
R value is the quantitative measure of resistance to heat flow or conductivity and is the reciprocal of U-factor. The units for R-value are ft² °F hr/Btu (English) or m² °K hr/W (SI or metric). A high R-value is not particularly important in North Central Texas.

Radiant Barrier
All materials emit energy by thermal radiation as a result of temperature. Radiant barriers or reflective barriers work by reducing the heat transfer from thermal radiation. Radiant barriers must face an open air space to perform properly as they transfer heat only across air spaces. Although radiant barrier works as heat control in Dallas Fort Worth, it is not the best heat control for attics in this area.

Radiation
Radiation is energy transmitted directly through space. Although it does not require matter in transmission, it does require a line of sight connection between the objects. All objects radiate energy or heat, which heats all cooler objects around it. Solar radiation passes through space to heat (and light) objects that it strikes.

Reclaimed wastewater:
Treated wastewater that can be used for beneficial purposes, such as irrigating certain plants.

Recycled water:
Water that is used more than one time before it passes back into the natural system.

Relative Humidity
Relative Humidity is the ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.

Renewable Energy
Renewable energy is energy generated from resources that are unlimited, rapidly replenished or naturally renewable such as wind, water, sun, wave and waste matter. It is energy flows which occur naturally and repeatedly in the environment. More about renewable energy.

Runoff:
Precipitation that flows over land to surface streams, rivers, and lakes.

Saturated
Being saturated is being the most concentrated solution possible at a given temperature; unable to dissolve still more of a substance. Filled to repletion; holding by absorption, or in solution, all that is possible.

Sealed Combustion
Sealed combustion means that a combustion appliance, such as a furnace, water heater, or fireplace, acquires all air for combustion though a dedicated sealed passage from the outside; combustion occurs in a sealed combustion chamber, and all combustion products are vented to the outside through a separate dedicated sealed vent.

SEER (Seasonal Energy Efficiency Ratio)
SEER is the most commonly used measure of the efficiency of consumer central air conditioning systems. A high SEER rating is not of prime importance in North Central Texas.

Septic system
Septic systems are used to treat household sewage and wastewater by allowing the solids to decompose and settle in a tank, then letting the liquid be absorbed by the soil in a drainage field. Septic systems are normally used in home construction only when a sewer line is not available.

Semi-Permeable
The term vapor semi-permeable describes a material with a water vapor permeance between 1 and 10 Perms. Water vapor can pass through a semi-permeable material but at a slow rate.

Semi-volatile Organic Compound (SVOC)
A substance that evaporates slowly at standard temperature (20° C and 1 atm pressure).

Shading Coefficient (SC)
The ratio of the total solar heat admittance through a given glazing product relative to the solar heat admittance of double-strength, clear glass at normal solar incidence (i.e. perpendicular to the glazing surface).

Skylight
A glazing and framing assembly consisting of sloped or possibly vertical surfaces. The assembly is generally inserted into a building to admit daylight. See haze factor, low-E, glazing, U-value.

Solar Energy
Energy derived from the sun in the form of solar radiation--electromagnetic radiation emitted by the sun. A form of renewable energy.

Solar Heat Gain Coefficient (SHGC)
SHGC measures how well a window blocks heat caused by sunlight. The lower the SHGC rating the less solar heat the window transmits. This rating is expressed as a fraction between 0 and 1. The number is the ratio of a window’s solar heat admittance compared to the total solar heat available on the exterior window surface at normal solar incidence (i.e. perpendicular to the glazing surface).

Solar Reflectance/Reflectivity
Solar reflectance is a decimal number less than one that represents the fraction of light reflected off the roof. Solar reflectivity is the percentage of sunlight that is reflected by an object when "hit" by sunlight.

Sone
A sound rating. Fans rated 1.5 sones and below are considered very quiet.

Sorption
The action by which molecules are attracted to or attach to solid particles, including soil.

Specific Heat
Specific heat is the amount of heat required to raise the temperature of one gram of a substance 1 degree Celsius

Specific Humidity
The water vapor content measured as weight of vapor per unit weight of air.

Spring
A spring is a point where groundwater flows or seeps from the ground.

Storm Drain
A storm drain allows water runoff from homes, streets, and other properties to flow into an underground system.

Sublimation
The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase is sublimation.

Subflorescence /Sub-efflorescence
A potentially harmful accumulation of water soluble salts that recrystalize beneath the masonry surface as moisture in the wall evaporates.

Supply ducts
The ducts in a forced air heating or cooling system that supply heated or cooled air from the or air conditioner to conditioned spaces.

Surface Water
Water above the surface of the land, including creeks, ponds, floodwater, and runoff is called surface water.

Temperature
Temperature is a measure of the heat content of a body. The molecular motion of a substance creates energy, which can be measured in terms of the heat it generates.

Temporary Wetland
A vernal pool or wetland in which water is present for only part of the year, usually during wet or rainy seasons.

Thermal Boundary
The border between conditioned and unconditioned space where insulation should be placed.

Thermal Chimney
The use of thermal chimneys in a home creates or reinforces the effect hot air rising to induce air movement for cooling purposes.

Thermal Mass
The use of thermal mass in a home prevents rapid temperature fluctuations by using materials such as masonry and water that can store heat energy for extended time.

Thermostat
A control device that measures the temperature of the air in a home or the water in a hot water tank and activates heating or cooling equipment to cause the air or water temperature to remain at a pre-specified value, normally called the set point temperature.

Tidal Energy
Tidal energy is from the ebb and flow of the tide. Tidal changes in sea level can be used to generate electricity by constructing dams across coastal bays or estuaries. A form of renewable energy.

Ton(s) of Refrigeration
Units used to characterize the cooling capacity of air conditioning equipment. One ton equals 12,000 Btu/h.

U-Factor or U-Value
The U-factor is a measure of heat flow or conductivity through a material, the reciprocal of R-value. Although R-values are used as for measures of the resistance to heat flow for individual building materials, U-factor is always used to measure the conductive energy of building enclosures.

Vapor
The state of water in the hydrologic cycle in which individual molecules are highly energized and move about freely; also known as gas/gaseous.

Vapor Barrier
A vapor barrier is a material that is vapor impermeable and has a permeance of 0.1 perm or less. A vapor barrier is a Class I vapor control layer.

Vapor Control Layer
The element (or elements) that is designed and installed in an assembly to control the movement of water by vapor diffusion.

Vapor Impermeable
Materials with a permeance of 0.1 perm or less such as glass, polyethylene film, and aluminum foil.

Vapor Permeable
Materials with a permeance of greater than 10 perms such as housewraps and building papers.

Vapor Permeance
Classes Test procedure for determining vapor permeance class is ASTM E-96 Test Method A-the desiccant or dry cup method.

Vapor Retarder
A vapor retarder is a material that has a permeance of 1.0 perm or less and greater than 0.1 perm. A vapor retarder is a material that is vapor semi-impermeable. A vapor retarder is a Class II vapor control layer.

Vapor Semi-Impermeable
Materials with a permeance of 1.0 perm or less and greater than 0.1 perm such as oil-based paints and many vinyl coverings.

Vapor Semi-Permeable
Materials with a permeance of 10 perms or less and greater than 1.0 perm such as plywood, OSB and most latex-based paints.

Ventilation
The controlled movement of air into and out of a house.

Volatile
Any substance which evaporates quickly.

Volatile Organic Compound (VOC)
Any organic compound which evaporates readily to the atmosphere. VOCs contribute significantly to photochemical smog production and certain health problems.

Volatilization
A process by which a chemical evaporates.

W (watt)
One of two (Btu/h is the other) standard units of measure for the rate at which energy is consumed by equipment or the rate at which energy moves from one location to another. It is also the standard unit of measure for electrical power.

Water (H2O)
Water is an odorless, tasteless, colorless liquid made up of a combination of hydrogen and oxygen. Water forms streams, lakes, and seas, and is a major constituent of all living matter. the conservation of water is part of green building.

Water Cycle
The paths water takes through its various states--vapor, liquid, solid--as it moves throughout the oceans, atmosphere, groundwater, and streams. Also known as the Hydrologic cycle.

Water Resistant Barrier (WRB)
A water resistant barrier is also referred to as a drainage plane.

Wastewater
Water that contains unwanted materials from homes and other human uses. It is a mixture of water and dissolved or suspended substances.

Wastewater Treatment
Any of the mechanical or chemical processes used to modify the quality of wastewater in order to make it more compatible or acceptable to humans and the environment.

Wave Energy
Tidal energy is from the ebb and flow of the tide. Tidal changes in sea level can be used to generate electricity by constructing dams across coastal bays or estuaries. A form of renewable energy.

Wet-Bulb Temperature
A measure of combined heat and humidity. At the same temperature, air with less relative humidity has a lower wet-bulb temperature.

Wetlands
Wetlands are areas where water saturation is the dominant factor in determining the nature of soil development and the types of plant and animal communities. A green developer will respect wetlands.

Wind Energy or Power
Energy obtained from turbine engines powered by wind. Wind is generated from the uneven solar heating of the earth. As air flows past the rotors of a turbine, the spinning rotor drives the shaft of an electric generator. A form of renewable energy.

Wind-Washing
Air movement due to increased pressure differences that occur at the outside corners and roof eaves of buildings. Wind-washing can have significant impact on thermal and moisture movement and hence thermal and moisture performance of exterior wall assemblies.

Wing Wall
A wing wall is a vertical exterior wall partition. When placed perpendicular to adjoining windows it will enhance ventilation through the windows. A side-opening casement window can create a mini-wing wall.

Xeriscaping
An environmentally friendly form of landscaping that uses a variety of locally-adapted and drought-tolerant plants, shrubs, and ground cover. Also known as smartscaping, drought-tolerant landscaping, and zeroscaping.

Zero Energy House
Any house that uses about half the energy of traditional construction be described as a zero energy house. It is more a definition of an energy-efficient house than an actual zero energy house which is now defined as a Net Zero Energy House.