Annual fuel utilization efficiency. A thermal efficiency measure of combustion equipment like furnaces, boilers, and water heaters.
British Thermal Unit, a traditional unit of energy equal to about 1.06 kilojoules. It is approximately the amount of energy needed to heat one pound of water one degree Fahrenheit.
A roof that is designed to keep solar heat out of buildings and to increase energy savings by using reflective roofing materials or cool roof systems.
Cool roofs (generally white) stay as much as 70° Fahrenheit cooler at peak times than traditional asphalt roofs, offering important benefits to building owners. Cool roofs also protect the environment from the negative effects of urban heat islands. The magnitude of energy savings depends on the building type, level of roof insulation, and ventilation rate.
To achieve the cool roof effect, specify a reflective, high emissivity single-ply elastomeric membrane, or coat the roof with a certified cool roof coating. Typical rolled roofs, built-up roofs, and composition roofs absorb 70-90% of incident solar radiation. Cool roofs can absorb as little as 20% of incident radiation (3-year aged performance).
Asphalt shingles are inexpensive, prevalent, and familiar to roofers. However, they are ineffective for reflecting incoming solar radiation. The solar reflectance of all commercial asphalt shingles is low. (Premium white shingles are only about 30% reflective, and other colors reflect less.)
In addition to reducing heat gain in a building, cool roofs may also benefit the immediate surroundings by reducing the heat island effect. This effect is the result of many dark surfaces in an urban environment that increase ambient air temperatures by as much as 2–8° F.
Coefficient of performance of a heat pump. The ratio of the change in heat at the output to the supplied work.
The temperature of air measured by a thermometer exposed to the air but shielded from radiation and moisture. It does not indicate the amount of moisture in the air.
Dry-bulb temperature is an important consideration when designing a building for a certain climate.
Compare with wet-bulb temperature.
Energy efficiency ratio. The ratio of output cooling in BTUs per hour to the input power in watts. The EER measures the efficiency of air conditioners.
Combustion efficiency.
The amount of heat necessary to raise the temperature of all components of a unit area in an assembly by 1° F (Imperial) or by 1° K (metric).
Heat capacity is calculated as the sum of the average thickness times the density times the specific heat for each component. A higher value of heat capacity indicates a greater thermal mass.
| Units | Heat Capacity |
|---|---|
| IP | Btu/(ft² •°F) |
| SI | J/(m² • °K) |
Heating, ventilation, and air conditioning.
Incident solar radiation. A measure of solar radiation energy received on a given surface area in a given time.
Imperial units of measure.
The derived unit of energy in the International System of Units (SI).
One joule is the amount of work done by a force of one newton moving an object through a distance of one meter, or the work required to continuously produce one watt of power for one second.
1000 British Thermal Units (BTUs).
Kilowatt-hour, a unit of energy equal to 1000 watt hours or 3.6 megajoules.
A megajoule, equal to one million joules, or approximately the kinetic energy of a one-ton vehicle moving at 100 miles per hour.
An SI measure of force per unit area, defined as one newton per square meter.
A measure of the insulating quality of a material. A higher R-value indicates a greater ability to insulate a space, preventing heat transfer through the material.
| Units | R-Value |
|---|---|
| IP | ft²-hr ºF/Btu |
| SI | W/(m² • °K) |
More insulation (a higher R-value) for walls and roofs does not necessarily result in higher performance. Higher levels of insulation can trap heat in a building. The optimal choice of construction depends on the building type, climate, and occupancy schedule. Perform an energy analysis to determine the best constructions for the project.
For example, a residential building in a cold climate benefits from higher R-value constructions. However, higher R-value constructions for a retail store in a warm climate results in increased energy use because cooler night-time air is prevented from cooling the building. Most non-residential projects have significant internal loads from people, lights, and equipment. In these cases, higher insulation may lead to increased cooling energy requirements.
Seasonal energy efficiency ratio. The SEER measures the efficiency of air conditioners. It is calculated as the cooling output in BTUs during a typical cooling season divided by the total electric energy input in watt-hours during the same period. A higher SEER rating indicates a more energy efficient unit.
The International System of Units, a modern form of the metric system.
A measure of a window's ability to block radiant heat transfer, typically from sunlight.
The SHGC is the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number between 0 and 1. A low SHGC indicates that a window transmits low amounts of solar heat.
The appropriate SHGC depends on the climate, the building type, and the amount of glass.
In hot climates, a low SHGC (0.20 - 0.35) is important for windows and skylights. In a cold climate or a situation where you want to take advantage of the sun for passive heating, a high SHGC of 0.5-0.7 is preferable.
A unit of heat energy equal to 100,000 British Thermal Units (BTUs). It is approximately the energy equivalent of burning 100 cubic feet of natural gas, or about 29.3 kilowatt-hours of electrical energy.
See visible light transmittance.
See dry-bulb temperature.
See wet-bulb temperature.
An indicator of how well a window resists conduction.
The rate of heat conductivity is indicated by the U-value of a window assembly. The lower the U-value, the greater a window’s resistance to conductive heat flow and the better its insulating value.
The desirable U-value depends on the climate, the building type, and the amount of glass.
For example, in a warm climate like Los Angeles, the U-value may be insignificant. In cold climates, however, a low U-value such as 0.25 - 0.40 (IP units) is beneficial. Skylights in cold climates should have a low U-value to address condensation issues.
The weight of the material per unit of area.
| Units | Unit Density |
|---|---|
| IP | lbm/ft² |
| SI | kg/m² |
A measure of the transmission of visible light. Most values are between 0.3 and 0.8.
Glass with a higher value transmits more light than glass with a lower value. A high value can help to maximize daylight. However, too much light transmission can cause glare.
The appropriate Tvis value depends on the climate, the building type, and the amount of glass.
For example, glare can be a problem if the value is too high and the glass area is large. If the value is too low and the glass area is too small, you don't get the benefit of natural daylight.
Selective coatings allow high Tvis and low SHGC. You do not need mirrored glass to achieve low heat gain. New glass can have 65% visible light transmittance, 30% solar heat gain, and a 0.30 U-value.
The lowest temperature that can be reached by the evaporation of water only. It is an indication of the amount of moisture in the air.
For air that has less than 100% relative humidity, the wet-bulb temperature is lower than the dry-bulb temperature, and the dew point temperature is lower than the wet-bulb temperature.