Annual fuel utilization efficiency. Measures how efficiently a gas furnace or boiler operates. AFUE is the percentage of energy consumed by the system that is converted to useful heat. For example, a 90% AFUE means that for every Btu of gas used, the system will provide 0.9 Btu of heat. The higher the AFUE, the more efficient the system.
British Thermal Unit. The quantity of heat required to raise the temperature of one pound of water (at or near 39.2 degrees Fahrenheit) by one degree Fahrenheit.
A measurement of output versus energy consumed for boilers and hot water heaters. Efficient burners using gaseous or liquid fuels operate at excess air levels of 15% or less and leave negligible amounts of unburned fuel.
A roof made of materials (typically light in color) that have a high thermal emittance and solar reflectance. Cool roofs can substantially reduce a building's cooling load. The emittance of a material refers to its ability to release absorbed heat. The higher the emittance value, the greater the material's ability to release heat. With the exception of a metallic surface, most roofing materials can have emittance values above 0.85 (85%).
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. Indicates how efficiently a heating or cooling system (a heat pump in heating mode and a chiller for cooling) will operate at a single outdoor temperature condition. Higher COP values indicate a more efficient system.
Dry-bulb temperature refers to the actual air temperature as measured by a thermometer.
It is called dry bulb because it is measured with a standard thermometer whose bulb is not wet - if it were wet, the evaporation of moisture from its surface would affect the reading and give something closer to the wet-bulb temperature.
Energy efficiency ratio. Measure of the relative efficiency of a cooling appliance or equipment, expressed as the ratio of output, in BTUs per hour to the energy consumed in Watts.
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) |
An acronym for heating, ventilation, and air-conditioning systems.
Incident solar radiation. Insolation refers to the amount of solar radiation energy falling on a flat surface which can be converted into other forms of energy, such as heat or electricity. It is not affected in any way by the surface properties of materials or by any internal refractive effects as it is concerned only with the radiation actually striking the surface.
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 common unit of electrical consumption. It is the equivalent of one kilowatt of power expended for one hour. 1kWh = 3.412 Btu.
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. This value measures how efficiently a smaller residential air conditioner or heat pump operates over an entire cooling season, as opposed to a single outdoor temperature. As with EER, a higher SEER reflects a more efficient cooling system. SEER is the ratio of the total amount of cooling Btu's the system provides over the entire season divided by the total number of watt-hours it consumes.
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 |
The percentage of visible light that passes through a window or similar glazed opening.
Most VLT values are between 0.3 and 0.8. The higher the VLT, the more light is transmitted. A high VLT is typically desirable 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 temperature obtained by evaporating water into the air at constant pressure. This is commonly measured with a thermometer that has a wetted wick at its base (bulb), hence the name web bulb.
The wet-bulb temperature (WBT) relates relative humidity to the ambient air or dry bulb temperature. When moisture evaporates, it absorbs heat energy from its environment in order to change phase (via latent heat of vaporization), thus reducing the temperature slightly. The WBT will vary with relative humidity.
The difference between the wet-bulb and dry-bulb temperature gives a measure of atmospheric humidity. When the air is at saturation point, there will be no moisture evaporation at all. Thus the wet-bulb will equal the dry-bulb temperature.