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The Basics
This section describes some of the basic concepts in the 2000 IECC. See our Building Energy Codes Glossary for a more complete list of terms and acronyms that are used in the code and throughout the BECP website and compliance tools.
Building Envelope
The "building envelope" is the area that separates conditioned space from unconditioned space. In this picture, the "building envelope" is the area surrounded by the insulation. The code is only concerned with the "building envelope".
SHGC
Solar Heat Gain Coefficient (SHGC) is:
- the glazing's effectiveness in blocking heat caused by sunlight
- expressed as a number between 0 and 1
- part of a system for rating window performance
- used by the National Fenestration Rating Council
- gradually replacing shading coefficient (SC) in product literature and design standards
- convert SC to SHGC by multiplying the SC value by 0.87
The lower the glazing's SHGC, the less solar heat it transmits.
The REScheck software does not currently have an option for inputting SHGC and projection factors (horizontal overhangs), nor does it allow for trade offs of this requirement. The SHGC is a mandatory requirement that is listed in the REScheck Inspection Checklist for those locations in which it is a requirement.
SHGC FAQs
HDD
An example of a typical heating degree-day is HDD65 which means for any one day, when the temperature is < 65°F, there are as many degree-days as degrees F temperature difference between mean temperature and 65. Annual heating degree-days are the sum of the degree-days over a calendar year.
R-Value
R-value is a measurement of a material's resistance to heat flow. Insulation materials have tiny pockets of trapped air that resist the transfer of heat through the material. (The code assumes that insulation is installed properly and is not compressed in any way.) The ability of insulation to slow the transfer of heat is measured in R-values. The higher the R-value, the better the insulation material's ability to resist the flow of heat through it. The picture below represents the relative thickness of insulation material at different R-values.
U-Factor
U-factor measures how well a product prevents heat from escaping. U-factor ratings generally fall between 0.20 and 1.20. The insulating value is indicated by the R-value, which is the inverse of the U-factor. The lower the U-factor, the greater a product's resistance to heat flow and the better its insulating value.
HVAC Equipment Efficiency Terms
AFUE
Annual fuel utilization efficiency; combustion heating equipment efficiency is expressed in terms of AFUE. New equipment typically ranges from about 78- to 96-percent AFUE. Higher AFUE ratings indicate more efficient equipment.
HSPF
Heating seasonal performance factor; heat pump heating is expressed in terms of HSPF. New equipment ranges from about 6.8 to 10.0 HSPF. Higher HSPF ratings indicate more efficient equipment.
SEER
Seasonal energy efficiency ratio; cooling efficiency for electric air conditioners and heat pumps is expressed in terms of SEER. Higher SEER ratings indicate more efficient equipment.
Area Takeoffs
Calculating the areas of the building components (e.g., windows, doors, exterior walls) is easily the most time-consuming step in energy code compliance. Below are some helpful hints for calculating area takeoffs. Note that the concept of building envelope is important. Takeoffs can be simplified by using the AreaCalc tool.
When calculating area takeoffs:
- use dimension lines when available
- scale with an architects' scale only if necessary
- estimate only if necessary.
Walls
- Only measure the wall area in the building envelope
- Measure the wall height from the finished floor to the insulation in the ceiling
- Include the wall area between finished floors (e.g., between first and second floor)
- Measure to the outside of the exterior wall for width
Windows
- Use the rough opening on the building floor plans or window schedule
- Only count the glazing in the building envelope surrounding conditioned space
Doors
- Use the rough opening on the building floor plans or door schedule
- Check the door type for opaque vs. glass
- Only count the doors in the building envelope - between conditioned space and unconditioned space
Roof
- Measure the roof area of the conditioned space at the insulation level
- Check for vaulted ceilings or other irregular ceiling heights in the building
- The roof area must be the same as or equal to the building footprint area
Skylight
- Use the rough opening dimensions from the roof plans, floor plans, or the window schedule
Raised Floor
- Measure the area of the floor over unconditioned space or outdoor air
Slab Perimeter
- Measure in linear feet the slab edge of the conditioned space
AreaCalc
AreaCalc is a tool packaged with the REScheck code compliance software. It can be accessed by selecting Start->REScheck->AreaCalc Take-off Tool. A spreadsheet-like interface is used to calculate window, door, skylight, roof, wall, and floor areas. These areas can then be transferred directly into REScheck where the code compliance results for these assemblies can be displayed.
AreaCalc also allows you to construct a library of commonly-used windows, skylights, and doors. Including a window, for example, is as easy as selecting the window from a menu and indicating how many windows of that type are to be installed. The areas of all windows are summed, and the total is displayed.
