RESFEN screenshot of house options.

A program to help consumers and builders pick the most energy-efficient and cost-effective window for a given application. RESFEN calculates the heating and cooling energy use and associated costs as well as the peak heating and cooling demand for specific window products. Users define a problem by specifying the house type (single story or two story), geographic location, orientation, electricity and gas cost, and building configuration details (such as wall type, floor type, and HVAC systems). Window options are defined by specifying the window's size, shading, and thermal properties: U-factor, Solar Heat Gain Coefficient, and air leakage rate. RESFEN calculates the energy and cost implications of the windows compared to insulated walls. The relative energy and cost impacts of two different windows can be compared against each other.

A major update to the program (RESFEN 5) is that it is now based on a database structure which allows for a much more flexible file system for storing data and libraries (see Input below). Screen Shots


fenestration, energy performance



Expertise Required

Minimal knowledge about windows.


Many thousand.


Window manufacturers, glass manufacturers, building contractors, architects, engineers, and homeowners who are interested in predicting the energy performance of residential window systems.


Geographic location; electricity and gas cost; infiltration; internal load levels; HVAC; wall and floor type; as well as window size, type, U-value, shading coefficient.

RESFEN5 is now made up of a set of "Libraries" which is where information is stored and input. Each library has a List view, where all the records (rows) in the library can be viewed, and a Detail view, where all the data (or fields) in each record can be viewed.

House Library: The House Library in RESFEN5 is similar to the main screen in RESFEN 3.1, but with the ability to see multiple input sets at the same time from the List View.

Locations Library: RESFEN5 has a Locations Library which can be used to create new locations that reference specific weather data and insulation packages. In the default database that is installed, called RESFEN5.mdb, there are 240 locations for the United States, as well as four for Canada. There are also other databases (for Canada, Chile and China) with more locations defined.

Window Library: RESFEN5 has a Window Library that allows data for specific windows to be imported from the WINDOW5 program. A default set of WINDOW5 data is installed with RESFEN5, and other data can be imported as needed or provided by manufacturers or other sources.

Packages Library: RESFEN5 has a Packages Library, which allows the user to create new insulation packages that can be associated with a Location to create customized modeling situations.

Results Library: RESFEN5 has a Results Library, which allows the user to view results from multiple runs, as well as export those results to comma separated text files that can be imported into a spreadsheet program.

Electric Rates Library: RESFEN5 has an Electric Rates Library, which allows the user to add new rates or change existing rates. The default library contains one rate for each US state, based on data from the Energy Information Administration (

Gas Rates Library: RESFEN5 has a Gas Rates Library, which allows the user to add new rates or change existing rates. The default library contains one rate for each US state, based on data from the Energy Information Administration.


Annual energy use and cost for heating and cooling; peak load increments due to the windows. Results given for two discrete sets of window orientations (north, east, south, west) or (northwest, southeast, southwest, northwest). Can also calculate incremental changes due to obstructions, overhangs, and interior shades.

Computer Platform

Microsoft Windows 2000, Windows XP, Windows, 98 or Windows NT, recommend Pentium 133 MHz or better, 16 MB memory (32 preferred) and 100 MB available hard disk space.

Programming Language



Easy to use, immediate, accurate.


Limited number of residential configurations, limited graphics output.



Lawrence Berkeley National Laboratory


Mail Stop 90-3111
1 Cyclotron Road
Berkeley, California 94720
United States


+1 (510) 486-4141


+1 (510) 486-4089




Available at no cost by downloading from the website.