DDY File

What are DDY files and how to open them

Are you having problems opening a DDY file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your DDY files.

What is a DDY file?

A .DDY file is an ASHRAE Design Conditions Design Day Data file.

The data format named ASHRAE Design Conditions Design Day Data stores "design day" climate definitions used in building energy modelling and HVAC equipment sizing. It describes extreme heating and cooling outdoor conditions - such as high dry-bulb, wet-bulb, dew point temperatures, wind speed and elevation - derived from long-term climate records per criteria set out in the ASHRAE Handbook Fundamentals. These design day definitions represent rare conditions (e.g. 0.4 %, 1 %, 2 %, 5 % exceedances) that engineers use to size systems so that they will operate adequately under worst case weather.

Software tools like EnergyPlus generate this file format using station weather data and ASHRAE-defined percentile statistics. Once created, it is read by HVAC or energy simulation software during "sizing periods" to define outdoor conditions for equipment load calculations; it is not meant to record continuous historical weather data.

Internally, the file begins with an ASCII line such as "!ThefollowingLocationandDesignDaydataareproducedaspossiblefromtheindicateddatasource". It includes fields labeled "- LONGITUDE {W- E+}", "- LATITUDE {N+ S-}", "LOCATION NAME", "ELEVATION {M}", and sections headed "DESIGN_CONDITIONS", with sublabels like "COOLING" and "HEATING" reflecting the type of extreme condition.

How to open DDY files

Important: Different programs may use files with the DDY file extension for different purposes, so unless you are sure which format your DDY file is, you may need to try a few different programs.

While we have not verified the app ourselves yet, our users have suggested a single DDY opener which you will find listed below.

Last updated: October 22, 2026

Various apps that use files with this extension

These apps are known to open certain types of DDY files. Remember, different programs may use DDY files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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