Building Envelope Calculations

The latest version of AIM's calculation engine includes terms representing building envelope conduction and solar heating of lab spaces.

Background on Envelope Loads for Lab Building Calculations

AIM's calculation engine is a bin sim calculator, which does not contain complex envelope geometry or related building physics features. In general, lab building savings calculations (especially for existing buildings) are not strongly dependent on the use of accurate envelope loads because lab building energy consumption is heavily governed by airflow and equipment loads. However, to improve calculation accuracy, the latest version of AIM incorporates building envelope terms. Note that AIM does not include any energy efficiency measures relating to envelope changes and it should not be used for calculation of envelope measure savings.

The Envelope Terms

The envelope loads within AIM are expressed as two separate terms:

• Conduction loads: this is the energy conducted through walls and windows due to the temperature difference between the air inside and outside of the building. This term may reflect a cooling or heating load, depending largely on the outside air temperature during a given hour of simulation. Within the AIM calculations, the load is calculated separately for each temperature bin used.
• Solar load: this term represents the heating occurring due to solar radiation transmitted through the windows (or absorbed and then conducted by the envelope). Within the AIM calculations, the median daytime solar load is added to all time-temperature bins representing occupied times; this is an approximate approach that is designed to represent typical (not peak) solar loading.

Envelope Properties

Building envelope properties in AIM are defined relative to the climate zone of the building. By default, buildings are assigned an envelope performance which corresponds approximately to the performance required by the version of ASHRAE 90.1 applicable to its climate zone in the year it was built. Within AIM, the user can specify the actual level of envelope performance, spanning from "Low" to "Very High"; this adjusts the size of the envelope loads in the calculations accordingly. The envelope performance selector is located in the Building Systems tab within Building Parameters.

Development of the Methodology

The magnitudes of the envelope terms and their dependence on building (code) age and location were developed using a suite of detailed energy models created and run by Labs2Zero Development Partner SmithGroup. I2SL is grateful to SmithGroup for their expertise in developing the calculation approach best suited for AIM's calculations.