Thermal Envelope

This section explains the use of the thermal envelope in the software’s analysis calculations.

Overview

A correct definition of the thermal envelope is required by the following xStar analyses:

Energy code compliance analysis (UA analysis)
Heating and cooling load analysis
Auto code minimum model

What is the thermal envelope?

The thermal envelope consists of the walls, roofs, ceilings, and/or floor elements that separate the conditioned and unconditioned spaces of the building. These elements often divide the exterior of the building from its interior. They are almost always required to have at most a maximum U-factor according to building energy codes.

How do I change the thermal envelope?

When first opening xStar for a Revit project, xStar raises the Edit Thermal Envelope dialog to let you verify the automatically detected thermal envelope. For buildings with complex geometry, the initial thermal envelope may be incorrect.

*Edit Thermal Envelope* dialog with automatically determined thermal envelope selected. In this case, you would likely want to remove the detached garage from the thermal envelope.

The dialog lets you:

Select Thermal Envelope: select the currently defined thermal envelope in the Revit viewport. If the Revit elements are not visible in the current view, you will not be able to see the selection.
Add Selection to Thermal Envelope: add the line items corresponding to the current Revit selection(s) to the thermal envelope.
Remove Selection from Thermal Envelope: remove the line items corresponding to the current Revit selection(s) to the thermal envelope.
Reset Thermal Envelope: reset the thermal envelope definition to its definition when the dialog was first opened.

The Edit Thermal Envelope dialog can be accessed at any time from the View menu.

Alternative method for a single line item

Each line item has a property called Part of Thermal Envelope. When viewing individual line items in the View Line Item dialog, you can check the Part of Thermal Envelope checkbox to add the line item to the thermal envelope. Unchecking the checkbox will remove the line item from the thermal envelope.

View Line Item dialog, accessible from the Project Browser when line items are displayed, shows properties of a single line item.

How is the thermal envelope used?

UA analysis: the thermal envelope line items are directly used in the UA calculation. See How xStar Calculates UA | xStar’s-UA-Calculation for more details.
Heating and cooling load analysis: the thermal envelope is used to determine the conductive heat losses from the interior of the building to the exterior.
Auto code minimum model: line items that are part of the thermal envelope are automatically assigned an exterior assembly, e.g. exterior wall, slab-on-grade, etc. Line items that are not part of the thermal envelope are automatically assigned an interior assembly.