How xStar Calculates UA
This page describes how xStar calculates the building UA. We implement REScheck’s approach for calculating the UA of the building.
What is UA?
“UA” stands for U-factor (“U”) multiplied by area (“A”). It is a measure of the heat transferred through the envelope (or part of the envelope) of a building given a temperature difference over a certain time period.
What is U?
The U-factor “U” is a heat transfer coefficient. It is a measure of how well a surface inhibits a 1D flow of heat through it’s thickness. Surfaces with low U-factors are more insulating than surfaces with high U-factors (note that this trend is the opposite of R-value, where high R-values are more insulating than low R-values). The units of U-factor are Btu/hr-°F-ft2 (or W/K-m2).
U-factors typically roll up multiple heat transfer processes into a single number: e.g. conduction, convection, and radiation effects are usually intermingled.
Windows
For example, manufacturers of windows are required to report a U-factor with their windows. The measured U-factor is dependent on the glazing properties, frame material, geometry and construction quality of the window. It accounts for various thermal effects like conduction through the frame, radiation between panes, and convection at the boundary layer of the exterior panes, etc., which are not easily separated.
Layered surfaces
Surfaces like walls usually contain layers of materials whose thermal properties are defined with an R-value (units hr-°F-ft2/Btu or K-m2/W). R-value measures 1D conduction of heat through a material’s thickness. Sometimes, equivalent R-values are assigned to layers that exhibit conductive, convective, and radiative heat transfer effects like air gaps in a cavity wall construction. Such R-values can be calculated with computer models.
When calculating U-factors for a surface defined with an R-value, air film R-values are added to the surface assembly definition. The air film value represents convective effects of the boundary layer at the surface. Air film values are determined from the table below (from REScheck).
Surface Type | Inside Air Film R-value | Outside Air Film R-value |
|---|---|---|
Ceiling/Roof | 0.61 | 0.61 |
Door/Window/Wall | 0.25 | 0.68 |
Floor | 0.92 | 0.92 |
When an assembly does not have multiple parallel heat transfer paths (e.g. there is no framing present), the total U-factor is calculated as
Thermal Bridging
The calculation becomes more complex when there are parallel heat transfer paths (e.g. a more conductive path through framing elements and a less conductive path through cavity insulation, namely thermal bridging occurs). See Schultz, Bartlett, & Taylor, REScheck Technical Support Document, 2019 for more details. We implement their method exactly.
Layers Modeled as Separate Revit Elements
Sometimes Revit users model walls using multiple Revit wall elements, drawing e.g. the insulation and structural layers separately. Sometimes a cathedral ceiling roof assembly includes everything but the interior finish, and the ceiling is modeled separately from the roof. In these cases, only the exterior-most Revit element should be marked as part of the thermal envelope. xStar will then detect and merge any interior parallel layers modeled as separate elements in order to accurately calculate an effective assembly U-factor.
What is A?
The area “A” is the area of the heat transfer surface.
xStar’s UA Calculation
To calculate the total UA for a building, xStar sums the UA for each line item that is part of the thermal envelope. U is calculated as described above (and in the REScheck documentation) and A is calculated directly from the polygon geometry extracted from Revit.
The “Total UA” metric excludes contributions from floor surfaces, even if they are part of the thermal envelope. This is in accordance with REScheck’s behavior for slabs. However, the “Total UA” chart shows the UA for floors for comparative purposes (this number is actually an “FP” number (same units) – the F-factor multiplied by the perimeter – which is the appropriate method for slabs). See REScheck for more details.
Why aren’t floors included?
Because they aren’t included by REScheck either. The reasoning is (at least) twofold. This discussion applies to floors with ground contact. First, there are diminishing returns on reducing heat transfer to the ground when foundation/slab insulation is increased above the prescriptive requirements of the building energy code. Second, ground heat transfer is challenging to estimate accurately.
Learn More
Schultz, Bartlett, & Taylor, REScheck Technical Support Document, 2019.
Straube, High Performance Enclosures, Building Science Press, 2012.
Huang et al, Whole House Simulation of Foundation Heat Flows Using the DOE 2.1-C Program, 1988.
Any textbook on building science.