# Base Envelope Performance Factor Calculation

The simulation model for calculating the base envelope performance factor shall modify the simulation model for calculating the proposed envelope performance factor as follows:

## Opaque Assemblies

All opaque assemblies shall be modeled with the maximum U-factor required [here](/user-guide/workflows/energy-efficiency/ashrae-90.1-2019/appendix-c-methodology-for-building-envelope-trade-off-option/prescriptive-building-envelope-compliance-path.md) for the appropriate class of construction, space conditioning category, and climate zone.

Mass walls and mass floors shall be modeled with heat capacity (HC) equal to 147 kJ/m²·K. All other opaque assemblies shall be modeled with the same HC as the proposed design.

Mass walls shall be modeled with equal mass on each side of the insulation. All other opaque assemblies shall be modeled with insulation on the exterior.

## Roof Surfaces

Exterior roof surfaces shall be modeled with the solar reflectance and thermal emittance required, where minimum three-year-aged solar *reflectance* of 0.55 and a minimum three-year-aged thermal *emittance* of 0.75 is adopted.

All other roofs, including those exempted from the requirements, shall be modeled the same as in the proposed design.

## Fenestration

Fenestration shall be assumed to be flush with the wall or roof.

### **Area Adjustments:**

* If fenestration area for new buildings or additions exceeds the maximum allowed by Section 5.5.4.2, the area shall be reduced proportionally along each exposure until the limit set in Section 5.5.4.2 is met
* If fenestration area facing west or east of the proposed design exceeds the area limit set in Section 5.5.4.5, the baseline building performance shall be generated by simulating the building with its actual orientation and again after rotating the entire building 90°, 180°, and 270°, then averaging the results of the four simulations

### **Thermal and Optical Properties:**

* Fenestration U-factor and SHGC shall be the maximum allowed for the appropriate class of construction, space conditioning category, and climate zone in accordance with Section 5.5.4
* Where there is no SHGC requirement, SHGC shall be 0.40 for all vertical fenestration and 0.55 for skylights
* Visible transmittance (VT) for fenestration in the base envelope design shall equal 1.10 × SHGC

### Shading Devices

Manually operated interior shades shall be modeled on all vertical fenestration as described in Section C3.5.5.1. Permanent shading devices, such as fins and overhangs, shall not be modeled.

### Daylighting

Daylight areas and photosensor locations shall be modeled as described in Section C3.5.3 after reducing the fenestration area as described in Section C3.6(c).

{% hint style="success" %}
These requirements establish the baseline envelope against which the proposed design is compared. The baseline uses prescriptive maximum U-factors, SHGCs, and other properties from code requirements, creating a standardized reference design. Modeling standardized thermal mass properties, insulation positions, and fenestration characteristics without permanent shading ensures fair comparison by isolating the performance benefits of the proposed envelope design decisions.
{% endhint %}


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