Define fenestration (glazed constructions)

Fenestration refers to glass constructions (windows, skylights, glass doors).

The input process to define fenestration (glazed constructions) consists of three steps:

1. Adding fenestration to the geometry model
2. Defining the thermal properties of the fenestration. Important performance characteristics include overall thermal transmittance (U-factor), solar heat gain coefficient (SHGC), and visible light transmittance (VT or VLT).
3. Assigning the glazed constructions to the geometrical surfaces of the building model.

Adding fenestration to the geometry model

Fenestration is generally added as a subsurface on exterior walls and roofs. For 3-D models, these subsurfaces are drawn onto the wall/roof. For "2-D" models (where users enter areas and orientations of surfaces without actually drawing them), the glazing surfaces are defined by specifying the surface area, and indicating which wall or roof the glazing is on.

Defining thermal properties of fenestration

Important performance characteristics of glazing include overall thermal transmittance (U-factor), solar heat gain coefficient (SHGC), and visible light transmittance (VT or VLT).

Window frame type has a big impact on performance, so it is important to pay attention to how the thermal impact of window frames is represented in the simulation tool. Aluminum is a common frame material for commercial windows, and it is an excellent conductor of heat, which means it is not very good from an efficiency perspective. Therefore, aluminum frames have a big impact on the overall window U-factor. Thermally broken aluminum frames mitigate some of this thermal bridging. Some tools model the frames explicitly and others expect that the U-factor input includes the framing impact.

Some simulation tools provide more than one method for entering window performance, and it is useful to understand the implications of choosing one over the other. Often there is a simple option to enter the overall performance characteristics: U-factor, SHGC and VLT. There may also be an option to enter more detailed performance information, either by importing a data file with information such as angular and spectral transmission or by defining layers of the window assembly as specific glass types and air gaps. The more detailed methods will likely provide a more accurate heat transfer estimate, but the simple method is a reasonable choice for simple box modeling. The simple input is typically quicker, and the accuracy is likely adequate for providing an understanding of the impact of varying window performance. The more detailed window input method may be appropriate for later modeling stages when comparing performance of specific glazing options.

Assigning fenestration constructions to the model surfaces

After the thermal properties of the fenestration constructions have been defined, you must assign them to the model. Many software tools can automate this process by automatically assigning a fenestration construction to all of the model's windows, or a selected subset. If different constructions are used on different orientations of the building, then a user may select all the zones on that orientation and assign a chosen set of constructions, then repeat this process for other orientations.

If there are unique constructions used only in certain places, then the assignment process may involve clicking an individual glazed surface on the model and manually assigning the construction. Some software tools allow you to select a surface (or surfaces) from a list - in this case, it is strongly recommended that, while creating the glazing geometry, you give any special surfaces a unique, easily-identifiable name such that you'll be able to find it on a long drop-down list.

Simple box model approach

Fenestration design is a very common and valuable subject for simple box model evaluation. The analysis can inform the design team about the impact of window area, window performance and shading alternatives. The analysis can also show how the impact varies on different orientations.

Geometry simplification

See also: Simple box model window simplification

Window area assumptions in very early stages

Window area is often a variable to explore using models representing a range of window-wall ratios. However, if this is not a subject of study, and if there is no information from the design team about the likely window area, then a reasonable choice may be taken from the tables below.

Baseline Window-Wall Ratio from ASHRAE Standard 90.1-2019, Appendix G

Building Area Type	Window Wall Ratio
Grocery store	7%
Healthcare (outpatient)	21%
Hospital 27	27%
Hotel/motel (≤75 rooms)	24%
Hotel/motel (>75 rooms)	34%
Office (≤5000 ft2)	19%
Office (5000 to 50,000 ft2)	31%
Office (>50,000 ft2)	40%
Restaurant (quick service)	34%
Restaurant (full service)	24%
Retail (stand alone)	11%
Retail (strip mall)	20%
School (primary)	22%
School (secondary and university)	22%
Warehouse (nonrefrigerated)	6%

Commercial Reference Buildings - Assumptions for Building Form (Source: U.S. Department of Energy Commercial Reference Building Models of the National Building Stock, Table 13. 2011)

Building Type	Floor Area (ft2)	Aspect Ratio	No. of Floors	Floor-to -Floor Height (ft)	Floor-to- Ceiling Height (ft)	Glazing Fraction
Small Office	5,500	1.5	1	10	10	0.21
Medium Office	53,628	1.5	3	13	9	0.33
Large Office	498,588	1.5	12	13	9	0.38
Primary School	73,960	E-Shape	1	13	13	0.35
Secondary School	210,887	E-Shape	2	13	13	0.33
Stand-Alone Retail	24,962	1.3	1	20	20	0.07
Strip Mall	22,500	4	1	17	17	0.11
Supermarket	45,000	1.5	1	20	20	0.11
Quick Service Restaurant	2,500	1	1	10	10	0.14
Full Service Restaurant	5,500	1	1	10	10	0.17
Small Hotel	43,200	3	4	11* 9	11* 9	0.11
Large Hotel	122,120	3.8* 5.1	6	13* 10	13* 10	0.27
Hospital	241,351	1.3	5	14	14	0.15
Outpatient Healthcare	40,946	1.4	3	10	10	0.19
Warehouse	52,045	2.2	1	28	28	0.006
Midrise Apartment	33,740	2.7	4	10	10	0.15

Window performance, represented by U-factor, solar heat gain coefficient (SHGC) and visible light transmittance (VLT), is also a common topic for study. A common starting point for window performance is energy code requirements, such as those listed below from ASHRAE Standard 90.1-2019, or for the energy code that applies to the building project. For information on the potential range of glazing performance, see the ASHRAE Handbook of Fundamentals, Chapter 15. Manufacturers also publish data for the performance of insulated glass units with different glass types and coatings.

Window Performance Requirements from ASHRAE Standard 90.1-2019

Climate	Fixed window
Zone	U-factor	SHGC
CZ0	0.50	0.22
CZ1	0.50	0.23
CZ2	0.45	0.25
CZ3	0.42	0.25
CZ4	0.36	0.36
CZ5	0.36	0.38
CZ6	0.34	0.38
CZ7	0.29	0.40
CZ8	0.26	0.40

Detailed design input data

Detailed design model fenestration properties should represent the project's actual window selections as specified in later phases of a project such as the design development and construction documents phases. Compliance models should use inputs that represent the final design.

Fenestration thermal properties should be based on architectural specifications, but the actual thermal inputs are generally not shown on the drawings or specs and will need to be gathered separately in order to define the model inputs appropriately. The architect can provide the name of the manufacturer and glass selection and this can be used to research the thermal properties.

Example of NFRC data for a window system. (Source: IBPSA-USA BEM Workshop)

NFRC data

The National Fenestration Rating Council (NFRC) is an independent non-profit organization that establishes objective window, door, and skylight energy performance ratings. NFRC-certified products are independently tested, certified, and labeled. The NFRC label includes detailed performance data for a selected glazing assembly, including:

• U-factor
• Solar Heat Gain Coefficient (SHGC)
• Visible Transmittance (VT)
• Air leakage
• Condensation resistance (optional for manufacturers to include)

The label data is derived from testing and is the combined performance of the glazing and the framing system.

Read more about fenestration performance.

Example of specifying window layers and framing design to calculate overall fenestration assembly performance. (Source: IBPSA-USA BEM Workshop)

Calculated properties

Sometimes, test data is not available such as in the case of custom window assemblies or site-built assemblies. In the absence of test data, software tools can be used to determine glazing performance.

These software tools allow users to enter details about a proposed window construction, including glass layers, gap thickness and frame type, and then it will calculate the overall thermal and optical performance properties.

“WINDOW” from Lawrence Berkeley National Laboratory is one example of a standalone software program that specializes in this calculation. Some BEM software tools offer similar capability built-in to the tool.

If you are working on a code compliance model, then you should check the calculation rules or check with the authority having jurisdiction to see what methods are acceptable. Compliance models will often force you to use default values for site-built assemblies if no NFRC label is available.