Curtain wall thermal calculation procedures

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Curtain wall system thermal property calculation procedures

For a better understanding of curtain wall energy efficiency, it is essential to determine the curtain wall system thermal properties (U-value, SHGC & shading coefficient) with the computer calculation method, in addition to the center-of-glazing glass optical & thermal properties.

This article aims to explain the step-by-step procedures in curtain wall thermal property calculations. An overview of the calculation principles is first provided and followed by the key steps:

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Curtain wall system thermal property calculation principles

The methods described in the NFRC simulation manual (based on NFRC 100 & NFRC 200) are used in the curtain wall system thermal property calculations.

Curtain wall systems are typically with repetitive patterns of lites. Two lites are selected in the thermal property calculation, as illustrated below.

The two-lite curtain wall section is calculated as a horizontal two-lite window, with the following frames modeled:

  • Sill (½ height)
  • Head (½ height)
  • Jamb (½ width, left and right)
  • Mullion (full width)

The sill, head, and jambs are modeled with ½ height or ½ width, as the horizontal mullions and vertical transoms are shared with adjacent lites.

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U-value calculation

For U-value calculations, the two-lite curtain wall section is divided into 3 parts:

  • Frame
  • Edge-of-glazing
  • Center-of-glazing

The frame part is the opaque part (below the sight line) of the curtain wall, including all non-glass materials (e.g. frame and gasket) and the glass portion below the sight line (e.g. spacer and a short segment of glass).

The edge-of-glazing part is the transition area from the frame to the glass center. It is measured 63.5 mm from the sight line.

The center-of-glazing part is the glass vision area excluding the edge-of-glazing part.

Please expand the block below for some notes:

Notes on frame cross section orientation and slight line
  • In this article, it is assumed that a window sill frame cross section is used in the context by default, with the frame at the bottom (glass at the top) and the outdoor side on the left (the indoor side on the right). With this convention, a frame is below the sight line (though it is not true for a jamb or head frame).
  • Sight line refers to a virtual horizontal line passing through the topmost point of the frame part. Refer to the example below for an illustration of sight line.

Refer to the sketch below with the 3 parts illustrated.

The center-of-glazing U-value is calculated in the WINDOW software, with the same procedures in our standard glass optical & thermal property testing services (single glazing, double glazing).

The frame and edge-of-glazing U-values are calculated in the THERM software. More details will be described below.

The overall curtain wall system U-value is calculated as the area-weighted average U-value of the frame, edge-of-glazing, and center-of-glazing parts in WINDOW.

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SHGC calculation

For SHGC calculations, a simplified approach is used. It is assumed that the SHGC of the edge-of-glazing part is the same as that of the center-of-glazing. Instead of 3 parts in U-value calculations, there are only 2 parts in SHGC calculations:

  • Frame
  • Glazing

Similarly, the frame part and the glazing part are divided by the slight line. The glazing SHGC is calculated in the WINDOW software.

Refer to the sketch below with the 2 parts illustrated.

The frame SHGC is not directly calculated in THERM. Instead, the U-value of the exterior frame surface is calculated in THERM. This information is then used to calculate the frame SHGC in WINDOW, with a used-defined solar absorptance.

Finally, the overall curtain wall system SHGC is calculated as the area-weighted average SHGC of the frame and glazing parts in WINDOW.

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Shading coefficient

The shading coefficient result is not directly calculated in both THERM and WINDOW. It should be manually converted from the relevant SHGC result with the simple relationship: shading coefficient = SHGC/0.87.

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Step 1. Glazing system thermal property calculation in WINDOW

The material properties and boundary conditions of the glazing system are required in frame modeling in Step 3. Therefore, it is necessary to perform the center-of-glazing thermal property calculation first.

If the glass data is available in the IGDB, the data can be directly used (referenced by the NFRC ID). Otherwise, the glass needs to be tested and added to a user database.

The calculation procedures of glazing system thermal properties are the same as in our standard glass optical & thermal property testing services (single glazing, double glazing). Shown below is a screenshot of the WINDOW software.

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Step 2. Frame CAD drawing preparation in CAD software

To calculate the frame and edge-of-glazing properties, the 2D model of the respective frame cross section needs to be created in the THERM software.

Though it is possible to directly draw cross sections in THERM, the usual practice is to use a CAD model as the underlay and then trace the cross section following the underlay.

THERM accepts R12 ASCII DXF file format only. The CAD files need to be edited and cleaned prior to the export. Below are some typical operations required:

  • Scale to 1:1 with mm unit
  • Orient the frame with the outdoor side on the left
  • Remove unnecessary components and hatches
  • Explode all parts

Shown below is a CAD underlay example in THERM (a vertical mullion of a curtain wall system).

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Step 3. Frame modeling in THERM

With the CAD underlay, it is convenient to trace the cross section geometry in THERM. All solid and air parts need to be modeled.

For the glass part, it should be calculated in WINDOW (refer to Step 1) and imported to THERM. The glass length in the model is 150 mm from the sight line:

  • The portion below the sight line is part of the frame (including the spacer and sealant of a DGU glass)
  • The portion of 63.5 mm from the sight line is the edge-of-glazing part.

Material properties

The material properties of the glass part (such as glass emissivity and effective thermal conductivity) are imported from WINDOW.

For typical solid materials, the material properties (thermal conductivity and emissivity) are available in the material library of THERM. For non-standard materials, the material properties need to be tested and defined by the user. For most curtain wall systems, the standard material library is sufficient, and additional material property testing is not required.

Air cavities are a special type of material in THERM and the properties are automatically determined by THERM with some built-in models.

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Boundary conditions

Besides the material properties, a set of boundary conditions should be defined at all boundary lines of the model. The boundary conditions specify how the parts modeled interact with the parts not modeled (e.g. indoor air and outdoor air).

For the indoor air and outdoor air boundaries, there is convective and radiative heat transfer only. The connective and radiative boundary conditions should be defined.

For the solid boundaries (typically the bottom and top boundaries), by default the adiabatic boundary condition is applied (i.e. there is no heat flow across the bottom and top boundaries).

The standard boundary conditions are available in the boundary condition library in THERM, which is based on the standard winter environmental conditions defined in NFRC 100. It is possible to define user boundary conditions for calculations based on different environmental conditions (e.g. the summer conditions in Singapore).

Additionally, it is also necessary to tag some boundaries, for THERM to identify the parts in a model. For example, the frame U-value calculation is based on the boundaries tagged “frame”. There are 3 system tags required for each frame cross section:

  • Frame: used for frame U-value calculation
  • Edge: used for edge-of-glazing U-value calculation
  • SHGC Exterior: used for frame exterior U-value calculation (which will be used in frame SHGC calculation)

Shown below is a frame cross section, created with the CAD underlay shown above, with material properties and boundary conditions defined.

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Step 4. Frame thermal property calculation in THERM

With the model created in THEM, the calculation can be performed with one click. Shown below is the calculation result window.

THERM uses a finite element solver and it typically takes just a few CPU seconds to complete a calculation. It is also possible to overlay the results on the model.

Shown below is an example with the temperature results overlayed. The calculation is with the summer environmental conditions (32 °C outdoors and 24 °C indoors). There is obvious thermal bridging at the aluminum frame part.

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Step 5. Curtain wall system thermal property calculation in WINDOW

With the center-of-glazing U-value calculated in Step 1 and the edge-of-glazing, frame, and frame exterior U-values calculated in Step 4, the curtain wall system thermal properties can be calculated in the WINDOW software.

The frame models should be imported to WINDOW. The frame SHGC is dependent on the solar absorptance of the frame surface. The default value is 0.5 for curtain wall frames (according to NFRC 200). It is possible to define a user value if it is available.

Additionally, the curtain wall system size and the associated THERM file to each frame need to be specified. WINDOW can then perform area-weighted averaging and report the overall curtain wall system U-value and SHGC. The shading coefficient is not calculated by WINDOW and needs to be manually calculated. Shown below is a screenshot of the WINDOW software with curtain wall system U-value & SHGC calculation results displayed.

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More information:

Can a calculation be performed with the summer environmental conditions?

The default environmental conditions used in the calculations are NFRC winter. It is possible to use the NFRC summer conditions or other user defined conditions in a calculation. They need to be defined as different boundary conditions in Step 3.

Please refer to this post for the difference between NFRC winter and summer U-values.

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Can spandrels be included in the modeling?

The modeling of spandrels is not supported in the current NFRC procedures.

A reference procedure for simulating spandrel U-factors was developed by the Fenestration Association of BC (FENBC).

With this procedure, it is possible to include the influence of spandrels in the modeling.

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