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Calculating Glazing Assembly Performance

RockfordConst

When it comes to energy modeling, specifically for LEED, the performance attributes of the glazing and the entire assembly are important to consider during design.

Because ASHRAE 90.1 (which dictates the baseline building in LEED) references assembly maximum performance criteria, we’re also required to model the assembly performance criteria for proposed building. This performance information accounts for the effects of the framing system as well as the glazing, and is preferably determined from NFRC test data. If NFRC data is not available, the proposed building must be modeled with the default values from ASHRAE 90.1 Appendix A, Table 8.2 (which can have a significantly negative impact on the proposed building model).

So if NFRC data isn’t available, we can use LBNL’s Window Software to quickly determine the true performance of a glazing system. If you’re interested in the steps associated with that calculation, we’ve provided a couple of brief sample tutorials below.

Tutorial A: Defining the performance of a vision glass system using LBNL’s Window 5 Software

Assumptions:

  • Glass Type = Viracon VUE 1-50 in a thermally broken aluminum frame
  • Opening Size = 5’ x 4’

Before starting, verify the units are in IP units, see lower right corner.  If not in IP units, click Tools > Switch Units

1. Go to the Glazing System Library (press F5 or use icon), define a new glass by clicking ‘New’

2. Enter a unique name and select the number of layers in the glazing unit

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3. Find the glass code for the first layer of glass from the Glass Library

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4. Find the glass code for the second layer of glass from the Glass Library in a similar manner  ( ¼” clear glass uses code ‘103’)

5. Find the gas code for the air layer from the Gas Library

6. Press F9 to calculate the glazing U-value and SHGC and check values against manufacturer’s data

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7. Go to Window Library, click ‘New’ to define the glazing assembly

8. Enter a unique name and choose Custom Single Vision for ‘Type’

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9. Enter the window size in inches

10. Define the glass type by clicking in the center of the window (diagonal lines will appear in the glass area once this is selected).  Then choose glass type under “Glazing System”.

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11. Click one segment of the frame (black box will appear around selected area once it is selected).  Then choose frame type under “Frame”.

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12. Repeat Step 11 for all frame segments.

13. Press F9 to calculate the assembly U-value and SHGC. 

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Center of Glass Performance:

U-factor = 0.118

SHGC = 0.210

Actual Performance:

U-factor = 0.312

SHGC = 0.216

 Tutorial B: Defining the performance of a spandrel glass system using LBNL’s Window 5 Software

Assumptions:

  • Glass Type = Viracon VUE 1-50 in a thermally broken aluminum frame
  • Insulation = 2” rigid polyisocyanurate (R-10)
  • Opening Size = 2’ x 4’

Before starting, verify the units are in IP units, see lower right corner.  If not in IP units, click Tools > Switch Units

  1. Go to the Glazing System Library (press F5 or use icon), define a new glass by clicking ‘New’
  2. Enter a unique name and select the number of layers in the glazing unit.  For a spandrel unit, the number of layers will be determined by iteration.  For this unit, we expect the spandrel unit to have a U-value of 0.1 Btu/h*ft2*F (inverse of R-10).  Let’s first try using 3 layers of glass.  When Viracon VUE 1-50 is used as the first layer and ¼” clear glass is used for layers 2 and 3 with air, the unit U-value is 0.215.
  3. In order to reduce the U-value to 0.1 Btu/h*ft2*F, the gas type will need to be changed and/or the number of layers will need to be increased.  By trial and error, it is found that five layers of glass and using Xenon in each air space produces a U-value of 0.104 Btu/h*ft2*F.  This closely replicates the expected performance of the spandrel unit and can be used to determine the composite U-value.
  4. Go to Window Library, click ‘New’ to define the glazing assembly.
  5. Repeat Step 8-13 from the vision glazing tutorial to build the spandrel assembly and calculate the composite U-value.  Please note that since this section is opaque, the SHGC is not used for any design calculations.