Fritted glass is a common and effective design strategy to reduce glare and maintain daylighting for commercial buildings. Read on to learn how to best simulate typical frit patterns for daylighting analysis.
Image source: The American Ceramic Society
What is frit?
Frit is a set of very small ceramic inserts between the glass panes of an insulated glazing unit (IGU) typically in the shape of round pellets that resemble dots. The density and size of the frit determine the percentage of light that is blocked and therefore impacts the visible transmittance of the glass. Some patterns are equally spaced grid patterns while others can be custom designs for aesthetics and performance. No matter the design the important performative aspects are based on the effective percentage of glass that is covered. The percentage of visual transmittance is used to simulate the impact of a frit pattern on glazing for daylight analysis.
Simulating fritted glazing
Users have two options to run a simulation of the desired frit pattern.
- Simulate the effective Visible Transmittance of the desired frit cover percentage.
- Simplify the frit geometry as shading devices.
Option 1 is the mathematical approach for a quick and easy way to test different design options of frit percentages across the whole project or an area-weighted average of the effective VT for the whole project. Option 2 is geometry-based providing specific options for specific window locations but can quickly become computationally taxing causing long analysis times. We'll cover both and demonstrate the fast way to model frit patterns for digital simulations.
Option 1 - Area Weighted Average Method
CASE 1: All Windows are Fitted Glazing
Every glass product has a visible transmittance (VT) or glass transmittance value ranging between 0 and 1 as the percentage of light that is let through the glass. A standard commercial IGU typically provides a VT of around 65% or 0.65 and is the default for our example here.
Users can simulate a 60% frit to all windows by calculating the 60% reduction from the windows VT, which in our example is 0.65. Some quick math tells us that 60% of 0.65 is 0.39, and subtracting 0.39 from 0.65 is the effective VT of 0.26 of 26%. Using the value of 0.26 for glass transmittance on the daylight page and recalculating will provide the effective impact of all windows that have a 60% frit.
CASE 2: Specific Windows or Areas have Fritted Glazing
Specifying a particular facade or a set of windows can get a bit trickier. Users can utilize the VT method to calculate an area-weighted average of the glazing that is going to be fitted against the glazing that is not, which provides an effective VT for all of the windows. All that is needed is the percentage of glazing to be fitted and the percentage that is not. For our example of glass with a 0.65 VT, of the total glazing area, 50% will be with a 60% frit and the remaining 50% will be without frit.
Next, calculate the effective VT of the fitted glass, which as we calculated earlier is 0.65 - (0.65 x 0.6) = 0.26. Now there are two VT values, 0.26 for half of the windows with frit and 0.65 for the other half of the windows without. Calculating the area-weighted average we multiply the VT value by the area percentage for each set of windows and add them together.
(0.5 x 0.26) + (0.5 x 0.65) = 0.49 VT%
The area-weighted average provides the effective VT input for the daylight analysis and represents the overall impact of that percentage of glazing covered in the specified percentage of the frit pattern.
Option 2 - Simplify Frit Geometry
Frit patterns are typically a very dense pattern of tiny objects that when modeled create a lot of geometry resulting in large models and long analysis times. As demonstrated below a grid of objects that generates a frit pattern can quickly increase in the number of objects or surfaces created but still cover the same amount of surface area. A single surface to cover 60% of a window has the same overall effect as 14,200 tiny surfaces. Therefore, a simulation with fewer objects but the same percentage of coverage will provide the effective impact of the desired frit pattern.
To test different frit densities on specific windows we've provided a set of different grid options with the same 60% coverage. Testing the results of four different frit pattern densities reveals the same overall impact on daylight and glare.
All four options are the same whole building sDA of 67% but the analysis heatmap appears slightly different between each design option. Each design option progressively increases in geometry count and analysis but does not change the overall effect. No matter what the surface size of the object representing the frit shading the effective result remains the same. The last two options on the right show more of what we expect for the analysis heatmap. They visually indicate a similar impact of the actual frit pattern with a grid of 1' and 6" square surfaces. We can see the same for glare and the ASE analysis.
Simulations are the simplification of reality. These studies present the result of that fact. Users can use both methods to simulate the effective impact of the fitted glazing. For gradient patterns use the geometry-base approach with a gradient pattern of larger to the smaller surfaces is possible but should maintain the total area percentage that would be covered.
Does it matter if I use squares or circles for my Frit geometry import?
> Yes, it is crucial you stick to a rectangle pattern. The file size of each object is dependent on the number of mesh points. The difference of mesh points between a circle and a square can be 90X larger. To ensure a speedy export and simulation run, stick to squares.
Can I do this with the Facade tool?
> Yes, but only with Option 1. Using the adjusted calculations for frit percentage as an area-weight average to calculate the effective visual transmittance can be used for the standardized room model as glass transmittance. Since the facade guidance feature is a rapid prototyping tool with fixed geometry there is no new geometry to upload to it. The geometry method is used with the Daylight page.