I am facing an issue creating custom construction sets for glazing based on specific project requirements. For example, if I want to create glazing with the following properties: a U-Factor of 6.88 W/m²K, SHGC of 0.25, and VT of 0.4, how would you go about it?
The problem is that OpenStudio uses the U-Factor as an input, which the Rhino plugin exports - the u-value is not used. It also appears that the Rhino plugin limits the U-Factor to 5.9 W/m²K. Trying to create a u-factor above 5.9 defaults to a skylights as well impractical u-value as shown in the image below.
Hi @carey, the auto-calculated U-Value is for users to do a quick QAQC in case of they are not familiar with the U-Factor. But it won’t apply to all cases. As long as you input the U-Factor 6.88 to the UI, it should be exported correctly as U-Factor to OpenStudio.
You can check the material’s U-Factor once you select it or check the material in the Material Manager:
If you want to model a single-pane window construction, then the best practice is to model it explicitly as such using a WindowMaterialGlazing and not try to figure out what the U-Factor should be with a WindowMaterialSimplyGlazSys.
When you have a single pane construction, effectively all of the thermal resistance comes from the air films on either side of the construction. This is why the U-Value (without air films) starts to approach infinity as the U-Factor (with air films) approaches 6 W/m2-K. To get a U-Factor higher than 6 for a typical window in a wall, you would essentially need to make your window out of a material with a negative R-value in order to negate the insulation that you get from the air films. So you’re breaking physics in that case.
Now, the resistance that you get from air films can vary a lot based on the orientation of the window. So you get better insulation/thermal resistance from the air films when the single pane glass is oriented vertically in a wall vs. horizontally in a skylight. This means that you can theoretically get a U-Factor as high as ~14 W/m2-K ONLY if the window construction is used as a horizontally-oriented skylight. This is why the interface changes to “Skylight only” when you plug in U-Factors above 6.
Granted, you don’t need to worry about any of these questions about air films if you just make your window construction out of a WindowMaterialGlazing. In that case, you just plug in the properties of what the glass is made out of and simulation engines like EnergyPlus will auto-calculate the air film resistance for you depending on the orientation of the Aperture geometry to which the construction is applied. So this is why I say that it’s best practice to use WindowMaterialGlazing.
