Modeling shadow boxes in EnergyPlus

asked 2016-01-19 08:48:15 -0600

Julien Marrec

29716 ●94 ●76 http://www.effibem.com/

In EnergyPlus, how do you typically model shadow boxes when you have curtain wall systems?

Example of a shadow box "construction":

image description

From outside to inside:

1" Insulated Glazing, $U_{IP} = 0.3 BTU.h^{-1}.ft^{-2}.R^{-1}$
5" Air Gap
1/8" Aluminum plate
Mineral Wool Insulation with Foil Vapor Barrier

You cannot use a WindowMaterial:SimpleGlazingSystem object with regular Material objects, I've tried.

How do you typically model shadow boxes in EnergyPlus?

Do you input the glazing as a Material:NoMass?
Or do you go to the trouble of inputing WindowMaterial:Glazing and WindowMaterial:Gas objects? Would it actually work?

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answered 2016-01-19 10:34:17 -0600

Aaron Boranian

14145 ●83 ●24 http://bigladdersoftware.com/

Since the Simple Glazing System won't work, you're stuck with using a Construction object, which requires consistent Material object references (all opaque materials or all window glass / gas / shade / screen / etc. materials). Modeling the assembly as an opaque construction usually means the GLAZING must be opaque and will only reflect or absorb heat, not let it pass through. You could use the Transparent Opaque Material object that converts received short-wave radiation into transmitted long-wave radiation. However, this object assumes the assigned transparent surface has no thermal resistance and zero convection on both sides of the surface (source: Engineering Reference).

Your other alternative is to create a window construction and model the aluminum plate and mineral wool as window layers. The best option for this is likely the Window Shade object, since you can input a thickness and conductivity for the layer and this object requires MUCH less transmittance / reflectance input fields than Window Glazing objects. The window construction approach would likely be more accurate than modeling an opaque construction, since it would allow sunlight to pass through the outside glazing layer where the opaque approach would not.

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I like your second idea better, I didn't think about using an interior shade. There's a third option which is to create shallow thermal zones representing the shadow box and to assign glazing outside and to assign the insulation to the surface between it and the plenum, but that's harder to setup and will considerably affect runtime...