LBT 1 Window Construction U-Factor or U-Value?

chris · November 5, 2020, 4:55am

Ah. In that case the math is extremely simple to convert between the U-factor (with film coefficients) that you input for an EnergyPlus SimpleGlazingSys and the U-value (without the films) that EnergyPlus actually uses in the simulation. I say “actually” because E+ dynamically computes the resistance of the air films at each timestep of the simulation and so it strips out a standard resistance of the U-factor before the simulation to find the U-Value. You can see the math right here:

github.com

ladybug-tools/honeybee-energy/blob/master/honeybee_energy/material/glazing.py#L559


    self._vt = float_in_range(vt, 0.0, 1.0, 'glazing material visible transmittance')

@property
def u_value(self):
    """U-value of the material layer [W/m2-K] (excluding air film resistance)."""
    return 1 / self.r_value

@property
def r_value(self):
    """R-value of the material layer [m2-K/W] (excluding air film resistance)."""
    return (1 / self.u_factor) - self._film_resistance

@classmethod
def from_idf(cls, idf_string):
    """Create EnergyWindowMaterialSimpleGlazSys from an EnergyPlus text string.

    Args:
        idf_string: A text string fully describing an EnergyPlus material.
    """
    ep_strs = parse_idf_string(idf_string, 'WindowMaterial:SimpleGlazingSystem,')
    idf_defaults = {3: 0.6}

And here is the standard value that EnergyPlus assumes for the _film_resistance of the simple glazing system material:

github.com

ladybug-tools/honeybee-energy/blob/master/honeybee_energy/material/glazing.py#L505


    * identifier
    * display_name
    * u_factor
    * shgc
    * vt
    * r_factor
    * u_value
    * r_value
"""
__slots__ = ('_u_factor', '_shgc', '_vt')
_film_resistance = (1 / 23) + (1 / 7)  # interior + exterior films resistance

def __init__(self, identifier, u_factor, shgc, vt=0.6):
    """Initialize energy window material simple glazing system."""
    _EnergyWindowMaterialGlazingBase.__init__(self, identifier)
    self.u_factor = u_factor
    self.shgc = shgc
    self.vt = vt

@property
def u_factor(self):