Rebuild on LBT an existing building adding current U-Factors

Barbarur · April 29, 2021, 9:31am

Hello all,

I have an old building where I know the current thermal transmittance of the walls, based on site analysis. I want to make a thermal analysis on HB, for later second analysis based on potential improvements on the building and make a comparison.

I wonder which the best way is to apply the construction specifications of the current building. I see the only HB Window Material allows me to input the U-factor. But I’m not sure it that will bring odd results during the calculation. Also, I don’t have the solar heat gain coefficient of the current wall construction.

Thanks

Erikbeeren · April 29, 2021, 2:30pm

@Barbarur,

You can set the conductivity of opaque materials. [W/mK]
You have to add thickness as well (energyplus way of defining material properties)

Barbarur · April 30, 2021, 5:05am

Hi @Erikbeeren ,

That’s not exactly what I’m looking for. The component you mention is for creating a new material, where I would need to input the thermal conductivity of the material (w/mK), and later the construction component, I guess, will calculate the U-Factor including the indoor and outdoor air layers resistance.

But I already have the U-Factor (W/m2K) based on site testing. I saw on the “HB Window Material (WindowMat)” I can input directly the U-Factor I have. But I don’t have clear how this component would perform for the task I’m looking for.

Or if there is a better alternative for simulation of existing buildings, as current performance of a 20 yo building it’s not the same as calculated on EP with the same materials/constructions. Introducing on-site collected data will improve the simulation result.

Erikbeeren · April 30, 2021, 7:40am

Hi @Barbarur,
As far as I know there is no standard tool for this in Honeybee. But with deconstruct construction you can check the output and look what the calculted U factor will be.

Barbarur · April 30, 2021, 10:23am

Hi @Erikbeeren ,

Thank you for the explanation. May I ask what construction component are you using on the screenshot I attached?
I can’t find it on my LBT.

Erikbeeren · April 30, 2021, 10:32am

home brewed. But it uses the LBT material and construction components inside. So you can use these.
It is basically a two layer construction.

SaeranVasanthakumar · April 30, 2021, 7:52pm

@Barbur,

FYI the reason that the u-factor isn’t exposed as an input is because the outside and inside air film resistances for EnergyPlus constructions are calculated during the simulation to better account for the evolution of radiant and convective heat transfer[1]. So your simulation will be more accurate if you could derive the existing building u-value (which, understandably, is difficult).

[1] Group – Surface Construction Elements: Input Output Reference — EnergyPlus 8.1

chris · May 4, 2021, 9:05pm

FYI, the air film thermal transmittances of the Honeybee-calculated U-factor are pretty simple. You can see the source code for them here:

github.com

ladybug-tools/honeybee-energy/blob/master/honeybee_energy/construction/_base.py#L162-L174


def out_h_simple(self):
    """Get the simple outdoor heat transfer coefficient according to ISO 10292.

    This is used for all opaque R-factor calculations.
    """
    return 23

def in_h_simple(self):
    """Get the simple indoor heat transfer coefficient according to ISO 10292.

    This is used for all opaque R-factor calculations.
    """
    return (3.6 + (4.4 * self.inside_emissivity / 0.84))

So you can just subtract those out from your R-factor if you want to know what R-value to plug in for the EnergyPlus material.