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What is the basis for Air Film Resistances used to calculate U-Factor with Film?

asked 2021-06-17 07:00:13 -0500

Keigo gravatar image

updated 2021-07-09 16:35:34 -0500

ASHRAE 90.1 prescribes assembly maximum U-factors for walls and roofs, and the U-factors need to be used for baseline case.

From the definition of U-factor in ASHRAE 90.1, it includes resistances of interior air film and exterior air film.

image description

U-Factor with Film [W/m2·K] can be found in Report: Envelope Summary. I think we must match U-Factor with Film with the assembly maximum U-factor prescribed in ASHRAE 90.1 for baseline case.

However, Thermal Resistance [m2·K/W] in Material:NoMass does Not include Interior/Exterior Air Film Resistances. The input Thermal Resistance is equal to the reciprocal of U-Factor no Film.

Therefore, in order to match U-Factor with Film with the assembly maximum U-factor prescribed in ASHRAE 90.1, we need to know the Interior Air Film Resistance and Exterior Air Film Resistance that are used to calculate U-Factor with Film. Please refer to my sketch below.

image description

I know that air film resistance is calculated for each time step based on weather data during energy simulation. But for this U-Factor with Film, I did some case studies with different Thermal Resistance and different weather data, calculated (Interior Air Film Resistance) + (Exterior Air Film Resistance) from the above equation, and found that (Interior Air Film Resistance) + (Exterior Air Film Resistance) was always about 0.150[m2·K/W] for external wall and about 0.189[m2·K/W] for roof. They had nothing to do with weather data. But I don't know what is the basis for these Air Film Resistances? Does anyone know how they are calculated?

I may have overlooked it, but I couldn't find the basis in EngineeringReference and I/O Reference. If possible, I want to know each value of Air Film Resistance: Interior Air Film Resistance and Exterior Air Film Resistance.


P.S.

I noticed my mistake, so please let me correct it.

(Interior Air Film Resistance) + (Exterior Air Film Resistance) for roof is not about 0.189[m2·K/W] but about 0.1373[m2·K/W]. Interior Air Film Resistance for roof is 0.1074271[m2K/W], and Exterior Air Film Resistance for roof is 0.0299387[m2K/W].

In my case study, I mistakenly selected Surface as Outside Boundary Condition of the roof in BuildingSurface:Detailed. Therefore, the roof was recognised as "semi-exterior" surface and 0.0810106[m2·K/W] was used, which resulted in 0.1074271+0.0810106≈0.189[m2·K/W]. After changing Outside Boundary Condition of the roof from Surface to Outdoor, 0.0299387[m2K/W] was used for Exterior Air Film Resistance, and (Interior Air Film Resistance) + (Exterior Air Film Resistance) = 0.1074271+0.0299387 ≈ 0.1373[m2K/W].

For the best answer, please refer to the link that @JasonGlazer attached in his comment. The basis for R-values (Air Film Resistances used to calculate U-Factor with Film) is ASHRAE Standard 90.1 (I-P Edition) Section A9.4.1. Then, the unit is converted from [h·ft2·°F/Btu] to [m2·K/W ... (more)

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answered 2021-06-17 07:55:23 -0500

The interior and exterior air films are documented in Appendix A of ASHRAE Standard 90.1. See A2.2.1, A2.4.1, A2.5.1 for different types of roofs and A3.1.1, A3.3.1, A3.4.1 for walls.

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Thank you for your reply. I saw Appendix A.

Roof: R-0.03 for exterior air film and R-0.11 for interior air film.

Wall: R-0.03 for exterior air film and R-0.12 for interior air film.

Does EnergyPlus actually use these values to output U-Factor with Film? Air Film Resistances for Wall seem to be the same, but those for Roof seem to be different. I think this may cause a problem because for example, when GBCI review our energy simulation for LEED certification, they will compare the U-factor in ASHRAE90.1 and U-Factor with Film in the summary report, and will notice the discrepancies.

Keigo gravatar image Keigo  ( 2021-06-17 10:31:47 -0500 )edit

@JasonGlazer Do you know what interior/exterior air film resistance values are actually used in EnergyPlus for the summary report? Or do you know if there is a way to manually change the settings of those values?

Keigo gravatar image Keigo  ( 2021-06-17 10:36:04 -0500 )edit
1

I believe they are set in EnergyPlus in this routine:

https://github.com/NREL/EnergyPlus/bl...

JasonGlazer gravatar image JasonGlazer  ( 2021-06-17 10:53:25 -0500 )edit

Film coefficients refer to added thermal insulation due to convection with indoor/outdoor air in contact with the surface's interior and exterior faces. U = [W/m^2K] = 1/(1/h1 + dxw/k + 1/h2), dxw = wall/roof thickness [m], k = wall/roof construction conductivity [W/mK], and h1, h2 = the individual convection heat transfer coefficient [W/m^2*K] with inside zone air/outside ambient air. You are looking for the values of 1/h1 and 1/h2. The surface.uFactor method in the OS SDK includes film coefficients, while the surface.thermalConductance method does not.

sashadf1 gravatar image sashadf1  ( 2021-06-17 12:18:29 -0500 )edit

@JasonGlazer Thank you so much! These values are exactly what are used in EnergyPlus for the summary report. And I noticed my mistake. In my case study, (Interior Air Film Resistance) + (Exterior Air Film Resistance) was about 0.189m2K/W for roof, which means that 0.1074 was used as Interior Air Film Resistance and 0.081 was used as "Semi-Exterior" Air Film Resistance beacuse I wrongly selected not Outdoors but Surface as Outside Boundary Condition in BuildingSurface:Detailed. After revising it to Outdoors, 0.0299 was used as "Exterior" Air Film Resistance.

Keigo gravatar image Keigo  ( 2021-06-17 12:40:29 -0500 )edit
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answered 2021-06-17 11:22:48 -0500

I believe this question is answered here.

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Oh, that's true. I'm sorry for asking a similar question.

Keigo gravatar image Keigo  ( 2021-06-17 12:50:56 -0500 )edit

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Asked: 2021-06-17 07:00:13 -0500

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Last updated: Jun 17