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How to exclude air film coefficient in constructions?

asked 2020-09-19 06:17:29 -0600

DavorJ's avatar

updated 2020-09-20 10:51:55 -0600

How can I exclude air film (U-value with film in envelope summary) in EnergyPlus simulations? It seems like EnergyPlus automatically calculates U-value of constructions with film and its whole output summary is based on those constructions, while I don't have constructions with air film.

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@DavorJ are you saying that you want to set a total U-value that already includes exterior air film so that EnergyPlus doesn't have to calculate and apply that? Or are you saying that you want to set the exterior air film loss to zero?

Aaron Boranian's avatar Aaron Boranian  ( 2020-09-19 17:10:30 -0600 )edit

Hi Aaron, thank you for your reply. I have construction layers in my technical data already and when I apply those layers in EnergyPlus, U-value is lower than defined (because EnergyPlus is applying air film?). I would like for a way to exclude air film from calculations, so I can have a U-value of an exterior wall or some other construction as it is defined in my technical sheet. Am I understanding construction with film wrong?

DavorJ's avatar DavorJ  ( 2020-09-19 18:24:42 -0600 )edit

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answered 2020-09-22 10:52:52 -0600

updated 2020-09-22 11:03:32 -0600

If you're trying to match the U-Factor with Film to a particular value, you can subtract the air film resistances from your target R-value to determine the R-value needed in your construction materials. The air film resistances are taken from ASHRAE Fundamentals, shown below. The exterior air film is R-0.17, and the interior is R-0.61 for roofs/floors or R-0.68 for walls (IP units).

ASHRAE Fundamentals 2009, Chapter 26, Table 1

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Hi, thanks for your answer. Which object should I use to subtract the R-value? Because I am using "Construction" where I input layers, how can I subtract it in E+ so it can give me output with the desired layers, is it possible?

DavorJ's avatar DavorJ  ( 2020-09-24 11:57:15 -0600 )edit

The Construction object lists the Material layers in your assembly. A typical assembly includes some materials that are fixed, like gypsum board, plywood, framing etc. Then you have an insulation layer, which you can vary the thickness of to achieve your desired R-value. Roofs are an easy example, because if you have insulation entirely above deck (IEAD), the layers can be as simple as insulation and metal decking. The metal decking is essentially R-0, so if you're trying to achieve R-30, the insulation layer would need to be 30 - 0.17 exterior film - 0.61 interior film = R-29.22.

ericmartinpe's avatar ericmartinpe  ( 2020-09-24 14:23:27 -0600 )edit

Eric, thank you very much for your answer. I understand what you are saying. It is definitely a good way to solve that issue. Thanks!

DavorJ's avatar DavorJ  ( 2020-09-24 14:48:13 -0600 )edit

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Asked: 2020-09-19 06:17:29 -0600

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Last updated: Sep 22 '20