Air Films in Building Construction Elements

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dancole
Posts: 9

Air Films in Building Construction Elements

Postby dancole » Thu Mar 15, 2007 1:11 am

Andy,

We are developing our own component library files at both the Materials and Constructions levels to enable us to use component properties consistent with those accepted by our governing industry body (BCA).

In the BCA deemed to satisfy provisions, indoor and outdoor air films are always included in the build-up of construction elements to achieve simple R values for walls, roofs, etc. This is also the case in the HVAC industry body AIRAH elements for heat load calculations.

In DB library construction elements I note that all materials are built up from thermal property files rather than simple R value files, even though simple R value selectability exists, and that indoor and outdoor air films are not included in any library construction elements.

As an experiment, I developed a number of typical construction elements in 2 different methods.
Method 1 using thermal properties for all materials similar to what is found in DB libraries, excluding indoor & outdoor air films from the materials used in the construction data file.
Method 2 using simple R values in the material files taking recommended R values from the BCA deemed to satisfy provisions and including R values for indoor & outdoor air films in the construction data file.

In comparing these files (Construction Data Calculated Tabs) I note that all values are identical except for the final R Value expression which is obviously influenced by the additional R values used in method 2 for air films, although the difference is not equal to the difference in values used in the build-up of the 2 different construction data files.

I also note the 'Calculated' tab displays expressions for Surface Resistance for the inner & outer surfaces although these values are not necessarilly consistent with the air film R values used to develop the individual construction files in method 2 above.

1)I assume that DB sees a layer which is actually an air film as a solid substance and applies its own surface resistance value, since I didn't observe any difference in these values between the 2 methods.

2) Can I assume that DB is using its own default or calculated values for surface air films and these should not be included where users want to build their own relevant construction data files?

3) In addition to the above observations, I note with interest that final R values calculated in DB do not match the R values given in built-up elements of the BCA provisions even though individual material R values for this exercise were taken directly from the BCA provisions.

I would appreciate your guidance as to the use of air films when building up construction data files and any comments you have as to discrepancies in R value totals mentioned above.
Andy Tindale
Posts: 2669
Location: Stroud

Postby Andy Tindale » Wed Mar 21, 2007 1:22 pm

Dan,

1)I assume that DB sees a layer which is actually an air film as a solid substance and applies its own surface resistance value, since I didn't observe any difference in these values between the 2 methods.

When defining constructions you should not add your own air film resistance layers because DesignBuilder does this internally. The layers you define should be just the solid layers plus any internal air gaps. Note that you can use the air gap materials under the Gases category or you can define your own using material R-value definitions. You should not use the gas materials flagged '**** DOES NOT INCLUDE EFFECT OF CONVECTION - NOT FOR GENERAL USE ****' for obvious reasons.

2) Can I assume that DB is using its own default or calculated values for surface air films and these should not be included where users want to build their own relevant construction data files?

Yes, see above. DesignBuilder calculates surface resistance values calculations requiring construction U-values or R-values. By default, EnergyPlus calculates its own surface resistance values dynamically based on surface inclination, temperatures, wind speeds etc. We will be providing facilities for entering your own surface resistance values in a future version.

3) In addition to the above observations, I note with interest that final R values calculated in DB do not match the R values given in built-up elements of the BCA provisions even though individual material R values for this exercise were taken directly from the BCA provisions.

Please check that the R-values you are comparing are consistent i.e. that they either both either include or exclude the surface film resistancees. You can obtain surface to surface R-value resistances calculated by DesignBuilder by taking the reciprocal of the surface to surface U-value data on the Calculated tab. I will make a note to display surface to surface R-values in a future version.

I hope this answers it.

Regards,

Andy
elmira2
Posts: 2

Re: Air Films in Building Construction Elements

Postby elmira2 » Fri Sep 21, 2018 10:14 am

Hi
What is the reference for the amount of air film resistance in the DB?

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