Re Composite Layers Representation
Another source of information how to approximate homogeneous layers properties is PLEA notes 2 Thermal Insulation by András Zöld and Steven V. Szokolay.
For example, for stud wall with thermal insulation; studs width x spaced at y. Homogenous layer conductivity [W/m*K] = (x/y)*studs conductivity + ((y-x)/y)*insulation conductivity.
Other example: insulation with steel pins layer conductivity =pins area/considered area * steel conductivity + ((considered area - pins area)/ considered area)* insulation conductivity.
The method from PLEA notes 2 is also used in New Zealand standard NZS 4214:2006, however the standard specifically states that the method is not intended for computer modeling.
I used this method anyway for timber framed wall with insulation and cavity. I divided the composite part into two layers: framing + insulation and framing + cavity and calculated the properties of the two homogenous layers according to PLEA notes. I’ve used to properties as EnergyPlus input.
To check the approximation, I modeled the wall in Therm software and computed the whole wall U value (using boundary conditions, surface resistance according to local standards). I compared the Therm calculated whole wall U value to U wall value calculated by EnergyPlus for my building model. The difference was around 1%.
The Therm software that uses a finite element method and calculates 2 dimensional heat transfer is available at http://windows.lbl.gov/software/therm/therm.html .
Comments and criticism of the procedure outlined above are welcomed.
Maciej
Dnia 13-02-2011 o godz. 3:49 Bereket Nigusse napisał(a):
For one dimensional approximation I recommend a paper by Karambakkam et al, at the following link.
Karambakkam, B.K., B. Nigusse, and J.D. Spitler. 2005. A One-dimensional Approximation for Transient Multi-dimensional Conduction Heat Transfer in Building Envelopes. Nordic Building Physics.
For stud walls, the procedure is as good as the accuracy of the U or R value of the composite or stud wall. To capture the peak conduction gain lag it is recommended to estimate the average value of the composite layer density and specific heat.
The composite layer properties calculation can easily be implemented as a spreadsheet application.
Bereket
From: jipp06 <jippnojunk@xxxxxxxxx>
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Sat, February 12, 2011 3:20:19 PM
Subject: [EnergyPlus_Support] stud wall construction
I need help to model a framed wall construction.
I have read the "Composite Wall Construction" section of the I/O manual and cannot imagine that one is limited to the limited Dataset provided. I would like to know what definitions you are using for studed walls and your opinions on the precision.
We use double framed walls or stud walls with exterior insulation so I will need to come up with some reasonable model.
Any help would be appreciated.
Thank you.
extract from I/O manual: "For stud walls, using these composite constructions [from DATASET] will give more accurate heat flow than
you would get by manually dividing the wall into a stud section and a non-stud section. If your wall�s exterior or interior roughness or thermal, solar or visible absorptances are different from those in the data set, you can make the appropriate changes to the first material (the outside layer) or the third material (the inside layer). None of the other values should be changed."
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