Okay, so you've got your building geometry setup, and now you are moving on to replacing the default construction types with your actual construction types (using actual roof, wall, window details).
You quickly notice that your software package does not have an input for stud type, nor any sort of inputs relating to joists/studs (this is true of many software packages).
So How do you model joist/framed walls or roofs (steel, wood, or otherwise)?
The most common answer is found in ASHRAE Standard 90.1, Appendix A.
Since many modeling programs can only models layers in 2 dimensions, the standard provides guidlines for this for roofs, walls and more. We will focus on an example of a Steel-Frame Wall.
Chiefly, you need to know the following:
Stud distance (16 on center/24 on center)
Depth of joist
R-value of continuous insulation
Knowing these numbers, you can lookup Table A3.3 in ASHRAE std 90.1 (for steel-frame walls).
From the table, you can find the overall U-factor of the assembly (and your modeling program should have an input for this!).
WARNING: it is not always best practice to simply model the U-value
The U-value is important, but the mass of the materials is typically important too. Thus, it is a good idea to model layers that yield a similar U-value to the one in Table A3.3, and that also have a similar thermal mass. Given the example of steel joist walls, an example would be to have the following layers:
Exterior layer (sometimes Air films need to be included)
Interior Layer (also may need to include an Air-film)
If you are not sure how to initiate a construction, a good way to proceed is to find a similar material in your software, and to copy it and make changes.
Why doesn't the software model the frame directly?
Some software packages do allow this. However, energy modeling software packages contain very complex algorithms (not just a U-factor delta T calculation). As such, the algorithms are quite complicated given a 2-dimensional stack of materials. If the actual 3d materials are modeled, the calculations are exponentially complex, and likely would not produce improved accuracy in 99% of models. Remember, the data from ASHRAE is well tested, and therefore a 2-dimensional stack of materials is sufficient to model. In the future, we might see the frames being modeled directly in most software packages, but I doubt that this will change the accuracy of energy models significantly (unless you are doing very specific research)
Side note - In my experience, inaccuracies in models stem from other places such as poor schedules, and improper zoning (or over-zoning, as I call it). I see a number of models where the U-value alone is used and the materials are altogether ignored. As such, modeling layers (in 2 dimensions) would be an improvement to the accuracy of many energy models.
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