Wall Thickness Impact on Zone Volume
Posted by Richard Raustad on 29 October 2010 11:27 AM
Question:

I am working with energy plus in sketchup on the quest for one model.

What I am trying to do is to trace the external geometry from a 3d daylighting model and to build my energy plus model and then apply thicknesses internally afterwards to account for the volume accurately.

I know I can apply the floor to ceiling height in energy plus (which will hopefully overule this issue in the Z plane), but I am struggling to find where I can apply the wall thickness without individually changing each co-ordinate.

Does anyone know if it is possible to assign a wall thickness specifically to go internally to ensure the internal volume will be accurate if I trace external walls?

Or do I have to trace internal walls. The issue with having to trace internal walls is that you are then left with gaps inbetween rooms.

Generally, how does energy plus account for wall thickness in the wall materials, does this just impact the thermal properties of the material, or does this also impact volume? If so, which direction will it impact the volume? Inside, outside, or half-half?

Thanks in advance.



Answer:

In EnergyPlus wall thickness only applies to heat transfer/thermal mass.

Geometrically, all walls in EnergyPlus are infinitesimally thin - i.e. they have no thickness. So, once each surface has been placed changing the material thickness will have no impact on zone volume, ceiling height, floor area, shading, or daylighting.

For most modern buildings the choice of inside vs outside vs centerline should have little impact on results so many modelers just pick one a let the volumes be off a little. Using centerlines throughout the model splits the difference. Or some modelers use outer edges for exterior walls and then use centerlines for interior walls.

If you are modeling a very thick wall, such as an old stone building, then you also have thermal mass considerations. If you use the outside edges there will be too much mass, inside will be too little. Again, centerline will split the difference and will be very close to the correct amount of thermal mass (possibly losing some corner mass).



Alternate Answer from Bldg-Sim group:

So if I understand this correctly you are starting with a thick wall model used for 3d daylighting. Tracing the exterior geometry is the correct approach for EnergyPlus modeling. The heat transfer surfaces should be at the exterior face of the wall vs. the interior. You are correct that the volume EnergyPlus calculates will change based on where you draw that boundary. Drawing the heat transfer surface at the interior face of wall, while creating more accurate volumes, introduces bigger issues such as changing the surface area of the heat transfer surfaces exposed to the outside air and ground.

Just as you can set the ceiling height in EnergyPlus, you can also set the zone volume. So if you use tools in SketchUp to help get this volume, you can manually enter it in your IDF. You will have to use an IDF or text editor for this vs. the SketchUp Plugin. Note that if you do this the volume is not dynamic. If you change your zone geometry EnergyPlus will still use the volume you set earlier. There is a warning when you run the simulation if the volumes differ drastically, but I’m not sure what that threshold is. Search for “field: volume” in the link below to learn more about the volume field.

http://apps1.eere.energy.gov/buildings/energyplus/pdfs/inputoutputreference.pdf

So, now on to the tools to use in SketchUp to determine accurate interior volume. If you are using SketchUp 7 one option you have is to make a copy of your zone floors outside of the EnergyPlus group and then use the offset too. The offset tool icon looks like two arcs with a red arrow. Click once near the edge of the tool, and then click again where you want your offset path to be. You can also type a distance after the initial click. If you select your new face you can right click and choose “entity info” to view the area of your new face. Multiply that times the height to get your volume. If you plan to do this on a model that is still changing I would locate these floor copies geometrically inside of the zone, but not physically in that group. You can also put the copies on a unique layer so that you can turn their visibility on and off. In SketchUp 8 you can use that same approach, or you can extrude the floor to a volume, and make it a group. Now you can right click to choose “entity info” and it will give you the volume of the group, assuming it is a clean manifold solid, airtight shape without extra surfaces or lines. Another option is to use a plugin called the “Joint Push Pull” plugin that among other things will perform a 3d offset. So if you have a 10 foot cube. You can offset all walls by 15 inches, and then get the volume of your new smaller cube (in SketchUp 8). Here is a link to the plugin, but you may have to register with the site to download the plugin and see the documentation.

http://forums.sketchucation.com/viewtopic.php?f=180&t=6708

Having shown you how you can use SketchUp to calculate the volume, and manually enter them into the IDF, I want to point out there are probably better places in the energy model for you to spend your effort rather than adjusting volumes. For example the effects of the internal mass in a zone would likely have a great impact on your energy analysis than minor adjustments to zone volumes.

I hope this helps, let me know if you have any additional questions.


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