Importance of ground heat transfer for building simulation

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The importance of ground heat transfer has a lot to do with the size of the building relative to the ground transfer area/perimeter. I've done a lot of work with high rise buildings in Chicago and I can honestly state that ignoring ground heat transfer is justified in most cases because for buildings more than 10 or so stories, the ground heat transfer is such a small fraction of the overall building energy use (well under 5%) that it can be ignored - especially when uncertainties in all the other energy uses is considered.

For smaller commercial buildings and residential buildings, ground heat transfer can be a far more important part of the building heat transfer, especially if the basement or foundation are not well insulated. If they are well insulated (meaning good details to avoid any thermal bridges), then, unless the building is very efficient, it is far less important and can be ignored for rough building estimates.

If the building is short and the ground connection is not well insulated, then you really do need to worry about ground heat transfer.

So, here is my checklist:

1. Is it a high rise? Unless you are designing a VERY high efficiency building, you can ignore it.

2. Is it a low rise but the ground connection is well insulated? You can ignore it for rough calculations of normal efficiency buildings but should include it for high efficiency buildings or more accurate calcs

3. Is it a low rise building without good basement/foundation insulation? You should try to include it.

Because the importance is generally fairly low, I think that using a "closest equivalent" design approach is usually close enough. By that I mean, pick out the closest equivalent to the actual slab/basement/foundation construction that the software you are using might include and use that rather than trying to get the exact details.

One problem with uninsulated basements is you really need the ground temp and not just the outside air temps. EnergyPlus has utility tools to estimate that from the weather files, but adding the ground heat transfer to the models is not particularly easy or streamlined. For insulated slab on grade floors, heat transfer is related to the perimeter of the basement/foundation and the difference between interior and exterior air temps.

One way to "fake" ground heat transfer is by adding an extra exterior wall that has the right overall U value and area such that the UAdelta T is equal to the FP delta T that you actually get from the perimeter heat transfer. You can look up F values from the ASHRAE handbook and if you know the perimeter, create a wall of unit height (1m or 1 ft depending on your units) around the perimeter and model as a simple U value that matches the F value for your foundation design. This will give a decent approximation to the heat transfer you would expect from a more complete analysis and ... (more)