For those of you building your own cellars (or at least thinking about it), I’m adding some documents to help guide you in calculating cooling needs.

First is a spreadsheet that lets you calculate your BTU needs, or at least approximate them.

https://www.dropbox.com/s/4ic366ysrw271tv/BTU%20Calculator.xlsx

(can’t upload here so link to Dropbox)

This may need a bit of explanation.

For “height and width” it should be obvious–that’s the dimensions of each wall (this assumes a rectangle). For ceiling and floor, use the length/width dimensions. All that really matters is that the two numbers when multiplied equal the square footage of that wall/floor/ceiling. Keep in mind that the “door wall” should omit the square feet for the door, and that should be entered separately.

Ambient temperature should be the highest typical temperature that will be on the other side of the wall. Because this has my figures in it (which you can replace after downloading) you can see I determined my basement doesn’t rise above 76 degrees. The room above it may rise to 82 if the AC is turned to its lowest point.

Note in the lower left you can set your preferred cellar temperature, which I put at 55. If you want it lower (or higher) that will affect the BTUs you need.

For the floor, I used ground temperatures, which I’ll explain how to calculate in the next post. (Here it is 67, which is the maximum soil temperature at that depth in Washington DC)

The temperature delta multiplied by square feet of space is what determines your heat gain (delta and BTU are both calculated for you).

Finally, enter the R value for the given wall. That depends on the amount of insulation provided by the wall, door, or flooring.

This all results in a total BTU need, which is the maximum you should need for your space. Most of the time you’ll need much less, because the temperature delta will be much smaller in the winter for example. The total BTU need can tell you the approximate size of AC that you need, and how often you can expect it to run.