Help me bust this comical Manual J calculation
Calling all energy nerds. Double check my math. I have a manual J calc in my hands with some weird results. I could go over every little error (some obviously intentional), but you have seen these kinds of things before. What I want to talk specifically is heat loss through the wall. His report does not show the design temps, so I am trying to work backwards through the calculations to figure it out. That’s where I need your help to double check my thinking.
Wall construction – 2×6 frame construction (assuming 16″oc), R-34 closed cell 2lb spray foam, R-2 Board insulation, brick finish. (this is not the actual insulation values. I told him the whole wall values were R36, but I guess the software didn’t have the exact option he was looking for so he made this up himself (actual wall construction is 2×6 studs, blown cellulose, 3″ polyiso exterior, brick).
Area – 1559 (close enough)
Sensible Loss = 5725 BTUs
So, forget the fact that the wall construction is way off (R24 vs R38), let’s focus on how he’s able to get that many BTU loss.
According to EngineeringToolbox.com, I see the formula to calculate heat loss through a material as…Heat Loss = (U-Value) x (wall area) x (temperature difference)
So if I want to solve for “Temperature difference”, the formula needs to be…
Temperature Difference = (U-Value x Wall Area) / Heat Loss
In this case, we know that he says the heat loss is 5725 BTUs. If my figuring is correct, his wall construction up there adds up to a whole wall value of R24, which makes the U value .0416.
So, if my basic math skills are right…
(.0416 x 1559) / 5725 = 89.45 degrees!
Am I thinking about this right?
For the record, the actual details are…
R38 wall, which has a U-Value of .0263
Area – 1613
Correct design temperature difference = 70F-13F = 57 degrees
That results in a 2400 BTU heat loss through the walls.
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