Manual J Heat Load Calculator

Occupants contributes a fixed 250 BTU/hr of internal heat gain, and this calculator only credits that gain toward Cooling Load, not Heating Load -- a simplified Manual J assumption that occupant body heat helps offset unwanted summer heat but isn't relied on to offset winter losses. Heating Load is driven entirely by wall, window, ceiling, and infiltration losses.

No -- wall U-value is 1 divided by R-value, so going from R-13 to R-26 cuts wall U-value from about 0.077 to about 0.038 (roughly half), but going from R-26 to R-52 only cuts it further to about 0.019. Each additional point of R-value buys a smaller absolute reduction than the last, even though Heating Load keeps falling monotonically across the full 1-60 R-value range.

It only affects Cooling Load, by assigning Window Area a solar heat gain factor that varies by orientation in this calculator's simplified model (South highest at 50 BTU/sqft/hr, North lowest at 20, with East and West in between). It has no effect on Heating Load, since this calculator treats solar gain purely as a summer cooling-season factor.

Design Temperature Difference reaches every envelope loss term (walls, windows, ceiling, infiltration) as a straight multiplier, while Wall Insulation R-Value only reaches wall loss and ceiling loss -- not window loss or infiltration. Even so, R-Value's full 1-60 range swings Heating Load more in absolute BTU/hr terms than Design Temperature Difference's full 10-100 deg F range does at this calculator's defaults (about 126,084 BTU/hr vs about 39,714 BTU/hr) -- not because wall and ceiling dominate the total near the default R-13 (they don't; at R-13 they're a minority, about 45%, of the total), but because at the low end of R-Value's range (near-uninsulated R-1) wall and ceiling loss balloon well above their default-R13 share, while window loss and infiltration stay fixed regardless of R-Value. Design Temperature Difference is set by your climate zone rather than a design choice, though, so R-Value is usually the more actionable lever for a specific building.

This calculator estimates total wall area from Floor Area (assuming a square footprint) times Ceiling Height, minus Window Area. If Window Area is larger than that estimate -- a very compact, oddly-shaped, or heavily-glazed floor plan -- the calculator floors wall area at 0 sq ft (treating the wall as entirely glass) rather than reporting a physically meaningless negative area. When this flag is true, treat the wall/window split as unreliable and double-check Window Area and Floor Area against the real building.