[stevehull]

Calculated heat loads (typically Manual J) and actual measured heat loads are often way, way off. I have done manual J for decades now and use this only as a wild guesstimate.

I have found that air infiltration is a FAR better predictor of heat load. Some examples:

Manual J says to put in a 4 tonn (output) heating unit for a maximum design temp of -10 F (1800 sq foot home). Builder did a tremendous job sealing home and blower door test shows less than two air exchanges per day! Air to air HRV provides necessary ventilation.

Standard 2x6 walls (probably net R15), R 40 in attic, R 20 below floor and double pane gas filled windows.

I suggested a 2 tonn unit. First HVAC installer would not do this as it was incompetance on my part. Had to find another installer that would do it

Actual field tests show unit running 100% at design temp with a 2 tonn unit maintaining an interior temp of 75 F.

Another similar sized home with poor air sealing (and yet more insulation) required a 4 tonn unit. No ERV, poor sealing around windows/doors. Blower door showed air exchange of once per hour . . .

Personally, I would take a less than adequately insulated home, but one with very low air infiltration over a higly insulated home any day.

Insulation is easy to measure, sealing cracks is hard to quantify, but sealing air leaks has a FAR larger effect.

Think about this . . . what is the point if insulating a long box with rolls and rolls of insulation on the outside - if both ends are left opened.