Manual J sanity check - GreenBuildingAdvisor

Manual J sanity check gary__b | Posted in Mechanicals on April 15, 2026 12:10pm Greetings GBA community. I’m in the late stages of a DIY major expansion and remodel (new second floor, rebuilt first floor) of a 1910 house in Portland OR. Unsatisfied with efforts to sub out the HVAC work, I’ve decided to add that to my DIY list. I’ve run my own Manual J (and S and D) calcs on a website called CoolCalc, which seems like an intuitive and capable platform. However, I’m having a little trouble believing the results and am hoping for just a quick sanity check on the results. The current workscope is just the upstairs, as I’m doing separate systems for up and down and tackling the up first. The footprint of the house is 1200 sf (30×40), and the design entails a dormered gable roof with cathedral ceilings throughout (my upstairs walls are notionally only 4 ft, but because essentially the entire upstairs is gable-ends the actual walls end up running from 4 ft up to 15 ft high). I’ve attached a model and upstairs floorplan to give you a sense of it.  Wall insulation is R19 fill + R6 exterior polysio; roof insulation is R30 fill + R13 polyiso. I’ve attempted to build it pretty tightly (taping all plywood seams on the roof and walls, caulk/foam gaps, etc), but am not able to do leak testing yet.  My Manual J is coming up with about 8k BTU heating and cooling for the entire upstairs (design temps here are a mild 88F/28F). So my question is: just based on experience or a back of the enve...