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Getting HeatCAD / Manual J estimate correct?

matt_65 Member Posts: 19
edited February 2019 in THE MAIN WALL
Getting ready to do HV air with hydronic heat coil, 1st floor joist trak, and basement baseboard for a 1920s colonial 2 floors plus walk up finished attic, Northern NJ using 10F out/70F in design day. No insulation in most exterior walls, R13 in the rafters, 3/4" xps foam board sandwiched between vinyl and original wood siding on 1st story walls and seems to be doubled up on 2nd story. Entered it as just R4 foam for all walls for now.

HeatCAD is giving ~30 btu/sq ft for normal sized rooms on 1st/2nd floor (the single story bumped out/vaulted to roof/3 exterior mostly glass walls study gets a whopping 61 btu/sq ft!) The whole 1st/2nd floor averages out to ~30 btu/sq ft.

Using a R7.4 value for uninsulated walls with R4 foamboard. I'm unsure about the 2nd floor being doubled up foam so left that R out and not accounting for the extra layer of siding. I did 3 stories to account for the converted attic and just fudged the wall and ceiling area to match total sq ft best I could, so looks like a squashed little floor if you go 3D.

Am I grossly miss-estimating heat loss by leaving the potentially thicker 2nd floor foam board out and fudging the attic space? The averages seem reasonable for an older house. I did a quick EDR for the original steam radiators on the first floor (two have been removed during renovations) and they could put out almost twice the heat loss I calculated (ran it again with no foam board at all to check), so that gives me a little pause but seems like everyone used to oversize this by a large margin? End up at ~67k btu/hr heating for ~2,700 sq ft of space (~750 sq ft is basement).

(edit attached the wrong images at first, heres the right ones)


  • Tinman
    Tinman Member Posts: 2,808
    Double EDR isn’t cause for concern in my opinion. I can safely guesstimate that every old building in Chicago with steam or hot water was figured at 50 BTU/sf, minimum.
    Steve Minnich