Manual J questions
I had a Manual J performed recently and also did one myself with http://www.loadcalc.net/ .
A few questions arose that I didn’t have good answers to. Some of these questions might help other people doing a Manual J in the future.
- Should the truss/joist space be included in the height of a room? I personally assumed yes if the truss was in the conditioned space but no if it was behind a good air barrier (roof, etc).
- Similarly, should the stud space be included in the square footage? I assumed no as long as there was an air barrier between the room and the trusses.
- Does anyone have any recommended ‘cheats’ to address windows that are shaded in the summer by mature vegetation? I have a number of NW facing windows that are shaded by mature trees on a house otherwise without shading (zero overhang roof). I’m wondering if I could improve the accuracy of the J by adding some shading to these windows.
- I have a black EPDM roof. I’m concerned the Manual J, other than a ‘dark’ roof material choice which didn’t seem to change anything, doesn’t seem to capture the cooling load of a black low slope roof.
- Natural cooling from an uninsulated basement floor: I have an uninsulated basement slab. The Manual J doesn’t seem to account for the natural cooling effect provided since it indicated significant (3k) cooling loads for these basement rooms. I know they are naturally lower than design temp so something is amiss here. It might just be the limits of the Manual J but if someone has a trick to account for this, I’d love to hear it. Otherwise, I’m just going to pull the basement cooling load off the equipment sizing.
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