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Community and Q&A

Manual J questions

KeithH | Posted in Mechanicals on

I had a Manual J performed recently and also did one myself with .

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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  1. GBA Editor
    Martin Holladay | | #1

    Like you, I welcome input from GBA readers who are knowledgeable about Manual J software programs.

    In the meantime, I think that your observations may be clues to defects in the software programs -- in other words, if the program doesn't appear to properly account for the difference between an insulated basement slab and an uninsulated slab, or between white roofing and black roofing, it's a possible indication of deficiencies in the software.

    That said, if your roof assembly is well insulated, it's possible that the color of the roofing is irrelevant. (Black roofing is much more likely to cause overheating if the roof assembly lacks insulation than if the roof assembly is well insulated.)

  2. Expert Member
    ARMANDO COBO | | #2

    I perform my own MJ8 with a professional software program just so I know where my building deficiencies are and so the HVAC contractor doesn’t pull a wool over my eyes. I just did a 2nd MJ with the program you linked to, and I was surprise how close the results were. My house has 4500 sf with 2.25t/cooling and 2.5t/heating; just slightly over the "real" MJ8.
    When I design a CONDITIONED attic/basement/crawl space, I always include the space in the calculations. Dimension are calculated to the outside framing walls. I do not account for landscape shading. Dark roof is good enough. I wouldn’t worry about the downsides of not accounting for summer cooling in a basement, it happens with winter heating.
    For a free quick analysis, I though it did a pretty good job on my calcs; and don’t forget to support the site!
    I forgot to mention that when you select the equipment, usually you are adding about a ½ ton to the system, in a high-performing house; maybe more in a leaky house. Usually, when doing a two-story house with these programs, is best to calculate separate loads per floor.

  3. KeithH | | #3


    I forgot to mention it here but my professional J and the load-calc were pretty much identical (and I know the inputs were slightly different) on heating and within 10% on cooling. For those on a budget especially with a more typical house (mine is an odd split level), the load-calc program might be adequate to meet your needs.

    Thanks for the reactions. I guess my modeling is adequate for my needs. Figuring out how to use it is another story.

  4. user-945061 | | #4

    I'm not familiar with the software at the link, but use Elite for my calcs.

    #3, There are all sorts of window properties that can be changed in the software, but I can't find anything on tree shading. My Man J trainer specifically didn't recommend factoring foliage owing to the potential for trees to be eliminated.

    #4 I just checked, and shingle color doesn't appear to affect load in my software, even at low levels of insulation. Would bet that it's set for worst case regardless of color.

    #5 AFAIK, Man J mainly loads the above grade foundation walls. A very common error is to specify uninsulated basement walls with significant portion above grade. You could probably 'lower' the basement walls to achieve the same result.

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