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  1. #1
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    Sep 2009
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    Manual J Calc question:

    When doing a load calc on a home, or room by room calc, where is the perimeter of the home or room measured, on the interior of the room, or the exterior?
    Take for example a room surrounded by 14" thick brick walls, the area measurements of the room are going to be completely different if you include the whole wall as being conditioned space.
    I'm not trained & experience in Manual J's at all, but it seems logical to measure the inside perimeter, because we are not trying to condition the 14" thick brick walls.
    Any help is appreciated.
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  2. #2
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    Aug 2007
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    Quote Originally Posted by tipsrfine View Post
    When doing a load calc on a home, or room by room calc, where is the perimeter of the home or room measured, on the interior of the room, or the exterior?
    Take for example a room surrounded by 14" thick brick walls, the area measurements of the room are going to be completely different if you include the whole wall as being conditioned space.
    I'm not trained & experience in Manual J's at all, but it seems logical to measure the inside perimeter, because we are not trying to condition the 14" thick brick walls.
    Any help is appreciated.
    you are correct, it is the interior dimensions used for the calculation.

  3. #3
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    Thanks. I have a contractor I'm working with who came in with a bid that appears to be oversized; he included the 16" thick walls in the rooms conditioned area.
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  4. #4
    Join Date
    May 2012
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    NC
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    I am currently enrolled in a Residential design class, so my real world experience is very limited but I have the book with me.. We are using Manual Jae(Abridged Edition). It is not as many pages and much easier to teach from. It says in Appendix 5 that "Exposed above grade wall is recorded to the nearest foot(use the outside dimensions for exposed walls and the centerline dimension for interior partitions)". I have class on Tuesdays and I also have access to a Manual J. I'll ask my instructor and look at my Manual J and let you know what else I find.

  5. #5
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    Dec 2002
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    Your splitting hairs! Your wall load is a small fraction of you total load..........so if your off by a few sq' feet its NOTHING! Worry about your windows, there direction,there placement in the structure, there type, overhangs, and interior and exterior shading and blinds. And as you know Tips your inflitration is very important, don't forget about that blower door!
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  6. #6
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    Dec 2002
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    Quote Originally Posted by alamo1718 View Post
    I am currently enrolled in a Residential design class, so my real world experience is very limited but I have the book with me.. We are using Manual Jae(Abridged Edition). It is not as many pages and much easier to teach from. It says in Appendix 5 that "Exposed above grade wall is recorded to the nearest foot(use the outside dimensions for exposed walls and the centerline dimension for interior partitions)". I have class on Tuesdays and I also have access to a Manual J. I'll ask my instructor and look at my Manual J and let you know what else I find.
    You are correct! And yes the full Manual J8 reads the same as Manual J Abridged.
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  7. #7
    Join Date
    Aug 2001
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    The OPs post is the kind of smart questions that need to be asked and answered. Splitting hairs? yes but a small miscalculation here and a mismeasurement there along with a few incorrect assumptions can send the end result far from acceptable accuracy. Be as precise as you can with measurements, and make as few as possible assumptions to get as close as you can to the actual load. Ideally: measure everything (twice) and assume nothing (if you can)

    Just don't follow the unofficial standard of measure sqft/ton and nothing else, or replace "Like with Like" and assume everything will be great.
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  8. #8
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    Feb 2010
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    Cal
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    Agreed, splitting hairs, but since he asked...

    Tips says hes not trying to condition the wall, but in reality you really are. Heat enters the wall structure and is transfered out at whatever rate. The surface area of both the interior and exterior matters, both conduct, and at different rates depending on which side is warmer. I cant comment on which dimension is the assumed dimension by the manJ folks.

    Me? Id measure the easiest way.

  9. #9
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    Jun 2001
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    Moore, Oklahoma, United States
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    Fudging input data on indoor/outdoor temps will seriously throw things off.

  10. #10
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    Dec 2002
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    Tips here is some good info for you. AreYouUsingManualJ8Correctly.pdf
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  11. #11
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    Aug 2011
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    Natchitoches, Louisiana, United States
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    Isn't that why we position the registers to sweep the perimeter walls?

  12. #12
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    Wall thickness may not have a big difference normally, but consider a finished attic with knee walls. The actual conditioned space area can be much less than if you measured at the exterior walls. Take ceilings and how they are entered into Manual J. You don't ignore the ceiling and enter the roof dimensions. Same goes for knee walls. Common sense says that all we care about is the load on the conditioned space. If you built a small dog house in an attic and conditioned it, you wouldn't throw 2 tons of a/c into it just because the attic it is in is bigger.
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  13. #13
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    Quote Originally Posted by second opinion View Post
    you are correct, it is the interior dimensions used for the calculation.
    According to appendix 5 it is the "outside dimensions" for exterior walls.
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