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  1. #1
    Join Date
    Aug 2006
    Location
    Carrollton, TX
    Posts
    44
    I was going over the final construction blueprints on my new house and realized that doing a room by room measurement of living space including closets, the square foot figure is less than what is on the drawings as they take into account all wall thickness as air conditioned space.

    Prints say 3240 feet of air conditioned area whereas measuring each room separately only yields 2800 square feet. How do you do an accurate Manual J based on these two figures?

  2. #2
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    Draw the rooms out with zero-width interior and exterior walls. Include the closets and any mechanical chases in the room areas. If the program you are using does not have a drawing feature, draw the house by hand and then measure each room. When you are done everyting should add up OK; if not, go find your errors.

  3. #3
    Join Date
    May 2006
    Location
    Tampa, FL
    Posts
    81
    You would only count the 2800 sq. feet as you are not air-conditioning the inside of the walls. But like he said, make sure you include all closets/pantries etc. in your sq. footage calcs even though you may not be supplying air directly to these small spaces. Just add the sq. footage from the small closets/pantries in with the adjacent space (kitchen, bedroom, hallway, etc).

  4. #4
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I had the same problem and had to conclude somebody fibbed about the true size of my house. This is not a solution to your problem, but you are not the only one to notice this.

    Nobody told me to make my walls zero width in HVAC-CALC and so I measured their real size. But I did measure the outside walls and allocated their length among the rooms. I believe that is about as accurate as you can practically go. In the end, remind yourself that this is just a model.

    Best of luck -- Pstu

  5. #5
    Join Date
    Apr 2002
    Posts
    11,808
    The square footage is not going to make much of a difference, as long as the length/area of exterior walls, or the area of ceiling under attic etc is correct.

  6. #6
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    By making the interior rooms and other spaces add up to the total square footage of the house you have an easy way to make sure you have included everything.

  7. #7
    Join Date
    Aug 2006
    Location
    Carrollton, TX
    Posts
    44
    Thanks for the replies. Boy, taxes are based on total square feet even if you can't live in the wall spaces

    It's just a model! So a 3,250 sq ft house is really a 2,800 ft house. Go figure.

    I'll make the neccesary tweaks. Thanbks again all...

  8. #8
    Join Date
    Apr 2006
    Posts
    143

    txborn

    Just a thought... all things being relative, do you think a 2x4x8 wall of air is easier to condition than a 2x4x8 wall considering that drywall can retain a calcuable amount of heat during the day if t-stat is not calling for cool??

  9. #9
    Join Date
    Aug 2006
    Location
    Carrollton, TX
    Posts
    44
    ddog1, not sure about that. Good question though. My walls will be packed with Icynene foam so I wouldn't even consider them to be an issue in the calcs. The drywall should be the same temp as the space in that case as no heat can get into that space between the sheetrock and outside sheathing.

    An interior wall uninsulated shouldn't be an issue if it is within the sealed building envelope.

    Going to visit a custom home (model home) tomorrow that has an Icynene sealed attic non vented and they have a piece of plexiglass in the ceiling so you can view a thermometer in the attic. On a 105 degree day, the attic temp is 4 degrees higher than the living area.

  10. #10
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    ALL the material mass within the conditioned space will be brought to the same temp, given enuf time -- furnishings & structure!
    yes, even the insulation will soak up some of the conditioned air. -- most at the conditioned side --

    will your inside walls be insulated? I swore I would do that for any bath rooms! -- against noise --

    the tax man uses the outside overall building dimensions!
    so does the initial house "developer" --

    "optimize" those numbers [to the desired benefit for the "cause"] -- told me by my first dealings with a V-P in 1960
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  11. #11
    Join Date
    Aug 2006
    Location
    Carrollton, TX
    Posts
    44
    Select inside walls insulated. Bathrooms, kid's walls facing the family room, master bedroom/family room wall downstairs. Will have returns in bedrooms.

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