GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Manual J calculations

mhenson | Posted in Energy Efficiency and Durability on

I have entered the exact same data on three “Free” manual J calculations sites. (by far more detailed than the others)

They all gave me completely different numbers.

coolcalc 90,000 btu’s per hour
servicetitan  18,000 btu’s per hour
pdhvac 48,000 btu’s per hour

This is totally inconsistent and thus useless.  They also don’t let me enter specific data for double stud walls, triple paned windows, etc.
Can someone help me find an accurate way to determine what the Manual J will be on the house I have started to build?  I want to size the HVAC system  correctly and am willing to pay a reasonably priced professional.  I also need to get the price/performance correct on insulating the hydronic slab under the whole house (no basement, single story).  The walls and roof will be more than adequate, but the cost of the rigid foam under the house is painfully expensive and a general 4-6″ is a very large price difference for me also my foundation engineer is not green experienced and wants me to put in a 5″ slab with 1/2 rebar 6″ on center because “the concrete may break because of the insulation”.  Any help is much appreciated.  Mind you, we are on almost solid rock on a mountain in Colorado.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. tneicna | | #1

    There are numerous places to do manual J/S/D (that you can pay for) is one that I have used.

  2. mhenson | | #2

    Thank you

  3. Tim_O | | #3

    I really like the calculators that Borst has. I think they allow more detail than the others. You have to find out your true R value to get accurate results, and they are a little more finicky than the others. The one spot I find is that it doesn't comprehend the basement/slab well. You have to use the slab on grade option to get it to calculate losses there. I still am sort of guessing on a few items on the cooling load calculator too, but it doesn't make much difference in the results.

  4. Expert Member
    Akos | | #4

    This is Dana's recommendation for something that is easy to use and reasonably accurate:

  5. onslow | | #5


    I am surprised at your engineers rebar plan. I have a 5" slab in my garage over 3" of foam and the only crack that has appeared in 6 years is a small one right by a door opening. It has 4x4 welded 10ga mesh and pex tubing for hydronic heat if I ever win the lotto. 1/2" rebar 6" on center seems very conservative unless you are floating the entire slab on foam with no foundation. Then perimeter loads could come into play, though I am straying above my pay grade.

    You will want to be very careful of the tubing placement and aim for consistent depth below the finished surface. I tied mine to the underside of mesh to keep the tubing from causing hairline cracks above them (a real PITA) . I was warned by my concrete guy that if the tubing gets any closer than 1" to the surface the drying process of the concrete will tend to make cracks over the tubing. PEX does not like to lie down quietly and be flat so plan on a trip to Harbor Freight for lots of cheap zip ties. Be sure to tie them tail ends down and clip the long ones.

    I would strongly recommend placing a wire along with the tubing to facilitate tracing later. The interior walls will need to be anchored and no amount of pictures will help after the concrete is poured and a wall or closet gets moved. If you aren't doing the PEX yourself, you can also count on some "better" path being used than shown on any plan. Ask me how I know.

    More important is the fact you are on almost solid rock. So am I and I will advise that spring runoff will very likely be tracking along that rock. Be sure to note the pitch of the rock and use washed rock under your foam to ensure fast drainage, to daylight if possible. I am anchored to sandstone bedrock and thanks to the pitch under the slab I have anywhere from 6 to 36" of washed rock under my foam and basement slab.

    Radon piping is installed, but unused. There is no practical way to check radon levels until a home is done. I would urge that you put radon piping in place regardless of what you might think of local risk. Here in Colorado the soils and rocks vary so quickly that readings can vary even over locations a few hundred feet apart. Plan on sealing all edges and penetrations after the slab is cured. You do not want to attempt radon control after building.

    Depending on slab and edge details your insulation dollars are best spent on protecting your outer edges over the main floor portions. At 10,500' you will have zero and lower temps sucking at the perimeter most. Once you have protected the perimeter or isolated the main slab from the perimeter you can probably expect R20 under slab to be a good point. You haven't said what heat source will be feeding your hydronics so the dollars spent on foam vs dollars spent on fuel/energy are unknowable at this point. Model the floor as 40F cold side 24/7 IF the slab is isolated from the perimeter.

    And please use at least CZ6 unless you can tell me where a mountain here is CZ5 at 10,500'.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |