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Calculating annual heating losses

davorradman | Posted in Energy Efficiency and Durability on

I’m trying to determine cost effectiveness and payback periods for going with thicker insulation on the house I am planning. My project manager does not offer services of energy modeling, so I would like to do some of it myself. I will probably end up paying an architect for it, but I’d like to give it a go myself.

I calculated U-values for several walls, floors, roofs and windows. I have areas. I have heating degree hour value for my climate, which is ~78 000 (typical central europe). i have insulation prices.

It’s extremely straightforward.

But it occurred to me that there has to be something else when calculating this for the roof.
I mean, heating degree hours, on account of there being somewhat bigger temperature difference between ceiling outside air, should be a bigger value for roof? Is there some delta t value between air at 1m height and 6.5m height, where the roof will be? Or some commonly agreed on factor that the value is multiplied by, which would account for people putting more insulation in the roof than on the wall?

And likewise, smaller for floor (no basement)?

Also, windows, are they calculated same as walls?

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  1. GBA Editor
    Martin Holladay | | #1

    The main reason that building codes call for higher R-values for roof assemblies than for walls is that it is relatively cheap to install extra insulation on an attic floor, but relatively expensive to add extra insulation to a wall.

    In fact, heat flows equally in all directions, so there is nothing wrong with a high-R wall (except the high cost).

    Here is the formula for determining annual heating energy use:
    BTU/year = HDD*24*(Area/R-value)

  2. davorradman | | #2

    Thanks Martin.

    My situation is such that I would be using ESP, on a flat roof (yes, I know). Since it has to be some "harder" EPS, the price difference between R50 R65 roof is not negligible for me (~800$ for 100m2). Payback period turns out to be ~50 years. R50 should be good enough for central europe though.

    Thanks for the formula.

  3. walta100 | | #3

    You should down load a free program called BEopt from the US government.

    It will calculate the energy related costs based your mortgage rate, local weather, orientation, Roof type/ area, wall type/ area, window type/ are at the your cost per square foot.

    You will need to invest about 20 hours learning the program and entering the data.


  4. davorradman | | #4

    Thanks Walter

    I do already have a similar free program of such kind that's used here, but I have a feeling this one might be better :)

    I will certainly give it a go over the weekend.

    As a side note, energy modeling certainly seems quite interesting.

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