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I keep seeing descriptions of Passivhaus buildings using 90% less heat

AlanB4 | Posted in PassivHaus on

But what does this equate in in btus at peak load or kWh/yr?
I know this is a broad question thats different in every climate but i just want some numbers for context, so lets compare two code minimum houses @1000sq/ft, one in climate zone 5 with 4000HDD and the other in climate zone 6 at 5000HDD compared to two identical location passivhaus’
For the sake of argument lets say both buildings have a design temp of -25C


Can these numbers scale with size (would they double at 2000sq/ft or be half at 500 sq/ft?

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  1. GBA Editor
    Martin Holladay | | #1

    The "90% less energy for space heating" is an old boast from the 1990s. It is obsolete and misleading.

    Originally, Wolfgang Feist made the boast -- comparing Passivhaus buildings in the 1990s to older buildings in Germany.

    A more relevant comparison would be to compare the heating energy required for a new Passivhaus building to a new code-minimum building. There are many variables, but most experts who have looked at the question come up with a more realistic percentage of about 50% savings.

  2. AlanB4 | | #2

    That makes sense, i am curious though about what actual loads and annual usage would be for a code minimum and passivhaus since the place i live in would not meet either!
    When i moved in Hot 2000 says it was 55k btu peak at -20C and 37300 kWh/yr in heating demand and since i upgraded the furnace and insulated the walls its telling me 45.5k btu peak and 24800 kWh annual heating total.

  3. vensonata | | #3

    Passivhaus figures are absolute rather than relative. So the colder climates make passivhaus harder to achieve. So ya, up in Alaska passivhaus would be 90% less than code. In Seattle, maybe 40% less than code.
    Also passivhaus would pecisely double with size doubling and half with reduction. What phyiscists know is big square houses are much easier to heat than small houses...per sq ft. passivhaus is very difficult for a 500sq ft, much easier for 4000sq ft.

  4. Dana1 | | #4

    I assume you're using Celsius, not Farenheit HDD? (5000HDD-F is zone 4, not 6.)

    I'm not sure if there is any zone 5 location with a 99% temperature bin of -25C/-13F, but many in zone 6.

    Heat load does not scale with the floor area of conditioned space very well, since heat loss is primarily a function of the exterior surface area and the U-factors & areas of the different assemblies & materials. Exterior surface area does not scale linearly with floor area- a 500' house will usually have a higher exterior surface/floor-area ratio than a 1000' house, and a much higher ratio than a 2500' house. At any floor area, shape of the house matters, of course.

  5. AlanB4 | | #5

    Yes Celsius HDD, to make it even simpler 5000HDD -25C 99% temp (thats what we are supposed to get here in a few hours with -40C/F windchill), 1000sq/ft single floor plus equivalent basement (concrete floor, no crawlspace), square or rectangle house, two standard windows on all 4 sides (is there a standard size window, if not reasonable size) code minimum and U0.15 for passivhaus, standard vented attic, insulation on attic floor, two doors which are code minimum, better if available for passivhaus (please state their R value), 1ACH, passivhaus level thermal bridging elimination for both, HRV at 0.3ACH and 75% efficiency (i have not seen any at big box store but assume it is available). Any other details needed?

    For both what would the R values be for subslab insulation, basement walls exterior insulation, above grade walls and attic. Would insulating the footings make an appreciable difference?

    Also peak load @ -25C and yearly kWh for heating at 5000HDD.

    Finally how many btu/hr are lost per whole window for the code minimum and passivhaus window at -25C


  6. GBA Editor
    Martin Holladay | | #6

    If you want the answers to all of those questions, and if you are planning to design a building, it's time for you to start using an energy modeling program -- either a free program (available online) or one you purchase. Then you can manipulate your specs and see the results.

    For more information, see Energy Modeling Software.

  7. AlanB4 | | #7

    Thanks for the link Martin, i'm not actually able to build a house, i am just curious about numbers for context compared to my current house, as i complete upgrades over the next few years i am curious when i will hit current code minimum (many years from now)

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