I'd have to add to what Vlad has said, by noting that his whole house is new-built with 6" studs, 24" spaced walls, insulated with mineral wool, double glazed windows.
And also Vlad is a total fiend when it comes to sealing air leaks. I've never seen anyone so thorough with a can of foam.
Please note that his HDD is a fairly moderate yearly average of 6399 Fahrenheit (based on 68F) or 3555 Centigrade (based on 20C).
[* EDIT above to include both Fahrenheit and Centigrade HDD *]
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Hey Vlad, how many Therms of gas did you burn during the months of December also for February? Please include the billing dates for Dec & Feb. Also your zip code or equivalent.
I've come up with a very interesting way to do a heating analysis for your house.
Best,
-AC