Posted By: ActivePassiveAs a would-be PH chaser, it *does* seem quite odd to have to adhere to a German baseline, whereas one's local climate might be far different...

The whole point of HDDs is that, but using the same baseline, you're comparing like with like. Over here, the baseline temperature for both HDDs and CDDs is 18C. If you have data for another location that's referenced to the same baseline temperature, you can see if your climate is warmer (fewer HDDs) or colder (more HDDs).

For example, last winter we had 4383 HDDs (baseline 18C) and this summer we've had 346 CDDs (baseline 18C), whereas winter 2009/2010 had only 3684 HDDs and summer 2011 had 388 CDDs.

Paul in Montreal

Posted By: gravelldBegs the question why the numbers aren't adjusted a bit?

Because it takes time and money and people to do the research to form the basis of any such decision.

At the moment you can't even use PHPP to demonstrate Part L compliance so until such time as the people of this country and their elected representatives and their civil servants, show some sign that there might be some official blessing and support I wouldn't hold out much hope of any UK specific changes, let alone southern England-specific changes.

Using standard (UK base 15.5C) degree days as an approximation for Passivhaus energy calculations is problematic. The 15.5C is the 'balance point' or 'base temperature' at which it is assumed in a 'normal house' heating would be turned on i.e. this is the outside temperature when the internal gains (solar, humans, electrical, hot water losses) offset the thermal losses through wall, windows, roof etc, below which heating is required to maintain the internal temperature. The 'balance point' for a Passivhaus is much lower, perhaps 5C. If you look at the way degree days scale because they are non linear you can't really use the standard 15.5C figures for approximate calculations on Passivhauses.

If you are using Degree Days for comparing different climatic areas for Passivhaus you should use a much lower 'balance point' or 'base temperature' and even then you need to be careful. For 'normal' poorly insulated homes in not too dissimilar regions then comparing Degree Days for example with a 15.5C base is a good approximation; it can also be used for estimating the impact of small changes in U values and air permeability.