Window installation Psi value in accordance to the PHI

[Anonymous]

I am calculating the psi value of a window installation and I would like to do it in accordance to the Passive House Institute conventions.
I refer to the PHI – Window frame certification schedule and the Calculating Window Performance Parameters for Passive House Energy Modeling documents.
In both documents, it is stated that the length of the wall is the “length of the external wall up to the edge of the window frame (including any fittings)” (page 3 in the PHI – Window frame certification schedule). I am a little bit perplexed how to interpret this.

Would please anybody comment up to where is the wall length? (please see the attached picture. I included 3 possible points. My feeling is that it is the case A. )..

Note – I understand that all the psi-values can be correct if relevant U x A are used later on in the building fabric heat loss calculations. I just would like to know how to keep to the PHI conventions.
Many thanks in advance
David

[Mark Siddall]

David,
The most important thing is to be consistent in your method and to document this in such a way that another person could replicate the results. Provided that the PHPP model using the psi-values uses the same dimensional methods used in the psi-value calcs then in terms of energy calcs the results will be the same. There are differing interpretations with regard to the appropriate point at which to determine the dimensions – and you cover almost all the bases.

EN standards, to which PHI refer, do not offer clarification that the level of detail that you are asking. PHPP refers to the 'rough opening' i.e. the line of structural opening for the window/door.

A) Pete Warm (Passivhaus Certifier) prefers this method as he can check dimensions against the window/door manufacturers documentation and use these dimensions in PHPP. Given that he's the certifier and I'm the designer his preference holds sway – so I use this method, however…
B) As an architect this is the option that I prefer as it is the actual structural opening and my opening schedules are based upon this dimension
C) Not appropriate as this is not the rough opening.

Edited 28/08/13

[Tom Foster]

Do you mean A and B reversed, Mark?

[Mark Siddall]

I don't understand your question Tom.

[Tom Foster]

It's OK, just nitpicking – diag B, not A, is the struct opg alternative which you prefer, and A is the joinery size one which PW likes.

I know it's PHPP but I have a doubt – surely the length of plain standard wall should end where it stops being plain and standard i.e. where the rounded reveals/modified stud begin. All that non-plain-wall detail should be part of what psi is being calcd for?

[Mark Siddall]

Tom,I
What is being discussed is not “PHPP” it is a convention for calculating thermal bridges. This information may then be entered into PHPP. The main thing is that consistent and clear points of reference are used so as to avoid error.

Thanks for nitpicking though. Error now ammended.

[Tom Foster]

OK not PHPP, and apparently not what it seems as a matter of principle. Some conventional convenience at work, which I will prob hear about next week when I re-sit the Therm training in Bristol, which I half-missed last time due to borrowed laptop problem.

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