Critique my Thermal Bridge modeling

[Duncan Fraser]

Hello, I'm a keen amateur and would appreciate some feedback on my analysis.

I'm Least sure about the simplification of the window model. 

Is modeling the window as all frame acceptable?

Also the internal and external dimensions appear to be the same in the window model?

I've attempted PHPP calculation with external dimensions.

Also attempted UK regulation calculation with internal dimensions.

Delta T 30 kelvin.

THERM 7.8.74.0

Nominal Structure U-value
Wall 0.112 W/(m²K)
Floor 0.123 W/(m²K)

Wall makeup:

60mm woodfibre 0.040

18mm OSB

300mm I Beam (97mmx45mm flanges)

Insulation filled void 0.038

15mm OSB

Homogenous I BEAM layer Equivalent U-value W/(m²K) 0.139

Homogenous I BEAM layer Equivalent Conductivity 0.0417

Floor makeup:

200mm concrete

300mm EPS

[Robin Mitchell]

I suspect that there are Passive House guidelines about how to model windows, but modeling the whole window as a frame is generally a pretty bad idea. I guess it will probably give you the worst possible outcome for the window, if that is what you are looking for, depending on what material you are using to model it. 

I would recommend using the WINDOW software to create a glazing system and then import that into your THERM file. You might want to take a look at this NFRC simulation manual, and I know there are several Passive House THERM tutorials on the web.

https://cdn.ymaws.com/nfrccommunity.org/resource/resmgr/simmanual/nfrcsim7-mar2023.pdf

But others on this forum should weigh in with their thoughts about this. 

Robin 

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Robin Mitchell

Building Technology and Urban Systems Division
Energy Technologies Area
Lawrence Berkeley National Laboratory

[Kevin D Brown]

There indeed is guidance, at least here in Canada ; ) There was a document created specifically for that purpose. It can likely be adapted for the UK situation, but I suspect that there’s plenty of people in that space in the UK. See particularly: https://peterwarm.co.uk

Attached is the Canadian document for your reference. It is specific for creating entries into the Passive House Planning Package (PHPP) using THERM. It may be of use.

Cheers,

Kevin.

[Duncan Fraser]

Thank you so much for the link to the Fenestration manual, I certainly would like to model the window to that level of detail but cannot justify it unfortunately!

If modeling the whole window as frame is the worst case and a plausible simplification, I'm content. (model using average frame UValue from manufacturer)
I took the idea from page12 of the NZ PH academy docment: https://passivehouse.nz/wp-content/uploads/2018/07/ThermInstructions.pdf

[Duncan Fraser]

Thanks Kevin for that helpful document link.

Section 3.6 is really helpful and confirms a lot of the methodology I used. Interestingly the window is also calculated as a L2D rather than my more simple Uwindow x L.

Interestingly: "window frame should be modelled 10 mm away from the window rough opening, and the cavity filled with whatever material will be placed between the window and the rough opening in construction"

That may make a small difference, depending on insulation used to fill the gap.