Linear Thermal Bridge at Fenestration Perimeter Transition

[Collin Robinson]

Hello everyone,

Although I deeply appreciate the thermalenvelope.ca database for linear thermal bridges, I would like to understand the calculation process better. I looked at ISO 15099 and some others but I could not understand how to do this at windows. I used a recent project as an example. In the attached file Thermal Bridging Therm Linear.pdf I show Therm U-factors for the clear wall, two proposed conditions, and a mitigated suggestion. I made the length of wall next to the window frame only 6" because I would like to understand the impact of these little 12" wide pilasters between windows. See the key plan in the "jamb detail" PDF. 

1. In the Metal Return, Brick Return, and Mitigated scenario, how do I use the values calculated to get a linear Psi-value in Btu/hr-ft-°F?

2. Does the scope of the fenestration perimeter transition thermal bridge start where the frame ends, since the frame is part of the window assembly U-factor, or does it overlap since the frame and wall thermal transmission affect each other?

3. When I tried to simulate both jambs meeting in the pilaster, it would start simulating but then hang. When I cut it in half so one adiabatic boundary was longer, it worked. Is this to be expected when the adiabatic boundaries are both edges of IGUs? That file is FullPilaster_Jamb1

Thanks very much for any help you all can provide. 

Best,

Collin

[Collin Robinson]

Update in case others share my question. I found some helpful resources in ISO 10211, including a spreadsheet for metric. Also helpful was the BC Reference Procedure for Passive House windows. The latter showed the exact cutoff location between wall and window, and some helpful tricks with U-factor tags. In ISO 10211, the width of both the flanking wall and the window should be 1 meter, or centerline, so that helped clarify that the frame wasn't supposed to dominate the U-window figure. The detail with metal closure had a Psi value of 0.142 W/m-K, not great but not as bad as I expected.

Collin 

[alberto morales samper]

Hi Colin,

Would it be possible to have a window thermal bridge modelling as a dample to follow your document and see what exactly to define in the model?

Thanks in advance.

Kind regards 

Enviado desde Outlook para Android

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From: lbnl-...@googlegroups.com <lbnl-...@googlegroups.com> on behalf of Collin Robinson <spiri...@gmail.com>
Sent: Monday, 20 July 2026 04:07:13
To: THERM <lbnl-...@googlegroups.com>
Subject: [LBNL-THERM] Re: Linear Thermal Bridge at Fenestration Perimeter Transition

 

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[Collin Robinson]

Hi Alberto, 

See attached file for the option shown, with updated XLS sheet. Please point out errors, I'm not the expert here, just trying to learn.  

Collin

[alberto morales samper]

Thanks. I really appreciate that you share It. I Will have a look.

Enviado desde Outlook para Android

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From: lbnl-...@googlegroups.com <lbnl-...@googlegroups.com> on behalf of Collin Robinson <spiri...@gmail.com>

Sent: Monday, 20 July 2026 18:53:43
To: THERM <lbnl-...@googlegroups.com>
Subject: Re: [LBNL-THERM] Re: Linear Thermal Bridge at Fenestration Perimeter Transition

 

To view this discussion visit https://groups.google.com/d/msgid/LBNL-THERM/f36d7f4d-fd2d-42a8-9b81-543e3da8011en%40googlegroups.com.