Thermal bridging model giving wrong results

[R K]

Hi

I am new to Therm and have modelled a repeating element of a flat roof using Therm v7.8 as 3 adjacent column of materials to deal with the cavities and timber joist.  The isotherms look good but I am getting very small U values for my U tags and I do not know how to combined these into an overall U value.  

I am trying to model Kingpan's Optim-R vacuum insulation and they indicate a Uvalue of 0.31.

Chatgpt and Gemini are not helping as they tend to invent Therm menus that dont exist and it seem the Therm manual is WIP.

Please could you take a look and point me in the right directiion.

Many thanks, R

[Robin Mitchell]

You have U-factor tags on both the interior and exterior Boundary Conditons. They should only be one one. Generally they are applied to the interior BCs. 

When I delete them from the exterior BCs and just have them on the interior, the U-factor looks better, although it isn't 0.31.

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Robin Mitchell

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