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Thermal Bridging Psi values for Isotex

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9 hours ago, Nickfromwales said:

I would expect them to be exaggerated tbh

Depends on the methodology used I think.  The formula is pretty simple, just converting an area thermal loss to be expressed as the perimeter length, as I understand it.

So if a wall 10m by 2.5 m wall looses 10W.K-1. [That is a U-Value of 0.25 W.m-2.K-1]

The perimeter is 25 m so 10 [W.K-1] / 25 [m] = 0.4 W.K-1

This is added to the 10 W.K-1, so 10.4 W.K-1


I think the problems start in defining the U-Value of the perimeter.  The easy way is to take the thickness of the wall, rotate it though 90 degrees, and assume that is the perimeter 'area'.  So it basically adds the wall thickness to the exposed area.

So in the above example, assuming the wall is 0.15 m thick, that will add an extra 0.75 m2 to the surface area, with a U-Value of 0.25 W.m-1.K-1, that is an extra 0.19 W.K-1 losses, giving a total of 10.19 W.K-1.

That is different from the initial crude calculation, but in the same ball park, but when you back calculate, 10.19 [W.K-1] / 26.2 [m], for the new perimeter length, you get a value of 0.39 W.K-1 extra losses, so all is well.  The difference may well be in the surface losses due to air flow, but I can never remember those values.


Your perimeter length may change depending on your wall design i.e. single, double or triple storey and where/how, joists and beams are connected.

Window and door openings (expletive deleted) it up as well as they really need to be added as well.

The ADDED is the important bit, we get very used to multiplying area by U-Value by temperature difference when calculating thermal looses, but the perimeter losses are an extra loss, so they are additional.


(I may be talking out my arse as it is now a good 15 years I since last looked at this, as part of may BSc, but I was thinking about it the other day, I know not why.)


Edited by SteamyTea
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10 hours ago, Nickfromwales said:

I would expect them to be exaggerated tbh. What are your specific concerns?


Need them for SAP calculations.


@SteamyTea thanks for your response. My SAP consultant tells me that the manufacturer/supplier would usually have a list of different non-repeating thermal bridging values - the measure of heat loss through the junctions between different construction elements expressed in w/mk.


I've found online a table for Durisol:



I have, of course, written to Isotex in Italy and asked them to provide but am hoping someone who's previously built in Isotex (or their SAP consultant) might already have such a table to hand.

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4 hours ago, Nickfromwales said:

I think you meant "loses".

More seriously, have I remembered how to calculate PSI near enough correctly.

If so, it is dead easy to set up a spreadsheet and calculate.

I think the ∆T will be the inside air temperature, minus the  median value of the ground and the outside air temperature for the perimeter.


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