TF suppliers being 'creative' with U values?

[Hilldes]

Got a number of quotes from timber frame suppliers. I have been checking a few of the claimed U values. One reason being my current spec is for render board external skin as opposed to brick or block. When I check the U values quoted by the supplier I get a higher figure than they state. Here is an example for an external wall...

 

Supplier offers a 140mm stud filled with 120mm of PIR (so about 85% PIR, 15% timber stud, plus a 20mm air gap). Across the inner face of the stud is 50mm PIR. Supplier claims overall U value for the wall will be 0.11 - but did not caveat this. I did a rough calc on just the elements mentioned and get approx U 0.16. I queried this disparity, the answer was "All our u-values quoted are all calculated based on a masonry external cladding". So I model a more complete, build up with brick outer skin and get U0.14. Is this a common problem?

 

Here are the details of the caluation of U0.14...

 

 

 

 

Edited May 29, 2020 by Hilldes

[ADLIan]

You should get U=0.13. Slightly better lambda for PUR and low emissivity air spaces next to foil faced PUR. Difference between brick outer leaf and render on board only impacts 3rd decimal place so still U=0.13.

 

Your TF supplier may not be using 15% bridging in TF as required under Regs but I still only get U=0.12 at best. Get a U-value calculation from them and post it here for comment

[Dan F]

 

32 minutes ago, ADLIan said:

foil

 

Some of the calculations I saw when we were looking at providers used a magic foil to improve u-values.  Our passive house consultant told us this was questionable and insisted on ignoring the the TF suppliers calculations (which were done by the foil manufactuer!)

[ADLIan]

Low e foil on pur is often certified by manufacturer, bba  or similar. Reflective breather membrane, again certified, can make useful reduction in uvalue for clear cavity and brick outer but little benefit with render on carrier board

[Hilldes]

Thanks both, if there was an explanation I thought it would be something like the breather membrane or foil on the insulation. This was not the first quote I've seen that seemed to good to be true.

 

P.S. I assumed PIR, the supplier just specified "rigid insulation".

Edited May 30, 2020 by Hilldes