Peter, What does the 0.4 represent?

Peter- what construction system are you using?

Get a peek at the book " Details for Passive Houses " if you can

Yep it answers all possible technical questions in full

..... *rant* cavity *rant* breather membrane *rant* third party certification *rant* .....

I think that people are confusing the psi value with the Y-value. The psi-value is the linear thermal transmittance (measured in W/mK). The Y-value is the total heat loss attributed to the linear thermal transmittance divided by the surface area of the thermal envelope (measured in W/m2K).

In a refurbed interwar semi-detached cavity wall home the Y-value is about 0.15W/m2K and in 2006 regs the Accredited details achieve 0.08W/m2K. In the UK, due to the calculation methodology, it is increasingly difficult to get below a Y-value of 0.03W/m2K but it can be done with good design and workmanship. I understand that the EST are developing details that will achieve 0.4W/m2K. (NOTE: PassivHaus Psi values are measured externally and thus can achieve thermal bridges of <0.01W/mK and a, in the Germans used a Y-value it would have a Y-value of ~0.003W/m2K).

With a high performance envelope, to rely upon a Y-value greater than 0.4W/m2K (i.e. Accredited details Y-value 0.8), is not cost effective. This is because in order to achieve the energy performance target, say 15kWh/m2/annum, the heat loss via the thermal bridges, becomes of disproportionate impact i.e. a wall U-value of 0.15W/m2K with a Y-value of 0.8W/m2K is degraded to that of 0.234W/m2K a reduction in performance of 53.6% (compared to a Y-value of 0.3W/m2K a mean reduction in performance of 27%).

In terms of design when seeking to achieve these performance targets you need to compensate for these additional losses as a consequence the calculated U-value of building elements would have to be improved; thus incurring the penalty of wider foundations, deeper excavation, additional labour etc., all of which result in additional cost (cost that can be avoided by using more appropriate Y-values). Thus addressing thermal bridging means good cost effective value engineering and, assuming appropriate skills on site, greater certainty of achieving the designed building performance.

From my own calcs I am mindful that using a notional semi-detached house (I understand that it's the BRE standard semi) the AECB y-value = ~0.027W/m2K (because the AECB have not currently studies the party wall condition I used accredited details for party junctions.)

Further improvements?
Sticking with the accredited details for party junctions, with a little tweaking by using some PH concepts can theoretically be dropped to about ~0.02W/m2K and perhaps even as low as 0.015 W/m2K! i.e. a 50-66% improvement upon EST and an 75-81% improvement upon Accredited details.

If you make some reasonable assumptions about possible improvements to the party wall details the y-value could get as low as 0.012W/m2K and perhaps even 0.008W/m2K. This is results in an improvement upon EST targets of 70-80% and an 85-90% improvement upon Accredited details…. Of course how easily these calculations translate to on site performance is another matter.

Unless relying upon some generic details, such as AECB Standards, that have been independently calculated then the psi-value for each junction needs to be calculated independently and fed into the modelling software. Ideally this is done using tools such as Trisco, Sisco or Bisco from Physibel .....saddly they don't do one called Disco ;-) .....as this will allow geometric thermal bridging to be appropriately considered.

Hope this helps.

Mark