From http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1771&page=2#Item_5

More specifically:
Re Basalt walll ties - I came across this aparently posted by a manufacturer on another forum. [See bottom of page 1] http://www.aecb.net/forum/index.php?topic=1498.0A relevant case study:a project designed in Pembrokeshire, with dense concrete block walls, a 300mm wide cavity fully filled with three 100mm mineral wool batts and Bassalt Ties (425mm long ties). The calculated U-Value for the walls is 0.12 W/(m2 K). The same walls with stainless steel wall ties would have a U-Value of 0.17 W/(m2 K) (~42% less thermally efficient).

I am interested in testing just what proportion of energy is lost through a cavity wall tie. Is it significant? How would one go about calculating this [without software].

The difficulty I see is that this cannot be accurately done in the conventional way ie 1/R as ties do not have a given uniform thickness with which to derive their R-value Or do they?

Thanks Ted,

I see from the example for Stainless Steel ties given that the calculated adjusment to a Wall of 0.32W/m2K is 0.003W/m2K. This is around 1%, significantly less than the 3% threshhold meaning that for the purpose of whole wall u-value they can be ignored in the same way as plastic ties. [maybe Basalt too?]

Obviously wall tie density increases with cavity width, but they would need to do so by a factor of 3 before inclusion becomes necessary at this u-value

So how important are wall ties in terms of thermal bridging?

But its nowhere near 42% is it.If we quadruple the number of ties and reduce the u-value of the wall to 0.1W/m2K, the contribution of the ties is still [only]10%

If we could build a wall with a u-value of 0.05, then 42% may be somewhere near accurate. The heat loss through the ties would still be exactly the same in both cases, though the latter may be percieved as being greater by a factor of 4.

So 42% appears to a bit of a porky.

The percentage measure is clearly a deceptive way of making a small figure appear to be large, It is not a true reflection of what is going on in real life.

That is a great link -- thanks. As I read it as more insulation is used the effects of thermal bridging increases. It would seem that even at our current building regulation standards we are at arround 10% greater heat losses through many masonry walls by ignoring the wall ties than we are allowed to think that we are. Lintels are another story.