As separating wall requirements increase, Colin Potter, Technical and Development Manager at Robust Details Ltd, discusses cavity treatments that can work for acoustics and thermals. “Cavity thermal bypass mechanism” is a wonderful term that following changes in Part L 2010, has become part of the construction lexicography. And in the true style of jargon, it has left some wondering “what is it, and what do I do with it?” To which the answers are: “it’s bad”; and “try to lose it.” Traditionally, SAP calculations and related U-values were used to evaluate heat transference directly through the external envelope, from somewhere warm to somewhere cold.
Walls separating dwellings did not form part of this, as the temperatures on both sides were assumed equal. However, recent research1 has identified that although there is no real thermal transfer through the separating wall, heat passes into the cooler cavity, rising up and out the top. This movement draws even colder air in from the ends, leading to a greater temperature differential and more heat being sucked from the rooms. The research has determined that this heat loss is equivalent to that lost through walls having a 0.5 U-value – hence the default in Part L.
Approval of Seal
Sealing the ends of cavities is paramount – without this, no improvements can be claimed, unless specific testing has shown otherwise. Simply closing empty cavities of separating walls prevents the ingress of colder air, and the heat loss reduces to levels equivalent to 0.2 U-value walls. But let’s not forget acoustics. Physically isolating one wall leaf from the other acoustically isolates one dwelling from the other. Closing the cavity with extruded plastic cavity closers for example, may seal the cavity but they can also rigidly connect the leafs and provide a sound transmission path. Using flexible materials, like membranes or non-rigid insulation, across the separating wall maintains the acoustic isolation. The Robust Details Handbook shows mineral wool closer at the ends of separating elements specifically to stop sound passing from one dwelling to another via the external wall cavity (fig.1). Testing by industry indicates that polythene-sleeved mineral wool may also provide an effective edge seal, but Building Control Bodies will need to be satisfied that this or any other method is a ‘robust’ sealing solution under normal site conditions.
The next level of thermal efficiency involves the cavity itself. The research demonstrated that fully insulating the sealed cavities resulted in minimal-to-zero air movement in the cavity, and a zero U-value can be claimed for the separating wall. The Building Control Alliance has agreed that: “A “fully filled” cavity separating wall (or party wall) means a cavity wall which has been insulated such that no continuous air path communicates between the top and bottom of the wall, nor are there any uninterrupted air paths between flanking elements at either end of the wall, whether or not such junctions are edge-sealed...” Although all RD cavity walls allow edge sealing, the majority were specified with clear cavities to maximise isolation, so only 0.2 U-value would be possible. However, manufacturers and trade associations have been undertaking extensive testing in liaison with Robust Details Limited to prove the acoustic performance of cavity separating walls fully filled with insulation. Consequently, there are now 25 Robust Details wall types that can achieve the zero U-value. But beware, fully insulating as the wall is built means mortar droppings can not collect at substructure level, so care is needed to ensure debris does not build up on the insulation, potentially connecting the leafs and destroying acoustic performance. And finally, we must be clear that following the Part E Robust Details have not been assessed in terms of compliance with Part L – so separate checks still need to be made.
Author, Colin Potter, Technical and Development Manager