When constructing cavity walls incorporating partial fill insulation, the insulation terminates at plate level. It is common practice to also terminate insulation at plate level across the gable ends rather than extend it up to the roofline. The question arises of whether the top of the insulation where it horizontally crosses each gable end should be protected against water ingress given the expanse of unfilled cavity wall rising above it? Could penetrating water use the top of the partial fill insulation as a bridge towards the inner skin? 

In theory rainwater penetrating an exterior skin of a correctly constructed cavity wall should not get anywhere near the insulation. 

In practice this is not the case. 

Within the triangle of masonry forming the gable are a large number of wall ties. Water will track along a perfectly positioned wall tie until the twist or feature of the tie directs the water to drip off it and fall into the cavity below. Within a 100mm cavity incorporating 50mm partial fill insulation, a conventional tie can discharge water halfway across the cavity. So where that partial fill insulation terminates horizontally across a gable, the top of it is exposed.

Water falling onto an exposed insulation top edge will either pool on it or it will gravitate into it. In both instances with the insulation secured against the inner skin, capillary permeation can result. This can occur where tie and insulation placement is perfect – the problem where ties are not perfectly centred or insulation placement is uneven is more extreme. 

Best Practice is to protect the top of the insulation to prevent this possibility happening.

Using a preformed Cavitray will ensure any water dripping from higher level ties cannot fall onto or into the top of the insulation. Cavity Trays Ltd manufactures several trays suitable for this purpose. 

A popular choice of contractors is the Type CD Cavity Dropcloak. This preformed tray conveniently does not require building into the exterior skin but importantly protects the insulation top and the inner skin.

‘There might be a temptation not to protect the top of partial fill insulation in such an application’ commented a Company spokesman ‘but it is essential as it takes only a couple of ties to cause a damp nuisance and rectification high on a gable wall once the building has been completed is costly’. 

The Type CD Cavity Dropcloak is listed within the new Technical Manual ‘ Protecting the Building Envelope’ the new publication released in May 2011 by Cavity Trays Ltd. 

Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU