Cavity Trays discusses partial fill insulation

It is common practice to also terminate insulation at plate level across the gable ends rather than extend it up to the roofline.

Cavity Trays poses the question 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? 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.

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 results.

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 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.

The Type CD Cavity Dropcloak is listed within the Technical Manual ’Protecting the Building Envelope’, released by Cavity Trays.