What is it?
A structural component that spans across the cavity at a door threshold, supporting the insulation and screed above it without filling the cavity with a solid, thermally conductive material.
What does it do?
At a door opening, the floor build-up (insulation, screed) needs to carry across the cavity to meet the threshold, but simply filling the cavity with concrete to support it, as used to be common practice, would create a direct thermal bridge from outside to in, right at a point people are already standing. A cavity bridge spans that gap structurally, in a material with low thermal conductivity, so it carries the load without carrying the heat loss.
Where is it used?
Beneath the floor build-up at external door thresholds, spanning the cavity at exactly the point a cavity infill would otherwise have been used lower down the wall.
Typical sizes
The Vulcon Dor Cav, the one in our external door model, is a 3 mm glass reinforced plastic plate 275 mm wide, in 900 mm and 1200 mm lengths cut to the door opening (longer openings take two, butted). It suits cavities of 50 to 150 mm, laid with 50 mm bearing on the outer leaf, and needs at least 75 mm of insulation and 40 mm of screed over it.
How to specify
Specified as a proprietary structural cavity closer product, sized to the cavity width and specified with the manufacturer's advice against the loads it needs to carry (foot traffic, the floor build-up above it). A block of rigid insulation, or a timber box filled with insulation, can serve the same purpose as a site-built alternative.
Interacting elements
- Rigid insulation / Screed: the flooring build-up bears on top of the cavity bridge at the threshold, completing it
- DPM: the damp-proof membrane laid directly over the top of the cavity bridge, completing the waterproofing line to the floor slab, then lapped into the DPC in the internal leaf of blockwork
- Masonry: the bridge is built into the low-level masonry as it's laid, spanning structurally between the two leaves
Popular manufacturers
Vulcon (Dor Cav) A structurally rated GRP cavity closer plate for door thresholds, laid on both leaves under the floor insulation and screed, with a nib that locates it against the outer leaf.