When I visit sites, snags with Hollow Core Slabs are common; air infiltration, junctions, penetrations at various points in their use. So here are my thoughts on how to fix the problems.
The most popular strategy at the moment is, to wrap the ends of pre-cast hollow core slabs with an air tightness membrane.
A membrane (or equivalent air tightness layer) is essential for structures with cavity walls because of permanent air circulation in the cavity. Without a membrane, air can infiltrate the building through the hollow cores and the junctions between the separate floor slabs leading to heat and energy loss
It´s definitely not the best practice anymore as there are too many details, which affect the final air tightness performance.
We prefer to close the hollow core slabs either on site by using a plastercoat or get them closed by the manufacturer in the factory prior to site delivery.
Engineers sometimes advise clients to shutter around the ceiling slabs and fill the remaining space with concrete. But this works only if your pre-cast ceiling slab does not finish flush with the block (block-on-flat or 150mm wide block required).
Is this pure overkill?
Where should the air get access when I have wrapped and sealed everything?
What you see on the image is air infiltration through a drill hole, probably made by the installer for the metal frame to carry the suspended ceiling. The client took all required attention to seal the pre-cast hollow core slabs and surrounding areas.
My builder is concerned that he will trap water in the hollow cores, by having the ends closed with rigid materials like plaster or concrete.
It can happen even when you wrap the slabs with air tightness membrane, as you can see in the short video.
I´ve been advised to use closed-cell sprayfoam for sealing down below the Ducon slabs.
We´re not quite sure whether the spray foam creates a durable air tightness layer. If you want to use this technology, please ensure to have the full wall area above your suspended ceiling at your building envelope covered. Bare block always creates air leakages.