Let’s talk about Hollow Core Slabs and the involved Air-Tightness Strategy for your project.
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.
What´s the current practice in the Irish building industry?
The most popular strategy at the moment is, to wrap the ends of pre-cast hollow core slabs with an air tightness membrane.
An air tightness membrane? Why?
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.
- The majority of air tightness membranes are not made for this application and therefore might do not resist the involved stress. That can easily lead to perforations, allowing air infiltration into the building.
- Even when you use more rigid materials for wrapping (e.g. DPC), you still need to tape/seal corners, overlaps and the end pieces. Keep in mind that this procedure has to be done on site without any weather protection.
- It´s not only the wrapping of the hollow core slab - the junctions down below and above need to be included in the air tightness strategy.
This is what Prodomo suggests:
- Create a resistant air tightness layer around your hollow core slab
- Search for any penetration into the cavity wall at suspended ceiling level
- Raise awareness of air infiltration with all involved trades (block layer, pre-cast slab manufacturer, electrician, plumber, installer for suspended ceilings)
- Get a preliminary air tightness test done when you still have access to the top and bottom of the pre-cast floor slab
Prodomo’s strategy for better air tightness around hollow core slabs
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).
Prodomo´s response to potential concerns
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 had wrapped the pre-cast hollow core slabs with airtightness membrane upfront.
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.
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 will happen even when you wrap the slabs with air tightness membrane, as you can see in the short video down below.
contact
prodomo Ireland
Unit 9 - Ridgeview
Monavalley Business Park
Tralee
Co. Kerry
V92 W2TH