Under Slab Membranes
This photo shows a plastic membrane underneath a slab. . . . but what sort of membrane?
Vapour Barrier
The Building Code of Australia requires the installation of a Vapour Barrier with the following characteristics
- Medium Impact Resistant Polyethylene Film 0.2mm thick, under slab and up sides to ground level.
- 200mm Overlaps at joints.
- All penetrations should be taped or sealed with a close fitting sleeve.
- Any damage should be repaired with an adequate sized patch and tape.
Damp-Proofing Membrane
The South Australian and New South Wales Authorities don’t think a Vapour barrier is enough; particularly if there is the potential for saline groundwater, or Acid Sulphate Soils.
They require the use of a High Impact Resistant Polyethylene Film 0.2mm thick as a damp proofing membrane.
They reason is type of membrane will be more resistant to puncture during construction.
Remember that during construction the workers will be walking on this membrane, and reinforcing steel may be dropped on it.
Some councils also further specify a layer of sand under the membrane to prevent penetration from stones under the membrane and provide a drainage layer.
Cost Difference
Two rolls of 50m x 4m Polyethylene film should be sufficient to protect the typical house slab.
Based on a quick internet search this week (June 2015) I found the following prices
- 2 rolls of Medium Impact Polyethylene would cost around $150
- 2 rolls of High Impact Polyethylene should cost around $220
For an extra $70 wouldn’t you prefer a better membrane?
Peculiar article, but exactly what I was looking for.