Vapour Barrier Issues During Build

Symmetryk · Oct 23, 2024 7:08 pm

Hi there!

We've started a build with Home Group Victoria (in Mernda), and have had a few problems along the way.

We don't have access to the site supervisor, we need to go through a client liaison officer (slowing things up, making it hard to co-ordinate private inspections).

They have been asking for payments before the private inspections have occurred. They initially started asking for payment before the stage began (I stupidly paid the pre-pour stage before the private inspection, and then had to fight to get issues fixed).

We are still waiting for the frame stage issues to be rectified.

The main question I have is this:

- the private inspector has identified a shortfall in the installation of the vapour barrier.

The site supervisor has indicated that there is no issue as per 2780.

I have googled the terms used to try and understand, as some folks have told me private inspectors don't always know best. It appears that there is no reason why an install shortfall should be acceptable. From Google and searching through this forum, at the very best a lacking vapour barrier is problematic and more work for us after handover, and at worst can cause major structural issues down the lines.

What is the best way to approach this rectification? I am in the middle of writing an email to the client liaison asking for clarification on the reasoning for a shortfall being okayed.

Any advice would be greatly appreciated. I don't want to make waves, I just want the build to be as good as it can be. Thanks for reading.

Eng. B A · Oct 24, 2024 1:00 pm

Vapour barriers are required under habitable slabs to stop moisture absorption through the slab to inside. This is expected to be picked up by your certifier for building approval. I’m not sure how you would install an adequate moisture barrier without constructing topping slab over existing with the use of vapour barrier under new slab. You could also have suspended timber floor over the slab.

Bimbob · Nov 21, 2024 7:07 pm

The only way to resolve the matter is to ask why the builder believes this is not an issue by asking how what has been done satisfies the standards they are referencing.
Simply saying “it’s fine because it was done to AS2780” is not a solution.
Ask:
This is what my certified says. Show me you havnt done this.

And do this at every stage.

BuildingandLegal · Nov 24, 2024 4:14 am

The vapour barrier detail in the NCC-BCA and AS2870 are the same! The NCC-BCA provides 1x option on where it is to lay, the width of any joints, how these joints are to be sealed together and at what height to terminate the barrier. (On top of the pad dirt/clay & under the concrete, the laps are to be a minimum of 200mm and taped together, any holes are to be taped to prevent water transferring through, the vapour barrier is to return back up the face of the slab and finish at FINAL ground level! This is the ONLY Deemed-to-Satisfy Solution (DTS) offered by the BCA. The purpose is to prevent the slab edge drawing water through the dirt and up into the slab, causing rotting of the bottom plate of the frame, the sharp edge of the carpet, creating mould etc etc. The issue is called Wet Slab Edge and is a massive problem in the UK. When we had droughts that lasted 5-8 years, then a wet period of 5 seconds then drought for 5-8 years again it wasn't much of a problem. Our environment has changes we now have constant wetting and drying periods throughout the year, and when it rains, we measure it in tens of millimeters rather than single millimeters.
The soil is staying wetter longer! = Slab Wetter = Mould, Rot, etc etc.

As your builder is smarter than the BCA and elected NOT to use the DTS, which he is allowed too, he needs to Document his Solution on how his system meets the minimum performance requirements of the BCA. This is a Performance Based Solution (PBS) and it involves 3x parties, The Builder, The Certifier & THE OWNER to agree on the solution.

Without being rude, you paid for an independent building inspection he should have Fully explained this in his report! That is what you are paying for!