Do you need a vapour barrier behind wall insulation batts?

[michaeljb]

I'm insulating all the walls. It's single brick . I will use rockwool r2.5 acoustic batts

Do I need a vapor barrier filt behind the batts for extra protection? I do live in melbourne

thanks  

  

[zippyf]

I am looking at retro fitting wall batts to a weatherboard house as we are getting rooms re-plastered.

We are in regional Victoria near Seymour. 

So would I be right to put vapour wrap between the weatherboards and the wall batt.

Do I need an air gap between the wrap and the weatherboard or is it ok to let the wrap be stapled up against the weatherboards. Then I would place the batts between the vapour wrap and plasterboard. 

There is only about a 75mm cavity

Cheers

[JacobZ]

Hi @zippyf,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

It would certainly be worthwhile to include a vapour barrier between the weatherboard and the batts, but you'll have to let me get in touch with Ametalin, our main supplier for building wraps and vapour barriers, when they reopen tomorrow for the particular method that they would recommend.

Please bear with me, I will get back to you as soon as possible.

Jacob

[zippyf]

Thanks JacobZ

And I guess if the vapour barrier is best or a breathable product. But the big question is the air gap or not.

Cheers

[JacobZ]

Hi @zippyf,

Unfortunately, the answer I received from Ametalin is that the only effective way to protect the cavity from moisture entry is to remove the weatherboard, install a membrane over the studs, with the Ametalin VapourTech Brane VHP being their recommendation for your climate area. You would then need to install battens over the studs to create an air gap, then reinstall the weatherboards over the battens.

This is because if you were to retrofit the membrane from the inside, it would not cover the outside of the timber studs, which would in turn allow moisture transfer through those timber studs, which would then end up in the cavity, which has the potential to cause mould and further issues with the insulation batts.

The other issue that arises, is that you have a 75mm cavity and the thinnest batts that are available are 75mm thick, so you would not have any room for an air gap anyway.

Unfortunately, the best suggestion I have is to speak with an insulation company for their advice. They may have experience insulating older homes that weren't designed for modern building standards and will be able to offer a solution that will work for you. 

Sorry I couldn't be more help.

Jacob