How to add vapour membrane without removing existing cladding?

hi_liza

We’re renovating the interior of our house in Brisbane and want to add insulation to all external walls. One of our main concerns is preventing mould inside the walls and within the insulation. Since we’re not removing the weatherboard cladding, would it be possible to install a vapour membrane from the inside? Some people have suggested painting the internal side of the cladding instead. What would be the best approach to ensure proper moisture control?

JacobZ

Hi @hi_liza,

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

I have had this exact conversation with our building wrap suppliers in the past, and the short answer is that it cannot really be done properly from the inside.

The issue is that a vapour membrane or wall wrap needs to form a continuous barrier across the outside of the framing, running over the studs. If you try to install it from the inside, you cannot achieve that continuous layer because the studs are in the way, so you end up with breaks all through the system. This allows the cold air to hit the studs, which creates a thermal bridge to the inside of the house. Warm air on the inside of the house can then hit the studs which have been cooled from contact with the outside air, which can then form condensation, which will then create a moist environment in the wall cavity.

You can add a vapour membrane between the studs, however it would not be 100% effective and as it is outside of the recommendations of the suppliers, it would not be warrantable. It would help to prevent the warm inside air hitting the back of the cold weatherboards, which is the main issue, but it will not stop the thermal bridging from the studs.

Your insulation will also do a lot of good preventing the warm air on the inside hitting the back of the weatherboards, but again, without that consistent membrane across the outside of the studs, there is still potential for condensation to form.

Unfortunately, paint will not stop this at all, and it really is just wasted money.

Realistically, the only effective way to prevent any kind of thermal bridging into the wall cavity is to install a membrane between the outside of the studs and the weatherboard, which unfortunately isn't possible from the inside and would require you to remove the weatherboards.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob