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Covering insulation

Do I need to cover my Rockwool insulation with a vapour control barrier before I put my wooden panels on it?

Asked by: Rebekah Watson  | 10.37pm, Monday 13 May

WW says:

Rockwool is a mineral fibre and provides insulation by trapping the air, preventing any convection flow, and has about half the insulation value of spray-foam but is considerably more eco-friendly.
Condensation will form on surfaces cooler than the air itself and the metal cabin sides should be properly painted, not just a quick coat of primer, to stop rust. The wooden panelling will not attract the condensation in the same way as the steel, but a vapour barrier will stop it. Over time any condensation, typically from the bracing, will stain the panels. The vapour barrier can be plastic sheet or a better solution is to paint the back of the panels to seal the wood.

Rupert Smedley  | 12.10PM, Tuesday 14 May

As Rupert says, condensation will form on any surface that is colder than the air that contains moisture vapour. It will also form inside porous materials as the moisture vapour passes through it if one side is above the air temperature and the other is below it. This is known as interstitial condensation and it occurs at what is called the 'dew point'.
This can result in the material becoming damp and losing some of its insulation value. The way to avoid it is to cover the warm side with a moisture vapour barrier to prevent the moisture from reaching the dew point. Unfortunately, this is easier said than done as the moisture vapour will find its way round any joints in the barrier if they are not well sealed. It will also seep through any punctures in the barrier caused by fixings. However, if you are using a porous insulation like Rockwool, a moisture vapour barrier is worth having but try to avoid any gaps or holes. If you have timber linings, a few good coats of varnish will also help to keep the moisture at bay.
Spray foam not only has a better insulation value but, because of its closed cell nature and the way in which it is applied, it acts as its own moisture vapour barrier and should prevent interstitial condensation.

Graham Booth  | 2.22PM, Tuesday 14 May

I suspect that Rupert meant at least a coat of gloss on the primer and, preferably, some undercoat too. Damp proof paint is usually used on plaster walls.
I was thinking of varnishing the front of the linings as you have more chance of sealing all the gaps once everything is in place. However, if you are having the timber stained, that will not be possible. Painting the backs of the boards is still a good idea as it will protect them.

Graham Booth  | 10.28AM, Wednesday 15 May

Red oxide is insufficent alone, it needs a sealing coat of gloss. As the colour is not a problem I wouldn't bother with undercoat.

Rupert Smedley  | 11.43AM, Wednesday 15 May

Readers say:

Thank you both very much for your speedy and very helpful replies. For a few reasons spray foam was not an option for me, hence Rockwool and as the installation can be troublesome in the future, i'm taking my time and making sure I get it right :o)
I have already painted the inside of the boat with a good coat of red oxide paint. Do I need anything else on top of it such as a damp proof paint? Or will the red oxide be sufficient?
Thanks for the tip on painting the backs of the wooden planks to seal the wood, to clarifiy is that the 'few coats of varnish' that you mentioned?The fronts will be stained.
Thanks again both of you for all your help!

Rebekah Watson  | 9.57AM, Wednesday 15 May

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