How to stop condensation when insulating a deck roof?

[helenmay]

Thanks in advance for any help with this. I have read a lot of the very helpful existing conversations about insulating deck/patio roofs and think I have a plan. However, I am new to all of this and just want to check if it works, and what I have to do/ be concerned about with water/condensation. 

So we are in a Queenslander, with a wood framed deck and simple corrugated metal roof. It gets incredibly hot out there and we want to insulate it so we can make good use of the space. Photos of the roof here

 and here 

From previous posts I know that some condensation is unavoidable, but I also don't want to cause problems with poor choices now! We do get some condensation and drips from time to time out there. 

We have good depth with the beams to be able to do something tidy. My plan currently is to use the following: 90mm earthwool batts and then plasterboard for a ceiling that can be fixed to the wooden beams.

Do I need to (or should I) coat the metal with something or add a layer that is water resistant--something like ametalin silverwrap between the metal and the batts? Or is there a better choice that I am missing?

Thanks for your help! I know there are a lot of posts asking about deck roofing, and I did try and see if there was an answer before I made a new post! 

[Jason]

Welcome to Bunnings Workshop @helenmay. We're so pleased to have you join the community, and glad to see that you've already found the advice shared by other members useful. You're right that this is a common and difficult challenge. We're here to help.

Apologies that a couple of our resident Bunnings D.I.Y. experts are taking a well-earned break at the moment, so there might be a bit of a delay before one can get back to you. 

In the meantime, let me tag some other experienced community members to see if they can share their thoughts with you: @Nailbag, @TedBear, @JDE.

Thanks,

Jason

[TedBear]

Hi @helenmay  Given that you would have read here that cold metal allows condensation to form, adding another metallic layer may not help. I suggest that you coat the underside of the roof metal with a bitumen based paint which is a form of insulation against the cold, so will resist formation of condensation. Dab it on roughly for best effect (more surface area & no smooth areas for drips to form on). Then add your insulation and ceiling material. But I'm always interested to know what others suggest... 

[Nailbag]

Hi @helenmay 

I would go with the Earthwool which would then be held in place with insulation foil and then the plasterboard. I would consider using Aquachek plasterboard which is for wet areas which maybe a more suitable product due to your humid climate over summer. Then install 2-3 evenly spaced whirlybirds over ceiling grills to allow rising heat to escape. These can make a significant difference to ambient room temp.

Nailbag

[EricL]

Hello @helenmay 

It's great that you've received excellent advice from @TedBear and @Nailbag. Your idea of using Ametalin is also a good idea as it will partially insulate the metal surface and prevent condensation from forming. Please make sure to let us know which system you'll be using; We look forward to seeing progress shots as you install the insulation.  

Here are some similar discussions that you might like to have a quick look at:

• How to insulate Colorbond patio? by @tanmaried

• How to stop condensation on a patio roof? by @condruss

• How to add insulation to a sunroom with Colorbond roof by @GaryWeiley

• How to insulate a patio roof? by @dingo22

If you have any other questions we can help, please let us know.

Eric

[helenmay]

Thanks all!

Really appreciate all your advice! 

There seems to be some contradiction about whether using ametalin or similar foil coated layer will help or make the condensation worse? @Nailbag  and @EricL suggest that adding it will help, while @TedBear's comment seems to suggest it could be an issue. I've read on some of the product guides that putting the foil out/ facing the roof is often better in more humid/tropic climates; which would be relevant to here in Brisbane. Is there a consensus on this or should I just have a go and hope for the best?

If I can also ask what may be a stupid question -- but if I did use ametalin I was going to use it as the first layer (directly under the roof), then earthwool then the plasterboard. From TedBear's comment, the order seems to be from the roof down: earthwool, foil, plasterboard. Am I misunderstanding this? 

I really appreciate your help! Hoping to get it done next week, and will post photos once it is finished. Thanks!

[Nailbag]

Hi @helenmay 

I would be inclined to speak to one of the insulation foil suppliers as obviously they will have the technical knowledge to advise with both project and local climate. 

Nailbag

[TedBear]

Hi @helenmay ,  I agree with @EricL that using the ametalin will be a good option for what you are wanting to achieve, given that it contains insulating material which would also stop any condensation build up.

[helenmay]

Thanks TedBear! Appreciate your help. 

Will chat to someone in store before I order, and will share once we’ve finished the deck!