How to insulate a Colorbond garage shed roof and walls?

Question

Hi everyone,

I’m planning to add insulation to my existing Colorbond garage shed, but I’m still unsure about the best approach and would really appreciate some advice from people who have done similar projects.

Roof insulation

My current idea is:

* Install timber battens along the roof purlins first to create an air gap
* Then install Foilboard onto the battens instead of attaching it directly to the metal roof sheets

From what I’ve read, this air gap should help improve the insulation performance and possibly reduce radiant heat transfer.

Would this also help prevent condensation issues, or is there a better approach for a metal shed roof?

Wall insulation

For the walls, I’ve seen people recommend either:

* Corflute panels: https://www.bunnings.com.au/3mm-corflute-white-900-x-600mm_p0390160?region_id=118196&gclsrc=aw.ds&gad_source=1&gad_campaignid=21122993677&gbraid=0AAAAADtbEB-cJ-1CkzzDnypSmMoOUvd-h&gclid=Cj0KCQjww8rQBhDjARIsAE43KPMLAfbNJl0aKKB7qTDbBXJxSGlH7iGfQ9kQ_7rppLIqQkFfCifsHMsaAmnwEALw_wcB

or
* A vapour barrier / builder’s plastic: https://www.bunnings.com.au/ametalin-1370mm-x-30m-vapourtech-brane-vhp-vapour-permeable-wall-insulation_p0811065?store=8196&gclsrc=aw.ds&gad_source=1&gad_campaignid=21122993677&gbraid=0AAAAADtbEB-cJ-1CkzzDnypSmMoOUvd-h&gclid=Cj0KCQjww8rQBhDjARIsAE43KPPMv7bqlQ4NVk2cqNlgSjqrw_GnHZl-7LM-lozDeli-f-a-LuAB5LUaAvF_EALw_wcB

But I’m not completely sure how these should be installed.

My current understanding is something like:

Metal wall cladding

↓

Corflute or vapour barrier

↓

Glasswool batts

↓

Plywood or plasterboard lining

My questions are:

* Should the Corflute or vapour barrier be installed directly against the metal wall, or should there also be an air gap between them and the metal cladding?
* Is Corflute mainly being used as a moisture barrier to stop condensation from reaching the glasswool?
* Should the glasswool directly against the Corflute or vapour? or should there also be an air gap between the glasswool and vapour?
* Would plywood be a better option than plasterboard for a workshop/garage environment?

Sealing gaps

I’ve also used expanding foam to seal the gaps between the roof and wall sheets (both the top and bottom part) to help stop insects and dust entering the shed.

Was this a good idea?

I’m now wondering whether sealing those gaps too much could reduce ventilation and potentially increase condensation issues inside the shed.

About the shed

* Existing Colorbond steel shed
* Steel frame construction
* Located in Brisbane
* Mainly used as a garage and workshop
* Priority is reducing summer heat while avoiding long-term condensation or mould problems

Any advice, recommended wall/roof build-ups, or lessons learned from your own shed insulation projects would be greatly appreciated.

Thanks in advance.

EricL · Answer

Hello @Danson

If you were to use

https://www.bunnings.com.au/70-x-35mm-outdoor-framing-h3-treated-pine-2-4m_p8032103

the compression of the batts would not be so great. The trade-off would be that the framing would occupy a bit of the flooring space. But you would end up with excellent insulation. You do have the option of using Foilboard on your wall as well, but it does not have the same "R" rating as batts.

Let me call on our other experienced members @melissathomp89, @Ritar, @denk and @arpad for their recommendations.

Eric

TedBear · Answer

Hi @Danson, have lined my metal workshop walls using MDF. Nothing between walls and cladding. I have found that the density of the mdf doesn't let any noticeable heat through. Like you, I sealed the gaps with expanding foam

Danson · Answer

Thanks everyone for the helpful advice so far. I’m starting to better understand that managing condensation is probably more important than simply adding more insulation in a steel shed.

After reading through the replies and some of the linked discussions, I think I’m getting closer to a workable plan, but I still have a few questions.

For the roof, I’m now leaning towards installing the Foilboard directly underneath the roof purlins rather than adding extra battens. The purlins already have enough depth to naturally create an air gap between the metal roof sheets and the Foilboard.

So the roof setup would roughly be:

Metal roof sheet

↓

Existing purlin cavity as air gap

↓

Foilboard fixed underneath the purlins

↓

Open shed ceiling (no plasterboard or plywood ceiling)

I already have two roof vents installed, so I’m hoping this open-ceiling design will still allow hot air and moisture to escape rather than trapping it above a ceiling cavity.

For the walls, I’m still trying to work out the best way to handle the condensation barrier and insulation layers.

At the moment, I’m considering either:

* either 2.5mm or 3mm Corflute

or
* builder’s plastic / vapour barrier

My current understanding is something like:

Metal wall cladding

↓

Corflute and/or builder’s plastic

↓

Existing steel frame

↓

Additional timber battens/frame

↓

Glasswool batts

↓

Plywood lining

One thing I’m unsure about is the Corflute installation itself.

If I use Corflute or vapour barrier, I would most likely need to cut it into separate sections to fit between the framing members rather than installing one continuous sheet across the entire wall.

Would this still work properly for condensation control, or does the Corflute or vapour barrier really need to be installed as one continuous layer?

I’m also running into a wall cavity depth issue.

My existing frame depth is only 70mm, and I found that:

* Corflute is around 3mm thick
* The thinnest Glasswool batts I can find at Bunnings are 75mm thick
* Additional timber battens would also slightly reduce the usable cavity depth

Because of this, the insulation thickness technically exceeds the wall cavity depth.

Would it generally be acceptable in this type of shed retrofit to slightly compress the Glasswool batts, or would it be better to look at a different insulation product altogether?

I’m also considering adding a shallow timber batten/frame layer over the existing steel frame so that:

* the Corflute can be fixed more securely
* the wall cavity depth is slightly extended
* plywood installation becomes easier later on

Has anyone here done something similar in an existing steel shed?

Really appreciate all the advice and shared experiences so far.