How to insulate a steel shed?

pricey333

Hi there,

I’ve recently had a 9x6m Titan steel shed built for storage / workshop / home gym use. 8mm reflective foam installed on the roof. Walls are uninsulated. Steel frame is 75mm deep on 600mm centres. 

I want to insulate the walls and probably add some more to the roof to make the space a bit more comfortable in the summer. 

After an extensive search of YouTube and this blog, I’ve found some great advice on how to do this. For the roof, I’m planning to add some kind of foil board, straight on top of the reflective foam - either screw directly into the steel roof supports or add a timber frame first. What’s best practice here? 

For the walls - I’d like to add earthwool between the steel supports, then OSB over the top. I understand that I need to install a vapour barrier directly onto the outer steel shell first, to control condensation before I add the wool. I’ve seen various advice saying to cut plastic sheet or vapour barrier sheet and attach to the frame, but I haven’t found explicit advice on how to do this for a metal framed shed. I am worried that I won’t get a proper seal with the vapour barrier and moisture will move into my wool insulation. Please can you give some guidance on how exactly this should be done? 

I should have thought ahead and had the walls wrapped in vapour barrier before the outer shell went on… but here we are. Hindsight is a great thing! 

Thanks so much. 

JacobZ

Hi @pricey333,

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

The unfortunate reality is that there is no perfect way to retrofit a vapour membrane from the inside, because the framing will always act as a thermal bridge between the warm inside of the shed and the cold outside of the shed because your vapour membrane does not run over the outside of the studs.

If you wanted to do things as effectively as possible, the best thing I could recommend is removing the wall sheets, installing the vapour membrane over the studs and then reinstalling the sheets. That way, you can guarantee that there is no thermal bridging happening where the framing is in contact with the wall sheets. Ultimately though, this is a lot of work, and you might not want to follow that route.

A more practical option would be to use Foilboard on the inside of the shed framing. Because Foilboard has an aluminium foil facing, it is highly resistant to water vapour, and when the sheets are installed with the joins, edges and penetrations properly sealed with Foilboard tape, it can form an effective vapour-control layer as well as providing insulation.

Foilboard’s large shed installation guide shows the panels being fitted between the wall girts or top hats, with the edges folded to hold them in place. All joins, exposed edges and gaps are then sealed with Foilboard tape. This would be a much more practical retrofit than trying to install separate pieces of membrane between each section of framing.

You would still have some thermal bridging through the steel framing itself, so it won't perform quite as well as having a continuous membrane and insulation layer outside the frame, but it should be a considerable improvement over leaving the steel walls uninsulated.

Foilboard can also be installed over the roof purlins using the method shown in the installation guide above, which utilises these https://www.bunnings.com.au/foilboard-ultra-steel-fasteners-250-pack_p0810882 as well as the tape mentioned above.

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

Jacob

A note for our New Zealand readers - The use of foil faced insulation products has been banned in New Zealand.