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How to install timber purlins to metal garage bracing?

Hi there, apologies if this has already been answered, couldn’t find any post on this already. I am going to follow the advice found here. Here are some photos of my garage.

I want to install foil boards to prevent condensation forming in the ceiling of my garage shed. 1st question is, how do I best seal the edges, especially for where the roof ends and the wall starts. I’m concerned some condensation might drip off and fall back into the garage.

2nd question is, I would like to clad the ceiling once I have been able to put all the foil board up. What would be the best cladding? I do like the look of plywood but understand it’s heavy. Would something like plywood work?

Finally, I was wondering whether I should install more purlins to take the weight of the foil board and the cladding. If so, how should I fix timber purlins to the metal bracing? Any advice on this would be awesome.

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aeosyyn

Another thing I forgot to mention is I was thinking of installing a roof vent first. If that didn’t work, then I’d look at installing the foil board. Would the roof vent have any impact with the foil boards’ insulation?

JacobZ

Hi @aeosyyn,

Many thanks for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.

It's great to hear you've had a look through our previous threads and found @bowmatty's example of a successful project.

A roof vent would certainly be a good first step. This will allow the trapped hot air to escape, which will help reduce the amount of condensation within your garage. The Bradford CSR SolarXVENT 150mm Solar Powered Vents are a great product that you might like to consider.

If you installed a vent and found that it wasn't doing enough to reduce the condensation, you would need to cut an opening in your foil board to allow air to still get to the vent.

The aim is to reduce any air transfer so reducing any gaps where hot air can contact the cold steel is going to be the most effective method for reducing the condensation you are experiencing.

The seams around your foil board can be sealed using Foilboard 50mm x 50m Aluminium Tape. This is an insulative tape designed to seal any seams between the foil board.

Where the foil board meets the purlin above the wall, cut the foil board at an angle using a Stanley 9mm Medium Duty Snap Blade Knife so the foil board meets snugly with the timber. Then the seam can be sealed with aluminium tape.

You might like to have a read through Foilboard's Installation Guide for some guidance.

There shouldn't be a need for additional purlins and at this point, it would be very difficult to install them in a way that actually supports instead of just adding more weight for the trusses to support.

Thinner plywood like the 3mm you've linked to would work, but you'd likely get some sagging. I'd actually suggest going to a thicker material such as this 2440 x 1200mm 7mm Braceboard Plywood Pine. It should be installed perpendicular to the purlins, attaching to 3 separate purlins wherever possible.

Allow me to tag some of our helpful members who I'm sure would be happy to offer their thoughts, @Dave-1, @ProjectPete, @TedBear, @DIYGnome.

Let me know if you have any further questions.

Jacob

A note for our New Zealand members: The use of foil insulation products in buildings is prohibited in New Zealand. See Foil insulation ban | Building Performance for further information.

Dave-1

Afternoon @aeosyyn

I must say that people have solved their condensation issues to a large degree by using foilboard. I wasnt sold on it being so but all the members that have tried are happy with it. I agree with @JacobZ about no extra timber purlins requried, The flatter and less cluttered the roof is the easier the sheets will be to mount.

As to mounting plywood or anything else to the ceil I ould be cautious. The shed roof has been designed to carry its load as is. Adding timber to it may structually weaken it, or weaken it enough for the next storm to push it past its limits.

Dave

aeosyyn

Thanks both for the advice, really appreciate it. Seems like I’ll go for the vent and then adding in foil boards. In terms of positioning, I’ve just drawn up a cross section of three options. I looked at the installation process but wasn’t sure exactly what it would look like.

in any case, once I install the roof vent, how would I go about sealing the air gap between the foil board and the roof?

Also, how would I fasten the foil board to the purlins?

MitchellMc

Hi @aeosyyn,

The boards will be installed closest to your second diagram. You'll adhere Spacer blocks to the roof sheeting with gutter silicone, and then, when dry, adhere the Foilboard to the spacer blocks. This creates an air gap between the roof sheeting and the Foilboard. The edges of the boards can be connected to the purlins with Foilboard Insulation Flexi-fast Fasteners or, if it's more convenient, blank plates can be nailed into the purlins face to capture the board's edge. Edges where the boards meet each other can be sealed and joined with Foilboard tape.

A short section of ducting can be used to seal this gap between the face of the Foilboard and the roof vent. Duct tape will help seal any cracks.

Please let me know if you have any questions.

Mitchell

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