How to add bracing for external wall?

[Sopwith]

The Bunnings workshop is so helpful. I hope you might be able to help solve a puzzle I have.

Two of my friends are each building a timber-framed shed, 3mx6m. In each case, the plan is taken from instructions online by (I can't recall the exact name) but think it is a Timber Industry association. The plans claim that they are suitable for the type of location my friends have.

One friend is using a technique I've seen many times - bracing the wall frame with galvanized straps, diagonally arranged, in a cross, with a butterfly tensioner.

The other - who has spent time in north America - is using structural ply. This is being nailed, as per manufacturer's instructions, which cite the NCC. I'd seen ply bracing near corners or door ways, but not an entire build, but gather that is usual in North America.

In both cases a vapour wrap will be fitted and then timber cladding, over battens. The timber frames will be stuffed with insulation, prior to the lining being fitted. One shed is to be a darkroom; the other, a writing space / hobby room.

The location of both sheds is southern Tasmania.

My questions are:

1. Is there any difference between the two bracing techniques - or is preference down to cost?

2. If bracing is via structural ply, is the vapour wrap still necessary?

The two approaches have provoked quite a bit of friendly discussion.

Thanks for reading.

[MitchellMc · Accepted solution]

Hi @Sopwith,

Both metal strap bracing and structural ply bracing are effective, with the choice depending on cost, material availability, and structural needs. Metal straps provide tension resistance, are lightweight, and allow for larger openings, while structural ply offers superior shear strength and can be easier to install. Ply bracing is more common in North America but is gaining popularity in Australia, especially in high-wind zones.

A vapour wrap is still necessary with ply bracing as it alone does not provide a weather barrier. Vapour wraps protect against moisture while allowing vapour to escape. This helps prevent condensation and improves the durability and thermal efficiency of the shed.

Please let me know if you have further questions.

Mitchell

[Sopwith]

Hello Mitchell

Thank you for a prompt reply and very informative too.

The area where the sheds are being built is N3, and both sheds have one end face into the prevailing winds. I recall the discussion did include a too and fro about shear forces.

Thanks again. I'll be interested in seeing how my friends' sheds turn out.

S.

[Sopwith]

Hi there.

I have an additional query. Watching my friends' respective builds is helping me decide what to do, in respect of my own garden shed. But,

I saw on another post on BW, (https://www.workshop.bunnings.com.au/t5/Living-and-Bedroom/How-to-insulate-a-wall/td-p/46797) where it's stated: "Vapour barriers need to be installed on the warm side of the insulation .... In cold climates, they are to be installed on the inside of the insulation (next to the internal wall lining)."

My friends, and I have to say, all the builds I've seen here in s. Tassie, the sequence is: internal lining; frame (stuffed with insulation) bracing; vapour barrier; batten; cladding [external world]. However the comment above seems to suggest the vapour barrier is next to the internal lining.

Maybe I've misunderstood.

In the cases I've seen the internal lining is hoop pine - sometimes T7G and other times, ply. One friend is using T7G' the other ply, btw.

Thanks for reading and any clarification.

[MitchellMc]

Hi @Sopwith,

To clarify, I checked with our permeable vapour barrier supplier, Ametalin, and they confirmed that their vapour barrier should be installed over the stud work and under the cladding. It must also be vapour-permeable and correctly oriented, as this allows internal moisture to escape while preventing external moisture from entering the home.

There are two main types of vapour barriers: non-permeable barriers, which block moisture in both directions and permeable vapour barriers, which allow moisture to escape but not enter. In colder climates, a permeable vapour barrier is sometimes placed on the warm side of the insulation (under the internal wall lining) to prevent internal moisture from condensing within the wall structure. However, you still need a separate sarking layer under the cladding to protect the timber frame from external moisture.

If you only installed a non-permeable vapour barrier under the cladding and there were high condensation levels within the room, it could trap moisture within the building, which might cause water damage within the wall structure. Some architects now call for a permeable vapour barrier under the internal lining and a secondary under the cladding to comply with the National Construction Code.

For your project, I'd recommend using  Ametalin 1370mm x 30m VapourTech Brane VHP Vapour Permeable Wall Insulation under the cladding. This product has one-way vapour permeability, allowing moisture inside the home to escape while preventing external moisture from entering.

Please let me know if you have any questions.

Mitchell

A note to our New Zealand members: The use of foil-based insulation products are prohibited from use.

[Sopwith]

Hello Mitchell

Thank you for the trouble you've taken answering my questions. Very much appreciated. 

You clarified something else I saw. I've just seen some architect plans that show a permeable vapour barrier behind the wall lining and then another permeable vapour barrier laid over the frame to the external wall, then battens and then cladding. I had not seen that in Australia before, but it makes sense.

On that, am I right in thinking that because the VapourTech Brane only allows moisture to flow one way, the material must be installed in the correct orientation, namely, the side that permits moisture to pass through is closest to the moisture source. So, in the case of a wrap on the internal side of a frame, the side of the wrap facing the room is the side that absorbs moisture. For an external wall, the side of the wrap facing the wall studs and insulation is the side that absorbs moisture?

Thanks for reading and your answers.

S

[JacobZ]

Hi @Sopwith,

Typically, the outside barrier is a permeable barrier that will allow moisture out and not in. The internal barrier would more often than not be non-permeable, and its intention is more around keeping air in than anything to do with moisture.

It's likely best to confirm with the architect as it is not typical in Australian construction and goes beyond what is usually required.

Let me know if you have any further questions.

Jacob

[Sopwith]

Hi Jacob

Thanks for your reply. Much appreciated.

I should say this excusrion started off with a conversation about how my friends were building their sheds, my interest in sometime building one, and I fell down the rabbit hole of building codes, construction practices, here in Oz and also North America - the the science of vapour movement! And the Passive House building techniques.  I think the vapour barriers next to the internal lining, are as you say, not so common here - but are used in Passive Houses (but then they have to dehumidify and treat the air).

Thank you to you and Mitchell for your replies.

S

[Sopwith]

Hello again, JacobZ and MitchellMc
Watching my friends build their sheds and observing the conversation between them has been I have to say, part of the fun of DIY! But now I have another query.
My friend who has used the bracing ply, put battens over the joins where the sheets meet - arranged vertically. Then over that the Ametalin, then battens, again vertically, but with some horizontal ones, that had channels cut in them. On top of that the cladding.
I've seen battens with horizontal channels, so any moisture can run down and out and not get trapped. But I have never seen the two batten sandwich.
So, to be clear the sequence is: internal lining attached to frame. Fame stuffed with rockwool, then bracing ply, then battens, then Ametalin, then batten and then cladding. This "shed" is actually more than a shed - its a hobby space so it has to be fairly well put together, so I understand the extra care.
So, I guess the question is: what is the correct or best way to lay the Ametalin over the bracing ply. As mentioned in one of my earlier posts, the climate zone is southern Tassie.
Thank you for reading and any reply.

[MitchellMc]

Hi @Sopwith,

There are many ways to construct a shed or small building, and there isn’t always a single “correct” method. What’s considered “right” is generally only as defined by the Building Code of Australia (BCA), and even that doesn’t always prescribe detailed techniques for every situation—particularly for hobby structures or unconventional builds. As long as the structure complies with the BCA and any relevant local requirements, there's often a fair bit of flexibility in how you approach the build.

In terms of vapour barriers or reflective foil like Ametalin, their general installation guidelines state that the product should be installed on the outside of the timber framing and underneath the external cladding. That allows the barrier to act as a weatherproofing layer and also reflect radiant heat away from the structure. In many standard builds, the barrier is applied directly over the bracing ply, followed by battens and then the cladding. This creates a cavity that allows for ventilation and moisture drainage, which is important—especially in a cooler, wetter climate like southern Tasmania.

Your friend's approach of may be aimed at improving ventilation and reducing the risk of trapped moisture. While it might not be conventional practice, it’s certainly not wrong either—it’s just another way of achieving good outcomes in a challenging climate. The extra care makes sense given it’s being used as a hobby space rather than just a garden shed.

So, while there’s no single best way, as long as the structure is weathertight, well-ventilated, and complies with code, then the method is valid. Your friend has likely just taken extra steps to ensure durability and comfort in the long term. 

Mitchell