Where Should OSB Go in a Timber Frame Wall? – Can I Just Ask? | Ep.20

In timber frame construction, orientated strand board (OSB) is typically used to provide racking strength. The common practice is to fix it to the outside of the frame, mainly for ease and speed on site. But that convenience can come at a cost. At Back to Earth, we almost always recommend placing OSB on the inner face instead – and for good reason.

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The conventional approach: why OSB ends up outside

It’s no mystery why timber framers like to put OSB on the outer face. It gives them a continuous surface they can access easily during assembly. It also provides something to fix wall ties and membranes to, helping streamline the build process. For the frame team, it’s the path of least resistance.

But from a building physics and durability standpoint, that location creates a few headaches down the line—especially in breathable, high-performance constructions.

Why we put it inside instead

We prefer to install the OSB on the internal face of the frame. While that might make things a touch more awkward for the carpenters, it has significant benefits for moisture control, airtightness detailing and even long-term aesthetics.

Let’s start with moisture. Buildings need to manage vapour movement as well as heat and air. One way to do that is by establishing a diffusion gradient—essentially a gradual easing of vapour resistance from inside to out. By placing the higher-resistance layer (in this case, the OSB) on the warm side of the wall, and using more vapour-open materials externally (like wood fibre or sheathing insulation), you encourage moisture to move in the right direction and avoid trapping it where it can do harm.

Airtightness made simpler

Modern timber buildings aim for high levels of airtightness. If you’re already using a membrane internally, having a rigid, continuous surface to stick and seal it to makes the job easier. OSB on the inside gives you just that.

It also opens the door to using OSB itself as part of the airtight layer. SmartPly Airtight or Durelis VapourBlock boards can be sealed at the joints and penetrations to form a robust and continuous airtight layer across the whole frame. No flimsy backing or fiddly corner detailing required.

Reducing cracks and cosmetic headaches

Timber frames move. As the wood dries and seasons, it shrinks and twists slightly, which can cause issues if the internal lining isn’t designed to accommodate that movement. If the OSB is fixed externally, any shifting in the structure tends to show up as cracking and gapping on the inside—especially where plasterboard is used as the only internal layer.

By putting the OSB on the inside, you absorb much of that movement before it reaches the plasterboard. We’ve seen buildings ten years down the line that still look freshly decorated because the board has buffered those seasonal shifts.

What if you have to put it outside?

Sometimes it’s unavoidable. Maybe a system supplier requires it. Maybe the sequencing on site demands it. If OSB has to go on the outer face, the key concern is moisture entrapment.

Standard OSB has relatively high vapour resistance. If warm, moist air from inside the building leaks out slowly through imperfect detailing, it can condense on the back of that board—especially in winter when it hits the dew point. That’s a recipe for mould, decay and delamination.

To mitigate that risk, you need two things: airtightness and a higher-resistance internal vapour retarder. Airtightness is your first line of defence, reducing the volume of moist air getting into the wall in the first place. On top of that, using a membrane with a vapour resistance of 20 metres or more (like Intello Plus or DB+ DV90) helps counterbalance the external OSB layer and restore the gradient.

Even then, it’s not ideal. OSB on the inside just makes more sense from a whole-building perspective.

Final thought

OSB placement might seem like a technical detail, but it’s one of those choices that has knock-on effects across the entire build. From durability to comfort to long-term appearance, getting it right makes a real difference. As with so much in natural and breathable construction, thinking from the inside out tends to deliver the best results.