Organic and absorbent materials in the wall
The rule
Section titled “The rule”No water-absorbing organic material goes into an external wall. Not OSB, not gypsum-based board.
This is not a matter of taste. In a thick brick wall moisture spreads out and the mass of the wall absorbs it: after a period of wetting the structure dries. A light gauge steel wall is closed, the layers are thin, there is no mass — whatever was built in stays there.
Why it is this categorical
Section titled “Why it is this categorical”The moisture cannot get lost
Section titled “The moisture cannot get lost”There is no buffer, and no route by which it would dry quickly.
The damage stays hidden
Section titled “The damage stays hidden”Wet OSB swells, loses strength and grows mould — all of it behind the lining. By the time it shows on the surface, the structure has been wet for years.
The repair is demolition
Section titled “The repair is demolition”Not patching, not repainting: opening up the build-up.
It takes the steel with it
Section titled “It takes the steel with it”Wet organic material also starts corrosion in the adjacent section.
What goes there instead
Section titled “What goes there instead”Mineral or cement-bonded board on the outside
Section titled “Mineral or cement-bonded board on the outside”Hard, impact-resistant, non-absorbent. And it improves the logic of the build-up too: if we already have a board that provides hardness, let it be the outermost layer — not the insulation.
The common counter-example, which we do not recommend
Section titled “The common counter-example, which we do not recommend”OSB outside, polystyrene on it, mineral wool between the sections. That puts the layers in exactly the wrong order: the hard material inside, the vulnerable one outside.
If the client’s expectation is low cost, we will design it that way too — but they have to know what they are taking on: a façade sensitive to impact and an absorbent layer on the cold side of the structure.
Inside is a different matter
Section titled “Inside is a different matter”The internal layers are on the protected side of the vapour control layer, where the risk from organic material is lower. Fire and acoustics still decide there, though.
What this principle really says
Section titled “What this principle really says”It does not condemn materials; it assigns them a place. The same board is a good choice in one plane and a bad one in another — the difference is which side of the vapour control layer it is on.
The point
Section titled “The point”Only materials that can bear being left there for decades go into a closed structure. Absorbent organic materials are not such materials — not in an external wall, at any rate.