Is Plywood breathable?

[Restoration man]

I am renovating a 90 year old house and insulating in someplaces as I go. I've been using sheep's wool and been very careful to make sure I don't throw much in the way of modern Un breathable products into the construction.

I have a flat zinc roof which has been repaired. It's stripped underneath because it had leaked and I was going to insulate with wool again. Then I was going to sheet it with plywood then finish with timber linings (t&g).

Just had a thought though is plywood with its glue construction breathable? I'm can't find a simple answer on the interweb.

Cheers

 

[^woody^]

Yes it is. Or no one would use it in timber frame construction.

 

[John D v2.0]

Everything is breathable to some extent, you just need to put the less breathable things towards the inside and the more breathable towards the outside, or have ventilation behind them.
But as said above, plywood will let moisture through, better than zinc but not as well as normal timber.

 

[tomfe]

Nah, all those layers of glue.
OSB3 is fairly breathable OSB2 is not.

 

[^woody^]

Nah, all those layers of glue.
OSB3 is fairly breathable OSB2 is not.

So it's ok to build a boat out of ply or OSB2? All that glue must surely keep the water out if it's impervious to moisture.

 

[tomfe]

Ply has a Typical resistivity r (MNs/gm) of 1000 (same as concrete) OSB3 400, mineral wool about 1.
You can build a boat out of ply and osb yes.
The inventor of osb made this out of osb. Though it was probably more because he could not because you should.

 

[^woody^]

Is it painted with breathable paint?

 

[tomfe]

I highly doubt it, it will be epoxied and fiber glassed to an inch of it's life otherwise it would swell and turn into day old weetabix (other bands available).

 

[John D v2.0]

A point of note is that breathable and waterproof aren't completely directly related. To be extreme Poly sheet with a pinhole is not breathable or water proof. Roofing membrane is breathable and water proof (under a given, small head of water)

 

[^woody^]

So as long as the op does not epoxy or fibreglass his ply, then it will be breathable.

 

[^woody^]

To be extreme Poly sheet with a pinhole is not breathable or water proof

No, it's defective.

 

[tomfe]

So as long as the op does not epoxy or fibreglass his ply, then it will be breathable.

So you are saying it's it not water proof then it's vapour open?

 

[^woody^]

So you are saying it's it not water proof then it's vapour open?

No I'm saying that as long as the OP does not get promoted to Admiral, and coat his plyboard in navy blue (or any marine themed colour scheme) epoxy fibreglass coating to keep water on one side of it, then it will be moisture permeable for his purpose.

 

[tomfe]

I actually read the op, for a roof ply is a good idea as it will work as a slight vcl but if you're trying to make your walls as vapour open as possible then don't use ply.
There are better products out there the cheapest one being OSB 3 but not great or Panelvent which is very open.

 

[^woody^]

you're trying to make your walls as vapour open as possible then don't use ply.

The main critera would be to prevent moisture getting into the frame in the first place. So the sheathing does not need to be highly permeable.

Moisture will wick through the ply naturally, driven by the higher intenal air temperature. Its a very, very tiny amount of moisture from the air.