Interstitial condensation - how DOES it work?

[Tim Watts]

OK -

I have a dormer end wall that's as manky as s**te.

3x3" verticals at various spacings between 1-2 ft. Random noggins.

Dodgey nailing and many previous repairs and extra bits of wood nailed in. Needs a bit of stiffening before it gets a new interior skin.

It now contains 50mm celotex between the beams and I aim to put another

50mm over the inside.

The obvious solution would be to plate it with ply, glued (PU foaming glue) and screwed.

Then cover that with celotex, tape joints and then plasterboard.

The outside is covered with tyvek type breathable membrane and hung concrete tiles.

Could doing this lead to interstitial condensation?

The beams themselves can freely breath in and out of the tyvek.

The ply probably won't - because of both the glue and the fact that large areas of ply will be sandwiched between two layers of 50mm celotex.

Obviously not something I can really "try" and see how it goes.

Am I right that this would be a bad idea?

[Martin Bonner]

It will *probably* be OK if you are scrupulous about maintaining a completely vapour-tight barrier on the inside face of the inner layer of celotex. As well as taping the bits of celotex together, you'll need to tape the edges of the celotex to the other walls.

I'd recommend WBP ply.

[Tim Watts]

Thanks - I can get perfect taping inside for sure.

Indeed - that will help.

I guess I'm imagining that moisture will come in from outside via the

3x3 timbers (it will), and travel sideways in the ply forming damp areas. Theoretically moisture could get out teh same way but it's a tortuous route and the ply is not full warm being halfway between 2 bits of insulation.

If it were an impervious material, there'd be no problem.

The beams themselves also absorb moistiure, but they are ventilated on the cold side fully so the path out for any moisture is not tortuous.

Tricky one to get my head around...

My fallback is to what a 6x2 beam along the top screwed through into the columns and leave a void around the back of it so cold side air can circulate all around it. The ceiling would meet the wall via a 45 degree bevel to achieve this.

Not as good as ply for adding stiffness, but would tie the columns together better than now.

[harryagain]

You need a vapour barrier on the warm side of the insulation (ie inside in the UK) This keeps out warm moist air which causes condensation within the insulation when dew point is reached.

It needs to be totally airtight, even a small hole will let air through in windy weather. So lots of attention to taping up. Special attention to piercings for wires. Special attention too if the room is a bathroom.

Any outer membrane should be "breathable". V important. If it gets wet, mould growth can defeat the breathability of it.

[Rick Hughes]

Finish your insulation .. apply vapour barrier (poly sheet) then fit ply Overlap poly by 150mm and any joints to be over a beam so they can be stapled down. Then fit inner celotex with no vapour barrier on inner face.

If you have already fitted ply ... different approach needed.

[Rick Hughes]

Actually test by TRADA proves that holes up to size of A4 in vapour barrier do not cause an issue ... however good practice is to make it air tight.

The comment that roofing sheet must be vapour barrier is 100% vital.

[Tim Watts]

Hi Rick,

Just to be clear: Screw the ply through 50mm of celotex?

Or lay more timbers over the existing beams and celotex between?