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PostPosted: Mon Nov 13, 2017 12:22 pm 
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We've long had condensation problems with the outside wall of our bedroom. I've just removed the plasterboard and taken out a laughable excuse for insulation. You can see the damp patches on the outer plywood skin of the wall where it had slumped and condensation/moisture gathered. The points of the nails you can see was how the builder had tried to support the old insulation.

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The object near the foot of the wall is expanded foam where a ventilation grill (put in for a previous gas fire) was closed off.

The timber is 100mm deep. I suspect I could put 75mm in, but I have a spare sheet of 50mm Ballytherm insulation so will probably use that. My plan is to push it between the timbers leaving an air gap between it and the outside skin.

Question: Do I need a vapour barrier and how would I fit it? And, of course, any other advice anyone might have will be welcome.


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PostPosted: Mon Nov 13, 2017 4:41 pm 
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Hi,before giving any advice could you tell me what directly on the outside wall of the timber frame?
What I mean by that is what directly on the outside of the ply I can see in the photos.
I can see an air vent that’s been foamed up also.


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PostPosted: Mon Nov 13, 2017 6:39 pm 
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As far as I know, there is some sort of builders paper then a cement screed (possibly on a mesh of some sort); ie no air gap. The ply is probably 3mm.

I put a vent over the door into the room and closed off the wall vent, which had been added when a previous owner installed a gas fire. Perhaps it needs opening again, but having some sort of control to prevent gales.

Having looked at a few web sites (EcoTherm), my current understanding is to use a plasterboard with a vapour barrier on the face towards the outside of the house and leave an air gap between the insulation and the ply.

Q) The insulation I have is the foil-faced variety. If I got some builders paper and wrapped it around the insulation before pushing it between the uprights, would that be of any benefit, or should the paper have air on both sides?


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PostPosted: Mon Nov 13, 2017 6:51 pm 
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steviejoiner74 wrote:
Hi,before giving any advice could you tell me what directly on the outside wall of the timber frame?
What I mean by that is what directly on the outside of the ply


You haven't answered the question asked by stevie headey

Is this an external or internal wall ?

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PostPosted: Mon Nov 13, 2017 6:57 pm 
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Quote:
You haven't answered the question asked by stevie headey


Actually, this was my reply:
Quote:
As far as I know, there is some sort of builders paper then a cement screed (possibly on a mesh of some sort); ie no air gap. The ply is probably 3mm.


True, I didn't state it was an outside wall (Oh, I did. First line of the original post). It is.


Last edited by headey on Mon Nov 13, 2017 7:26 pm, edited 1 time in total.

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PostPosted: Mon Nov 13, 2017 7:14 pm 
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You’re always going to have problems with this. If a mesh has been nailed directly onto the ply then rendered then it’s going to draw moisture onto the ply then into the insulation and studs and give you damp and mould problems. A vapour barrier won’t do anything,it’ll just make the problem worse as the wall will become saturated.
When rendering onto a timber frame you need to fix battens vertically then fix more battens horrizontally then renderlathe then the render.
However in 99% of buildings you have the timber frame,50mm cavity,then blockwork,then render.
Weep vents are installed into the blockwork to allow any moisture to escape.



For this message the author steviejoiner74 has received gratitude : headey
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PostPosted: Mon Nov 13, 2017 7:37 pm 
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Each time I work on this house I seem to find something wrong. I'm pretty sure the render is directly on the ply. This means there may be no barrier between the outside and in. Perhaps the builder put an impermeable membrane on top of the ply to stop any moisture ingress. However, that would not deal with any moisture trying to get out.

Each 'box' visible in my photos is therefore closed.

Q) Would there be any merit in fitting weep vents top and bottom of each 'box', vented to the outside?


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PostPosted: Mon Nov 13, 2017 9:29 pm 
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headey wrote:
Each time I work on this house I seem to find something wrong. I'm pretty sure the render is directly on the ply. This means there may be no barrier between the outside and in. Perhaps the builder put an impermeable membrane on top of the ply to stop any moisture ingress. However, that would not deal with any moisture trying to get out.

Each 'box' visible in my photos is therefore closed.

Q) Would there be any merit in fitting weep vents top and bottom of each 'box', vented to the outside?


Tbh you really need to make a small hole on the outside wall to expose what’s underneath and see the make-up externally. If the mesh is nailed directly to the ply then the moisture will travel in,hit warm air then condensate resulting in a damp,mouldy wall.



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PostPosted: Mon Nov 13, 2017 11:47 pm 
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Quote:
If the mesh is nailed directly to the ply

Many years ago I`removed a patio door at the back of the house and replaced it with a low wall and a window. I built the wall to the same pattern I saw when I removed the door: wooden frame->ply->building paper->mesh->render (although I had bought 18mm ply, not knowing what was actually there). Consequently, I'm pretty sure the construction will be the same at the front.

Amazingly, this is only one of lots in the neighbourhood built in the same way and which have been here since 1972. They may be crap, but they still stand.

I cannot completely re-do the outside of the house, not least because it's a semi, so I'm going to have to see what I can do to mitigate the problem. I'm tempted to ask my local builders merchant for a small vent I can insert into the front wall just above the bottom piece of wood (possibly in all the cells I can see). I think it was EcoTherm that suggested one only needs a lower vent if the wall is not subject to direct rain (and ours is a low bungalow with a big overhanging roof). I'm tempted to go with that solution for the moment and see how it works out.


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PostPosted: Mon Nov 13, 2017 11:57 pm 
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Using rigid insulation is a must when redoing.


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PostPosted: Tue Nov 14, 2017 3:10 pm 
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Quote:
..rigid insulation is a must...

Like this, I hope.

I've bought plasterboard with a foil face, but I'm assuming the foil face of the insulation is also a vapour barrier.


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PostPosted: Tue Nov 14, 2017 3:56 pm 
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Fit a vapour barrier before plaster boarding.

Why not hack the render of and fit cedral or similar cladding


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PostPosted: Tue Nov 14, 2017 6:44 pm 
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a vapour barrier is a must in timber frame construction but in your case it’s going to make the problem worse as it’s likely you have penetrating damp.


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PostPosted: Tue Nov 14, 2017 9:02 pm 
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Thanks for all the advice. Seems to be a bit of a difference of opinion (cotswold saying 'use a vapour barrier', stevie saying don't). The plasterboard I've just screwed up (not yet sealed in) has a foil backing, as, obviously, does the insulation.

I've bought some weep vents for brickwork. I plan to chop off the backs, as they are too long, and stick them into the outside render (vents pointing down!) with silicone.


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PostPosted: Fri Nov 17, 2017 11:37 am 
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If anyone is still watching this thread - I drilled into the outside wall to install the vents and confirmed what I suspected about the construction:

cement render -> weldmesh -> building paper -> plywood. Absolutely no air gaps!


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