How to tell if moisture behind dry lining is condensation?

Blisterman · 11-01-2021 1:49pm #1

Have had some dry lining put in to a blockwork wall, consisting of battened out 25mm air gap, PIR insulation between studs and an inner layer of insulated plasterboard. The blockwork is due to be rendered, but hasn't yet been.

A reasonable amount of care has been taken to ensure air tightness at the various junctions.

Nonetheless we spottted some dampess coming through the OSB subfloor at first floor level. We cut a hole through the wall, and the inner face of the blockwork is quite damp.

The builder believes that this is water making its way through the blockwork, and once its rendered, that will be stopped.

My concern is that this moisture is coming from through air leakage inside the house condensing, and therefore rendering will make no difference.

Is there any way of telling whether moisture is condensation or not.

Also, what can be done once the problem is rectified to ensure that the cavity behind the drylining is dried out and the water is not trapped?

mickdw · 11-01-2021 9:05pm

Is it a double wall or a hollow block wall.

10-10-20 · 11-01-2021 9:39pm

Assuming this is a double-leaf wall (which isn't clear), and if I follow you correctly, the make-up of the wall is roughly:

Plasterboard - 12mm
PIR - 25mm (guess)
Cavity - 25mm
Inner Block - 100mm
Cavity - 100mm (you have no cavity insulation?)
Outer Brick - 100mm

At Internal 20 degrees C / 50% RH & external 2 degrees C @ 90% RH.

...then the red astrix is the point where the "condensation risk" roughly sits, and in this case it lands at the inner block. I'd guess that especially if the mortar/blocks are still drying out, you have residual dampness in the wall, increasing the humidity level of the blockwork.

Was a DPM installed and is it sloping away from the inner block?

Blisterman · 12-01-2021 10:18am

10-10-20 wrote: »

Assuming this is a double-leaf wall (which isn't clear), and if I follow you correctly, the make-up of the wall is roughly:

Plasterboard - 12mm
PIR - 25mm (guess)
Cavity - 25mm
Inner Block - 100mm
Cavity - 100mm (you have no cavity insulation?)
Outer Brick - 100mm

At Internal 20 degrees C / 50% RH & external 2 degrees C @ 90% RH.

...then the red astrix is the point where the "condensation risk" roughly sits, and in this case it lands at the inner block. I'd guess that especially if the mortar/blocks are still drying out, you have residual dampness in the wall, increasing the humidity level of the blockwork.

Was a DPM installed and is it sloping away from the inner block?

The buildup is
10mm render (not done yet)
230mm solid blockwork
25mm cavity with battens
100mm PIR insulation between studs
72.5mm insulated plasterboard

sydthebeat · 12-01-2021 10:28am

I assume there's no heating turned on right now??

Condensation is formed when there is sufficient temp difference between a hot and cold surface that water vapour turns to moisture on contact with the cold surface.

If your external blockwork is not yet rendered, it's unlikely you've got your heating turned on, thus it's unlikely the moisture you are seeing now is condensation.

Have you weep holes draining that 25mm cavity to the external?

Where is your vapour barrier in your construction, there doesn't appear to be one.

When the wall is rendered, and the heating is turned on, the bad news is you are probably guaranteed to get condensation issues on the cold face of your insulation anyway.

Blisterman · 12-01-2021 12:48pm

There is heating on, but as all the insulation is on the inside face of the blockwork, the blockwork is cold. I understand it's a backwards way of sequencing the works, and the wall should really have been rendered first. But we are where we are now.

The buildup has been worked out with a condensation risk analysis. There is no separate vapour barrier, although one is integrated into the insualted plasterboard.

I understand some level of condensation is inevitable, but my intention is to minimise the risk by carefully detailing junctions to reduce air ingress from inside.

There are no weep holes in the blockwork. Would this help to ventilate and allow the cavity to dry out?

Calahonda52 · 12-01-2021 12:56pm

10 mm render is not enough, it needs to be scud/base/final.
My guess is that the walls are wet from rain and that they will eventually dry out.
However, one concern is for south facing walls is solar drive

https://www.greenbuildingadvisor.com/article/when-sunshine-drives-moisture-into-walls

Blisterman · 13-01-2021 11:58am

So does anyone know if there's a way of telling where specifically the moisture is coming from? From condensation or from outside?

BryanF · 13-01-2021 2:09pm

Yes. Site assessment required, Installing internal and external sensors over a number of days.

Didn’t you have a thread sometime back where we gave options on breathable insulation systems to avoid this problem?

Metric Tensor · 13-01-2021 8:17pm

You could take a small core out of the blockwork after a few wet days and see if it has a dry section in the middle or if its wet all the way through. It's obviously a fairly destructive way of doing things though.

Have you been plastering/skimming/pouring floors the inside? That sort of work generates a lot of moisture.

Also - is the house urban or rural as this affects wind driven rain.

Magiflo · 18-01-2021 9:09pm

Blisterman wrote: »

Have had some dry lining put in to a blockwork wall, consisting of battened out 25mm air gap, PIR insulation between studs and an inner layer of insulated plasterboard. The blockwork is due to be rendered, but hasn't yet been.

A reasonable amount of care has been taken to ensure air tightness at the various junctions.

Nonetheless we spottted some dampess coming through the OSB subfloor at first floor level. We cut a hole through the wall, and the inner face of the blockwork is quite damp.

The builder believes that this is water making its way through the blockwork, and once its rendered, that will be stopped.

My concern is that this moisture is coming from through air leakage inside the house condensing, and therefore rendering will make no difference.

Is there any way of telling whether moisture is condensation or not.

Also, what can be done once the problem is rectified to ensure that the cavity behind the drylining is dried out and the water is not trapped?

You should be able to get a damp surveyor to test the water gathering to see if it is condensate or water leaking through blocks.

How to tell if moisture behind dry lining is condensation?

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