Ventilation of Dry Lining Cavity in Old Property

[pbstev]

Hi,

I have just bought an old house with solid brick walls which has been suffering from lack of ventilation (mould on walls). Having stripped the walls I am going to fix battens to the wall, use counterbattens to form an air gap and then use a Kingspan K18 Insulated plasterboard panel to finish off,

I need to ventilate the cavity between the insulation and wall to allow it to breath but I am not sure if I can ventilate into the room by using vents in the plasterboard.. Does anyone know if venting into the room is possible so that the warm air dries the cavity or will this cause more condensation? Does this mean I have to vent to outside (tricky since the walls are 327mm wide?)

Thanks

 

[goldeagle]

I need to ventilate the cavity between the insulation and wall to allow it to breath

Why?

I wouldn't recommend fitting the cavity into the room - the warm air will just condense on the cold solid wall causing you the same problems, plus you will make the insulation less effective by losing heat.

Personally I wouldn't worry about venting at all - if you look at Kingspan/Celotex installation guides there is nowhere that says you have to vent the 'cavity', just that you need to leave an air gap. You should use a vapour barrier on the warm side of the insulation to prevent warm air getting through and condensing.

I will be doing something similar soon in a solid wall building which has had bad condensation problems in the past. I plan on battening the walls with treated timber, and also use DPC behind the battens to ensure potential damp cannot pass on to the timber. If a little bit of moisture does end up on the walls I'm not going to be too worried.

Of course if you have other damp problems you should get this resolved before insulating the walls

 

[bernardgreen]

Unless you create a perfect vapour barrier between the room and the wall you will get condensation on the wall. That moisture will need to be removed otherwise it will collect as dampness or in extreme conditions as puddles on the floor between wall and plaster board.

Then there is the moisture that comes through the wall from outside.

The basic rule is that any cavity has to be able to breathe or be totally sealed to prevent any ingress of moisture or vapour into the cavity. If you cannot ensure that seal then venting is essential.

A neighbour has installed mechanical ventilation to extract air from the cavity between insulation and an outside wall to prevent condensation in the cavity of vapour from the room and remove vapour from evaporation of moisture coming through the wall. It is an extreme case as the wall is 3/4 below ground level and therefore damp.

 

[pbstev]

Thanks for the responses, I am finding it all very confusing as everywhere provides different advice!

The wall is cold and I am sure the moisture is coming in from the outside. I was wanting to dry line because the wall had wallpaper on and by removing this some of the lime plaster has come away. Instead of finding a specialist who can repair/skim the lime plaster I thought the dry lining would be a good idea and by adding the insulation it may make the room warmer.

I am happy just to use a foil backed plasterboard with no insulation but with this being non porous it would just trap the moisture in the air gap behind. The way I was thinking is that by providing vents into the room at the top and bottom the air flow would dry any moisture out but I appreciate the warm air hitting the cold wall could also form condensation. This is where I am confused because this is not different from the solid wall drying naturally into the ventilated house. So is using ventilation grills at the top/bottom into the room a good or bad idea?

 

[pbstev]

One other thing I could have said is that there is currently no air bricks to the outside (this is a first floor gable wall) and installing air bricks to the outside is going to be very difficult and not really an option.

 

[bernardgreen]

If there was lime plaster on the wall then lime plaster may be the best option to allow the wall to breathe. Then use a breathing paint on the plaster.

Lime plastering is no more difficult than ordinary plaster, if you can get a smooth finish with normal plaster then you can with lime plaster. Some may say a slightly rough finish is more attractive than a perfect smooth finish.

I managed to get a reasonable finish on my first attempt at plastering. I was using lime with horse hair on two walls of my cottage.

So any reasonable plasterer should be able to get a good finish with lime after a bit of trial and error.