Do cavity walls require airbrick ventilation ?

[Little Plumb]

Hi, I live in a 3 storey block of flats built circa 1970. It's flat roofed and all the floors are solid.

3 of the flats (2 second floor, 1 ground)are reporting damp problems which could be condensation, ie mould on outside walls usually near window.

My question relates to the outside walls which are quite large areas of brickwork, without an airbrick in sight. If damp gets into the cavity I assume because of the flat roof & solid floors there will be nowhere for it to escape to.

I know it might not ultimately solve the damp problems in the 3 flats but should airbricks be installed in the walls to allow the them to breath?

 

[^woody^]

Cavity walls are not ventilated

The whole concept of the cavity wall is that moisture does not travel to the inside leaf. Your mention of mould is an indication that the problem is condensation.

 

[Little Plumb]

Thanks Woody I didn't know that. I believe in the past the residents managment committee have had air bricks inserted to try to cure damp problems.

As regards ventilating the cavity, I came across this site.

http://www.schrijver-system.com/

It appears to offer a cure for damp problems by what appears to be a cavity ventilation system.

Anyone come across this system before?

 

[masona]

Before you do anything, have a good read here first

 

[^woody^]

As regards ventilating the cavity, I came across this site.

http://www.schrijver-system.com/

It appears to offer a cure for damp problems by what appears to be a cavity ventilation system.

My first impression is that it is a system set up for specific problems in european buildings, and has no relevance to UK construction or climate. Alternatively its just a pointless system - sold along the lines of the other miracle cures in the Sunday supplements.

I'm not sure what problems it is supposed to cure, as cavity wall construction does not have damp problems in the first place. If there are, then it is due to other causes - non of which will be cured by ventilation.

A cavity gives a bit of insulation from the air within it. If this is ventilated, then this insulation is lost, the internal leaf is even colder, and your condensation problem worsened.

 

[Little Plumb]

I notice from Masons link to the condensation site, it mentions

Consider applying a surface finish to outside walls to prevent rain penetrating them. Either a clear waterproofing finish which can be brushed on or a paint/textured finish .

I assume this means rain penetrating into the cavity. If rain does get into the cavity what happens to it and wouldn't it make the cavity, with no ventilation, a moist area?

 

[JohnD]

"Rectangular holes are drilled out of the exterior wall 14ins above ground level at approximately 14in intervals, into which hollow ceramic bricks are cemented. The cold spots created by air being forced into the hollows attract damp, which condenses and evaporates. "

hmmmmmmm.

 

[freddiemercurystwin]

Cavities are ventilated (nowadays anyway) via the weep holes normally installed above cavity trays, moisture can penetrate through a single skin of masonry either by being absorbed through it in times of continuous damp weather or possibly though gaps in the joints (I've seen this on many an inspection of new blockwork walls). If they were 100% impervious you wouldn't need cavity trays or drips on the wall ties for the water to drip off! Hence cavity walls do need ventilation of some kind. Its difficult for anyone to say categorically whether this would solve the problem without surveying it and even then I doubt a builder would give you a guarantee!

 

[^woody^]

Weep holes are not for ventilation (and provide negligible air flow due to cavity air pressure) and no-one says that the external leaf is impervious, however the concept of the cavity wall is that the outer leaf can get saturated, but no water gets to the inner leaf.

Any moisture which gets to the back of the outer leaf runs down the back of the bricks. But this is very little - most is held within the bricks and mortar, and is drawn out by evaporation on the outside face.

Very little is evaporated in the cavity itself, due to factors relating to the design/concept of cavity wall construction, so no ventilation is needed to dry it off.