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We have a problem with mould on an external wall of an upstairs bedroom. There is no airbrick in the wall and the windows do not have that feature where there is a small air gap even when shut.

So, we considered installing an airbrick in the wall, but then had an idea for a simpler/cheaper option. My question is - would it work?

There is a chimney shaft on an inside wall. The fireplace is completely sealed up. Presumably the shaft is still open at the top - at the chimney itself. The idea is to drill holes through to the chimney shaft, either through the skirting board, or at ceiling level.

Would that be as good as an airbrick to the outside, or are there any problems with this approach?

The mould is caused by water vapour created in the home.
Being a bedroom, it is either breath and sweat, or water vapour from the kitchen or bathroom.
The external wall is too cold and is attracting the water vapour, which always moves from warm to cold.
If you make a hole in the chimney or in an outer wall, the water vapour in the air will disappear through the hole/s as will any heat you have in the room and the rest of the house. The problem with holes is that the wind will suck all the warm air in the room/or your home through the hole and it cannot be controlled.

From 2016 all new homes will have to be of Passive House standard, this means they will have to be virtually air tight as part of the Government/EU attempts to save on fuel and power.
This also applies to all new office blocks, new Government buildings are already being designed to this standard.

Your mould is probably caused by turning the heating down or off, warm air holds more water vapour than cold air, when a room cools down the air cools and water vapour held in the air condenses onto the nearest cold surface, usually a window but, if a wall is colder then the wall.

You can solve the problem by keeping the temperature of the room steady, by opening the window for a short time to let the water vapour out (this means loosing some of your heat)or by using a dehumidifier to collect the water vapour, or if the water vapour is coming from the kitchen or bathroom keeping the doors shut. Kitchens and bathrooms should be fitted with extractor fans to move the water vapour from the room to the outside.

So are airbricks a thing of the past now? If so, how do you deal with the water vapour that is in the air? I do take your point about keeping the bathroom door closed and opening windows regularly - I'll do that.

As I wrote ealier:


You can solve the problem by keeping the temperature of the room steady,
by opening the window for a short time to let the water vapour out (this means loosing some of your heat)
or by using a dehumidifier to collect the water vapour,
or if the water vapour is coming from the kitchen or bathroom keeping the doors shut.
Kitchens and bathrooms should be fitted with extractor fans to move the water vapour from the room to the outside.

In the 2016 regulations the EU require forced mechanical ventilation.

From a practical point of view this means an extractor fan fitted in the wall, ceiling or window, that is equiped with an automatic shutter to keep out the wind and preferably a heat exchanger, and is controlled by a humidistat.
An automatic extractor fan may not always come on automatically as there needs to be water vapour in the sensor, best practice is to turn it on, then once the water vapour is moving through the sensor, it will turn off by its self. This means comfortable dry rooms and a big saving on heating as the exchanger will save about 90% of your heat and the motor will last longer, as it does not have to struggle to find air to eject.

Ineville, Perry makes some valid comments. I have a very similar problem where I blocked up an existing fireplace at Ground Floor Level and staining at high level started shortly afterwards on the chimney breast over in the First Floor Bedroom, unoccupied so not due to breathing, sweat etc.. Heat in the room is maintained and there is no significant moisture from Bathroom/Kitchen. I was just about to spend thousands of Pounds removing the Chimney above roof level as I thought it was from a leak. I am now looking at ventilation as a cause.
You could try your first suggestion to drill some holes in the old existing fireplace. Blocked up Fireplaces should have vents fitted as standard to allow ventilation through the flue and to external. If not, it can result in condensation within the chimney flue which can cause staining on the walls. This may or may not be related to your particular problem in the Bedroom, depends if it is localised to the Chimney.
You should provide a series of small holes within approx 1 brick size. You can cover these holes with a standard metal or PVC vent facing from any DIY shop. These should be located at low level in the region of the original fireplace opening above skirting level. You could also fit a capping to the chimney flue externally to prevent rain from entering the chimney. Ensure there is no working fireplace below sharing this flue or you could have smoke ingress!

I wouldn't worry too much about loss of heat etc in this instance, and in any case you could always reverse these very minor works without incurring expense. If you want to control the airflow/heat loss, you could fit a hit-and-miss vent which allows you to open or close the vent. I would keep it open, clean off and paint internally and monitor for 6-12 months before incurring any costs for any more major works.

Hope this is of some use.

Thanks for the tips. I think I'll go ahead as, as you say, it is easily reversible.