External wall vents closed off - WHY?

There are basically two ways of getting rid of condensation: better ventilation or better insulation (or both.)
Traditionally Britain had lots of cheap coal and wasted it on open fires. That produced LOTS and LOTS of ventilation (and cinders for subsidising the brick makers but that is another story).
In the 1930's new concepts of comfort were expected and maids to attend to a fire in every room got expensive. So houses were built with rooms with no open fire (but cavity walls came in improving insulation). People started worrying about occupants not getting enough "good fresh air" and ventilation bricks started to be put into all rooms with out an open fire.
(My family built a home in the 1950's with primitive central heating and cavity walls incorporating an inner leaf of hollow breeze blocks - there were regulation ventilation bricks in nearly every room. My mother promptly blocked them all but it was still freezing indoors in the winter and the condensation running down the single glazing used to freeze the curtains to the panes!).
Since then houses have increasingly been better insulated in walls, windows and ceilings; condensation is rarely seen BUT for inside fitted cupboards.
If the house if full of warm moist air that percolates into a cupboard, perhaps full of books or blankets; when it makes its way to the outside wall, the wall temperature will fall low enough for condensation to form.

Personally I would block the air bricks and see what happens. If the house smells musty and bronchitis / asthma breaks out, caused by spores in the air from the black mould growing in the cupboards, then further thought will be needed.
Extractor fans in the bathroom and over the cooker in the kitchen are a must. Modern bedroom windows should be fitted with "trickle vents" as sweaty breath is also a major source of overnight moisture. Throw the windows open in the morning to give the house a five minute air change - there is little heat stored in the air and a morning airing will NOT reduce the heat stored in the structure by much.

Best of luck, Harry.

PS Do not block air bricks allowing ventilation under a wooden suspended ground floor - falling through floors, rotten through moisture rising from the ground, is not recommended.
PPS Beware of blocking a vent intended to provide air to a gas appliance, that needs to get air for combustion from inside the room; dying of carbon monoxide poisoning is also not recommended.

Picket_Fence wrote: »

Hello. I think my query is sort of linked to this topic. I have a 3 bed semi built in the 1930s. In each bedroom and the bathroom there is a vent (approx 150mm x 200mm) in the upper corner of the external wall.

What are these for? Do they still serve any purpose?
Can I block them? I have one that is very drafty particularly when we have a cold northerly wind.
An insulation company has checked and told me I already have cavity insulation - I'm not 100% convinced. Should insulation eliminate the draft or do the vents go right through the cavity and external wall?

Any thoughts appreciated.

Cavity wall insulation requires building regulation approval. You can check for old applications by calling your council or by checking online if they support it. When you bought the house it might have shown up in the solicitors searches too.

I'd assume the upstairs vents are for ventilation. I wouldn't expect the insulation to be installed blocking the vents, but I'm not an expert. If there is a draught it clearly isn't doing so.

In older houses vents are also often found near ground level to ventilate the underside of suspended wooden ground floors, don't block them or the floor joists can rot!