Advice needed – air bricks / subfloor ventilation

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18 Feb 2015
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United Kingdom
Hi – I’m slowly renovating a small 2 bed Victorian terrace and need some advice about air bricks and the number I need at the front of the house.

I’ve read up on all the reasons for having good air circulation in the sub floor of an old house and get it, I’m just confused as to how many vent points are adequate, whether I need to add any more at this stage of the project and who to approach to carry out the work if i do, given our new floorboards are now fitted and access to the subfloor from inside the house impossible

My concerns began with our pre purchase survey, it was a bit of a cut and paste job to be honest and stated the obligatory ‘inadequate sub floor ventilation… possible sub floor rot… better get an expert in to check blah blah’. A clause i’ve since learnt is common with surveyors wanting to cover their own back without really having to check anything.

As mentioned, we’ve since had the old (broken) floorboards ripped out and replaced with new / reclaimed timber boards, subfloor and underfloor insulation. When the boards were up and the joists exposed, we checked for rot and there was nothing.

We currently only have one air vent in the front of the house (centred below bay window). It’s a slightly strange set up in that the vent sits slightly above ground level outside, but feed air into the sub floor by a very rudimentary gap in the brickwork meaning air can get under the floorboards:





I presume they felt this was enough when the house was built in 1895… but I’m wandering whether I have to add more before the skirting boards go on and we can no longer access the brickwork (from inside at least)??

If possible I’d like to avoid adding any more in the front wall.

Another option is to add in a vent/grill under the front door threshold as we’re having this door replaced shortly:


Air bricks at the rear of the house are adequate (I know you need both).
We have no dwarf walls or other obstructions in the sub floor.
Other houses on our street are a real mixed bag, some have 1 air brick, some have 2, some even 3!
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Front door drill a line of 1/2" holes through it. That window bay, these are well known to be rotten, shallow foundations , indifferent damp course. That air vent is not Victorian, its later, I suspect it was put in to cure the damp bay. I would chisel away the lower inner brickwork, so as to put a chamfer on the brickwork to help the air flow downwards. And to stop the room having a cold spot on the wall insert a piece of 2" foam across the inner side of the hole, to just allow for 3/4" of render on the inside across the foam and edge of the bricks before plastering.
If you have no visible signs of damp or rot, if there are air bricks at the back of the house, and if air can flow beteen the voids under front and back rooms (via gaps in the under-floor brickwork), then I wouldn't worry too much.

Another one under the front door threshhold is never going to hurt.


You "wouldn't worry too much" - well i would.

Why do you think other houses in the street have 2 or 3 air bricks?

Can you see any possible damp issues with the bay render? Or the condition of the bay cill?

Did you notice that the first, bay joist is close to the wall & obstructing thro ventilation from the air brick?
There is a similarly positioned joist behind the front door threshold.

FWIW: What are these "gaps in the underfloor-brickwork" you talk about?
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I would generally expect to see TWO air bricks at front and same at the back.

But, like others, I wonder why you had all the advice about ventilation and then chose to ignore it and put in a new floor.

Ventilation is not very localised and so the only advantage of something under the front door is to increase total ventilation.

But the real advice would depend on the damp proof arrangements for the house. At that time a layer of slate was common as well as a row of harder ( engineering ) bricks.

In my current house, in which I've had all the floors up to insulate, the walls between rooms are constructed with gaps in the bonding, beneath floor level, to allow air flow from one end of the house to the other. I was suggesting the OP checks to see if this is the case. This would also answer the question about the hallway. if it isn't then he doesn't have through ventilation and the current arrangements are probably not adequate - however, I maintain that the first question to answer is whether there is actually a problem. It's a shame this question wasn't posed before the new floor went down...

Of course the render could cause a damp problem, as it's continuous to the ground.

The OP asked why the external vent is higher than the under floor.

That's simply to put the outside above ground level to avoid water entering.

Sometimes you see vents below the ground level, probably because the outside level has been increased after the house was built.

That's bad practice though as water can easily get in !


<Moderators note>
A number of unhelpful squabbling posts have been removed

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