New air bricks... how many and where?!

[bpk101]

As part of the next stage in our refurb of a 2-bed victorian terrace (circa 1880) we'd like to tackle sub floor ventilation and damp proofing in the front bay area of our house.

Our structural survey highlighted 'inadequate sub floor ventilation' and ground floor damp in the front bay area caused by the lower bay having been rendered poorly and a raised external concrete hard-standing blocking the sub vent and bridging the DPC. Here's a photo of our current bay area:

http://bpkersey.com/images/front-bay-1.jpg
http://bpkersey.com/images/front-bay-2.jpg

Our builders plan is to completely gut the concrete hardstanding and replace it with a new level hard-standing lowering it to the required depth to stop it bridging the DPC. Is this the right course of action?

Our survey also recommended installing more air bricks (even one under the front door sill). Neighbouring houses all seem to have 2 or 3 in that front bay area as seen here:

http://bpkersey.com/images/neighbouring-front-bay-1.jpg
http://bpkersey.com/images/neighbouring-front-bay-2.jpg

... but our builder says that isn't necessary and we only need the one?!

Any thoughts on this, should we get more air bricks installed as per the survey or listen to our builder?
And should these bricks enter and ventilate beneath the internal floorboards, or should the vents be above floor level with a visible internal grill on the wall below the bay window? I presume below floorboards to ventilate the timbers but our builder mentioned above floorboards!

Again... confused about air bricks and where they should go?!

 

[oldbutnotdead]

Is that vent brick in the 1st picture above internal floor level or below? (if below then I don't see the problem with the hardstanding blocking the vent?). And are you sure there's actually a DPC to be bridged (if original it'll be slate or a shoddy looking bituminous layer). I'm wondering whether your survey is like one I had done- got a refund on it after pointing out to the 'surveyor' that there were unlikely to be rusted wall ties (as per his report) in a 200 year old house with no wall cavity.

Air bricks (in older houses on the ground floor) are mostly for ventilating the subfloor to prevent the timbers from rotting (they get damp from the walls etc but as long as there's air movement then they dry out and last for ages). Modern houses (being better sealed against draughts) sometimes require airbricks above floor level if you're using a woodburner or some gas fires

Current building regs require openings of either 1500sq mm/metre of wall OR 500 sq mm per square metre of floor to be ventilated (do both sums and use the larger number). Modern plastic airbricks are rated at 6000 sq mm. Or a 20mm diameter hole gives you 314 square millimetres

It's a good idea to have an airbrick or 2 under the front door- usually the hallway floor is a bit of a blind avenue as far as ventilation goes.

If you're in a flood-prone area then think about sealable airbricks (they won't help if the water table rises but they can reduce damage from surface water flooding)

And if you are ferreting round in the subfloor, rake any loose junk out (typical lazy mans trick when hacking plaster off or knocking walls out is to dump the spoil under the floor).

 

[theprinceofdarkness]

You don't say if they are cavity walls or not. I think the problem is more of through ventilation. Often the kitchen floor is solid leaving only the bit behind the dining room as a possible "air outlet". Also make sure that any walls that cut the house in two (front to back)have air ways left open in them. Normally this is done by central heating installers.
Frank

 

[bpk101]

I'm not sure where that external air brick in the 1st picture leads to although it's position on the outer wall is higher than the internal floor level ... but there's definitely no sign of it exiting on the interior wall above floor level.

Is it possible that this air brick is above ground on the outside and below floorboard level on the inside, using some sort of periscopic channel??
I'm at the house this afternoon and will check if i can visibly see where that brick exits (the electricians are in and some of the floorboards are up).

We have solid walls, not cavity.

Air bricks at the back of the property have been blocked by a kitchen extension built around 15-20years ago. We are having a new kitchen extension built in May and will have the rear property ventilation issues solved then to achieve air flow right through the house.

The builders are working on the front exterior (brickwork and hard-standing repairs) next week so we'd like to tackle the front air bricks now.

So... do you think 2 x new air bricks in the bay (LHS + RHS when looking at house), the current air brick in the middle of bay and one more under the front door sill would be the right thing to ask for? As in this:

http://bpkersey.com/images/neighbouring-front-bay-1.jpg

Or is it too many / too little?

 

[oldbutnotdead]

Yes that brick could be venting below the floor internally- probably not via a periscopix device thought, just some chopped brickwork. If that is the case, have a prod about with something (knitting needle?) to make sure the bricks aren't blocked with rubbish between them.

3 air bricks in the bay window is a bit overkill- you'd be better in my opinion having airbricks in the walls either side of the bay (and the one in the centre of course)- these will give more even ventilation under the floor and (if there's a chimney in that room) will improve ventilation in the recess between chimney and outside wall- a favourite nook for damp. Make sure there isn't a dwarf wall between the window bay and the rest of the underfloor (or if there is a wall there make sure it has plenty of holes in it).