Airbrick advice please

[Hooo]

I've been poking around under my floorboards trying to stop the small furry visitors getting in, and I noticed something odd about the airbicks.
There are two at the front of the house that vent to the cavity, but one has a brick removed on the inside skin so that it vents right through to under the floor. It looks like there used to be a brick there, as there's mortar round the hole. There was a gas fire in the front room (since removed), would they have taken out the brick to ventilate it?
From browsing the forums it looks like I should have several airbricks ventilating under the floor, but it looks like I have only one, and that didn't used to vent under the floor.
I had the cavity insulated with rockwool last year, and they kept the airbricks clear.
Someone built a solid floored extension to the side and rear about 30 years ago, so if there were any airbricks there they have been covered up.
It's semi-detached, so no ventilation to the other side.
The floorboards were T&G, although there's quite a few gaps now since the new plumbing and electrics went in.
From the forums I get the impression that I should have rotten joists from the lack of ventilation, but it all seems fine under the floor.
It's 1930s, brick outer skin and breeze inner.

So should I open up the second airbrick right through, or fill in the missing brick?
I suspect that the visitors are getting in via the cavity, so I want to block them, would a wire mesh over the missing brick be OK?

 

[JerryM]

sounds like you are lucky in one respect - the joists

the air vents to the under floor are normally crucial. it does depend on how much moisture there is (they are intended to stop the wood getting above 20% saturation for more than 8 wks). i've not come across an "old" house with wood floor where they are not needed.

i would say that u would be wise to keep them.

there is no problem putting a vent on the inner leaf or covering with mesh.

the fire is irrelevant to the air vents for the under floor void.

there should be 1 single bricks worth of air vent every 2 to 3 metres of external wall.

 

[chessspy]

hi, hoo (who?)
It sounds like the air brick originally had a clay sleve to vent through to under the floor, (that would explain the mortar which would have been round the liner not a brick) there would not be any point in venting the cavity as it was designed to be a static air pocket and insulate, the fact that it didn't really work properly due to convection currents is neither here nor there, ( that's how and why filling the cavity with foam does, it keeps the air still (dry air is a nearly perfect insulator))
And yes you should have more air bricks, but if you have gaps in the floor boards here and there then there is plenty of unplanned ventilation going on.
clear any air bricks you have, reinstate the sleves and put in vermin mesh
job done, cost c £20

 

[Hooo]

So I should open up the second airbrick so that it ventilates under the floor?
Any idea why it was built that way in the first place? The inner skin is just continuous brick over the airbrick. It looks like it was built that way, and hasn't been filled in at a later date.
Is it OK just to knock out the brick, and is there an easy way of doing it? It's going to be a very awkward job, as the pillar that supports the joist is in front of where the hole should be.

 

[chessspy]

Hi, hoo,
If there is no evidence of rot or fungal ingression, I wouldn't bother.
If you want to be up and doing have a look around, I'm sure you'll find something.

 

[JerryM]

i'd go along with chessspy's advice too - if it's not broke don't fix it

it must have been intended to ventilate the cavity only.

i know when coal fires existed a lot of rooms had internal vents but all that i've seen were sleeved so i don't feel it has anything to do with that.

 

[chessspy]

Hi, jerry,
I don't know for sure that the cavities were never vented, but as far as I am aware the whole idea was to seal the cavity and make it air tight, that's one reason for closing the cavity at the eaves on top of the wall plate in between the roof joists, and a bloody pain it was too.
I would again suggest, that the air brick in question was sleeved, either by a terracotta sleeve or by slate made up into a sleeve which was often done as sleeves cost money and there were usually plenty of old slates about.
that would account, (as I suggested before) for the mortar round the hole making it look as if a brick had been removed.
The idea of a sealed air filled cavity was a good one, except for the convection currents which were set up and diminished the effect, and that is why the foam filled cavities work better, as I also said before.

 

[JerryM]

apologies chessspy & Hooo - i've not been clear enough - i was referring to the other vent than the one i think your referring to - the one described as "The inner skin is just continuous brick over the airbrick"

i don't think it matters i think our collective advise is put a new sleeve in the open vent and ignore the closed vent (albeit i amazed the joists are OK with just one single vent) - i guess those holes in the floorboards are pretty cold in the winter.

 

[chessspy]

Hi, jerry,
Aha I get it now.
yes I bet. I insulated under the floor of my place while I had the floorboards up, costs almost nothing, (ball of garden twine, staple gun, (£5) and the cost of the fleece, (glass wool) )
my heating bills are almost nothing.

 

[Hooo]

Don't notice any draughts through the floor boards, but then there's only one airbrick to ventilate the whole space
I'd pretty much decided to leave well alone. It's lasted 70 years as it is, so it must be OK.
I've never heard of insulating under the floor, do you just put the stuff between the joists? Isn't there a risk of damp in the joists then, due to lack of ventilation?

 

[chessspy]

Hi, hoo,
We used to do it to timber framed houses mostly, as it goes.
But I decided to do it to mine,
so what I did, is to staple the rotproof twine to the underside of the joists every 12" or so in lines to support the glass wool, and then just lay it in between the joists
the underside of the joist is open to the ventilation, and the air can travel through the wool, albeit slowly.
If it doesn't rot the roof rafters It can't very well rot the floor joists can it?
25% of heat is lost through the floor in a suspended timber floored house.

 

[JerryM]

the insulation would be well worth it in terms of heat saving (for most houses there is not enough depth to get underneath to fit and taking up the floorboards is too much hard work).

the insulation won't affect the risk of dampness in the joists (as long as u don't cover it say with plastic or similar). the method chessspy suggests or similar would be spot on.

you've either got dampness their or not and its sounds like your single vent is doing just fine.

 

[Hooo]

Insulating under the floor sounds like a good idea, wish I'd heard of it before I sanded mine. There's no way they're coming up now, they're softwood and it'd trash them.