Advice needed – air bricks / subfloor ventilation

bpk101

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:




NB...
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!

firefox1956

My 1910 built detached houe has loads of air bricks like yours.
All I would say is you are probaby better with more underfloor ventilation than less.

bpk101

Can the air brick installation work be carried out from the outside of the house though? Meaning can i go ahead and get the new floors waxed and finished and the skirting boards installed and still have new air bricks installed?

Obviously i should have got these vents sorted when the floors were up but we missed the opportunity. Now the new floors are laid :mad:

I think we'd have to go for the periscope vents explained here:

http://www.diydoctor.org.uk/projects/airbricks.htm

TheCyclingProgrammer

I definitely think you'd be best going with some kind of ducting to help the air from outside reach the voice below the floor.

Do you have any air bricks at the other end of the room?

When you had the floorboards up, did it feel like there was good air circulation below the floor? When we had the floorboards up in our house (1930s) we were checking ventilation as our survey said similar things about air flow. Our joists were fine however an air brick to the rear of the room had been blocked by a conservatory. We had a new one put in the back, an extra one in the front bay window and two new ones on the side of the house.

How did you insulate the floor? I'm assuming there was plenty of space below the joists to allow airflow? Did you put a breather membrane underneath the insulation to protect it from drafts, with a vapour control layer between the floorboards and insulation to prevent condensation issues?

bpk101

TheCyclingProgrammer wrote: »

Do you have any air bricks at the other end of the room?

Yes

TheCyclingProgrammer wrote: »

When you had the floorboards up, did it feel like there was good air circulation below the floor?

Yes, pretty good considering the one vent.

TheCyclingProgrammer wrote: »

How did you insulate the floor? I'm assuming there was plenty of space below the joists to allow airflow? Did you put a breather membrane underneath the insulation to protect it from drafts, with a vapour control layer between the floorboards and insulation to prevent condensation issues?

It was fairly basic by all accounts, the floor company put Celotex between the joists (similar to this), laid the ply subfloor on top and the reclaimed floorboards on top of that.

TheCyclingProgrammer

bpk101 wrote: »

It was fairly basic by all accounts, the floor company put Celotex between the joists (similar to this), laid the ply subfloor on top and the reclaimed floorboards on top of that.

Hmm. I'm not an expert so I'm sure somebody more experienced could chip in, but I spent quite a lot of time researching how to insulate a timber sub-floor before we bought our current house.

By introducing sub-floor insulation, you make the room above warmer and the void below colder. This increases the risk of interstitial condensation when the warm air meets the cold void below and if condensation is forming on the joists you could have issues.

The general approach is to have a vapour barrier above the insulation (under the floorboards, lapped up the walls and taped behind the skirting, to prevent moisture from entering the subfloor. The ventilation can reduce the thermal efficiency of the insulation (more so with mineral/fibre insulation I think) so its normal to have a vapour open "breather" membrane below the joists.

Celotex is foil backed which I believe should act as a form of vapour control layer but I'm not sure how effective that would be if its only between the joists - I'd have thought a polythene vapour barrier over the whole lot would have been better, but maybe I'm wrong. Were all the gaps between the insulation and the joists filled for air tightness?

I'd say the more ventilation you can have below the floor the better, if only to minimise the risk of condensation issues.