Unventilated under-floor cavity

[sidd]

In a part of the house that is probably from 1840, with 40cm thick brick walls, I have floors that are level with the outside, and full height french windows. The only sub-floor ventilation seems to have been cut in the last 30 years just outside the french windows. The timber is all in very good condition, considering it has been covered up for about 30 years, but there is some rot in the oak behind the french windows. I am beginning to wonder whether these vents were necessary and whether they have actually caused the rot, since the vents are in the ground and probably attract rainwater. There is generally quite a large space under the floors.

There is also another area which has rot after all these years, but that is next to a raised level floor in another room.

I had been planning to cut above ground vents in the walls with airbricks and tunnel down at an angle to end up below the floor to ventilate, but I am beginning to think that would be a waste of time. Does anyone have any views?

Either way, I am looking to put new flooring down over the hotch potch of lovely old oak boards mixed with dodgy newish ones. Ideally, I would put a layer of oak boards over an insulating underlay. But I think all insulating underlays are also vapout barriers... again, any views would be greatly appreciated.

 

[Penners]

You should definitely have effective ventilation of the void below a suspended wooden floor. Preferably a cross-flow of air.

If you're worried that water is getting into the ventilation grilles, the best thing is to create some sort of little roof over each grille, to keep the rain out without interrupting air flow. This is particularly important if the grilles are below ground level, in little "wells", which could fill up with water.

 

[sidd]

Penners, yes that makes conventional sense, but having seen Matt Greene's opinions about ventilation, I'm less sure. As it is, the current ventilation consists of 4"x10" holes cut in the york paving directly in front of the french window steps. Clearly it is not part of the original design, and putting little roofs in the paving in the middle of the doorway would make the aesthetics even worse. The rooms like this each have two external walls with one french window in each external wall.

My thinking is that conventional wisdom is at odds with Matt, probably because with the right conditions in old houses his view is right, but for 95% of houses, building regs approach is best.

 

[Penners]

I don't know what building regs have to say about suspended floors, and I respect Matt's views on matters of rot, etc.

Given the conformation of your underfloor ventilation, I could well see that it might provide an easy entry point for surface water, and that this might itself create the conditions for wet rot. And I suppose it's also possible that - if you're lucky - an unventilated underfloor void could by dry enough that rot would not become an issue. The trouble is, how can you be sure of this?

I can only say that, if it were my floor, I'd prefer to play it safe. I would want to ensure adequate ventilation of the void, with the vents arranged in such a way that water ingress could not take place.

By the way, I agree with you about a cushioned underlay. It would almost certainly have a vapour barrier effect

 

[sidd]

Yes. That was very much my view until today, when some installer of engineered floors told me to block up any ventilation I had. I walked away from him quite fast on both counts, but it still set me thinking. And Matt's postings make me really think that core drilling 8x airbricks diagonally down to below the sub-floor to do exactly what you suggest, is questionable to say the least if the building has existed for 170 years without such ventilation.

So I'm looking for some kind of confirmation that reinstating the lack of ventilation is the best thing to do. With an unventilated floor, I definitely don't want to put a vapour barrier down (and I probably need less insulation anyway), but if I do the modern thing, and ventilate properly, then I don't have a problem with putting underlay down.

My CO has said he doesn't mind if I put such ventilation in, but I think he has worked out that I put more time into finding the best course of action than he can justify on council time.

 

[Flyfisher]

Have you seen 'telescope vents' that can be used where floor levels are very near external ground levels?

Some info here that might help: http://www.vents.co.uk/pdf/pufv.pdf

 

[Schoolmarm]

Is that the original outside ground level?

Is that perhaps a step outside that window which has had the floor raised to match? (wrongly)


Matt says to keep wood dry but he doesn't say no sub floor ventilation.

If your current ventilation is letting water in then it needs sorting.

Is there a route under the house to get cross flow of air?

 

[sidd]

Thanks flyfisher. I had seen them. But I think they would be even more disruptive than core drilling. As it is, with the lime plaster, the walls are 45cm thick, and I'm hoping they are solid and not filled with rubble, as otherwise the core drilling will be problematic.

 

[Matt Green]

Greetings.

With no sub-floor ventilation and RH at as close to 100% you can get to, softwood timbers can get to about 23% moisture content, enough for furniture beetle damage, not enough for a fungal spore to germinate. You sir, have an oak floor which is much more durable. I don't have my books in front of me to see what oak will get to at 100% RH and I can't remember off the top of my head.

Check the joists and plates if you can and if you can keep a hatch or some screwed-down boards then that will be much more use in the long run. A totally inaccessible floor is a lot worse than an unventilated one, occassional checking can sve a lot of cash in you start to notice conditions changing under the floor. Please don't smack holes in your building to ventilate a floor that is fine, though.

A rational approach here ladies and gents please- dedicate energies to fixing actual problems.


Matt

 

[sidd]

Schoolmarm, those are some good questions. I think there is an archway there somewhere that the previous owners covered up. But on the other side of the house, which we have had to excavate, there is no evidence of any blocked up airbricks. I assume there is flow of air through the house, as you can see there are french windows on both sides. The only rot we have found is by the left and right windows, and in the middle of the wall next to the kitchen. But none of it is serious, and the floors seem very well supported. The drawing room has had a layer of engineered parquet for at least 30 years, with carpet above that, so there has not been much airflow up into the room. The dining room is mainly original oak boards, so there will have been a bit more flow.

 

[Schoolmarm]

Hmm interesting.

Well Matt says you would be ok with oak if inspected regularly.

What if the extra floors are laid on top Matt?

 

[sidd]

The plan with the floor is to cover it with new 200mm wide oak, secret nailed and nailed through the face with trad nails to make sure they don't bend. It's relatively straightforward to have access hatches inside the window alcoves. The existing boards are c. 180mm oak. I might put some hit and miss vents in the floors round the chimneys to make it look as if the rooms are ventilated for combustion purposes. But if I can use insulation, that would be good.

Thanks Matt. It is hard to step away from conventional wisdom, but the simple fact that there has not been a catastrophe in c. 170 years makes me think you are right. It is sooo ironic that technology made everyone forget the traditional ways of doing things, and now technology is helping us un-learn the mistakes of the last sixty or so years.