cavity wall question

Hello,

When building a cavity wall, how deep should the cavity go, i.e. how far below ground level?

I ask because you may remember I was posting about removing asbestos floor tiles. I wanted to remove them because the concrete floor beneath them was not level and this had caused some of the tiles to crack.

I have now safely removed the tiles thanks to all your help but I have found that the concrete is cracked beneath them. In some places this is because the central heating pipes were buried directly into the concrete and over the years the heat must have caused the concrete and tiles to crack. BTW I have read that some people worry that concrete causes pipes to corrode. There doesn't seem to be any leakage but the copper had turned green. Does that mean anything? I realise it is green will be copper carbonate from a reaction with the concrete but would it have affected the integrity of the pipes?

My plan is to replace these pipes with ones that are lagged.

But I have digressed. What about cavity walls? Well, there is a crack in the concrete in front of my patio door. I have chiseled away the concrete and there are no pipes underneath, so it's a bit of a mystery why it has cracked. I have noticed that there is concrete up to a course of bricks, then a gap full of rubble, sand, and concrete dust, then another brick course. It seems to me that this would have been the inner wall and the cavity, had there been a wall there, only there isn't because there's a door instead. I hope that makes sense!

Should the cavity gap be full of rubble? Should I remove it and what should I put there instead? Should I plug it with polystyrene and then concrete over the top or can it remain full of rubble?

Thanks in advance.

Reply to
Fred
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I'd remove the rubble, there is small risk that it could form a damp bridge. If you have some suitable insulation put it in the gap. Think about damp proof courses and not bridging the cavity when you concrete over.

Reply to
Dave Liquorice

That's what I was thinking, thanks for confirming that. I don't have anything suitable to hand, so what do you suggest: a small sheet of polystyrene from Wickes? Thanks again.

Reply to
Fred

It is always best to keep the gap clear of any debris, rubble, muck, brick bits, everything else. The cavity is there to stop damp coming through the walls and to keep an air gap for the walls to breath and sweat any moisture away.

Damp comes from both inside and outside, so the cavity helps to keep both skins of the wall dry by allowing air to travel through from the outside air bricks and any inside vents. It should have a natural chimney effect to the eaves of the roof space (warmer air rises naturally, hence the chimney effect). So best to keep the cavity as clear and clean as you possibly can.

Reply to
BigWallop

What you said makes perfect sense but if the cavity is filled, then there will be no chimney effect.

The cavity of the walls is filled with some sort of white substance. It is like a lump of foam but it crumbles on touch. I think they drilled holes into the breeze blocks and filled the cavity through those, hence this section in front of the door was never done. I agree it could become a bridge for damp so was considering polystyrene and some DPM.

Reply to
Fred

Best to leave it empty unless you have proper cavity wall insulation material, like the rest of the walls have. Something like this

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help you find out more about the materials that are used for this purpose. Quite an interesting site to read through also.

Reply to
BigWallop

So you wouldn't recommend cavity wall insulation, then?

MBQ

Reply to
Man at B&Q

So you wouldn't recommend cavity wall insulation, then?

MBQ

In some cases. No, I wouldn't. It is possible to create more problems with indoor condensation and outside retention of water, if the job of filling the cavity is not done properly. A true cavity wall expert is also looking at the ventilation from inside and outside the property. If there is no adequate ventilation, things like cooking, washing and even breathing would build up loads of moisture into the air inside the building. Blocking the air flow through the cavity may actually stop a lot of the moisture from escaping through the inner skin ventilation of the walls. In such circumstances, extra ventilation, properly installed, may have to be advised along with the insulation works.

So don't go thinking that the cavity isn't there for a worthy cause. It wouldn't be designed into a building if it wasn't actually doing something purposeful. Who wants to pay for a second skin of brick or block if one skin would be enough. It is there for an important purpose and should be treated as an important part of the whole structure of the build. Don't think it can just be filled with any old water resistant material and Bobs your proverbial Uncle. Cavity wall insulation is also designed to allow air to flow past it or through it, all be it a much slower flow of air, but it still allows a flow of air through the cavity. If it was hard packed, it would create a worse bridge between skins and cause rot and damp to run rife through the whole house.

So. No. I wouldn't advise cavity wall insulation in all cases. I would also advise that if you would like such works to be done on your property, that it be done by properly trained persons, with the proper materials and equipment.

Reply to
BigWallop

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