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.

On Fri, 07 Aug 2009 20:51:42 +0100, Fred wrote:

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?

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.

--
Cheers
Dave.

On Sat, 08 Aug 2009 10:57:33 +0100 (BST), "Dave Liquorice"
wrote:

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.

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.

"Fred" wrote in message
...
On Sat, 08 Aug 2009 10:57:33 +0100 (BST), "Dave Liquorice"
wrote:

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.

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.


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.

On Mon, 10 Aug 2009 01:31:13 GMT, "BigWallop"
wrote:

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.


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.

"Fred" wrote in message
...
On Mon, 10 Aug 2009 01:31:13 GMT, "BigWallop"
wrote:

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.


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.


Best to leave it empty unless you have proper cavity wall insulation
material, like the rest of the walls have. Something like this
http://www.nationalinsulationassocia...itywall .html
will help you find out more about the materials that are used for this
purpose. Quite an interesting site to read through also.

On Aug 10, 2:31*am, "BigWallop"
wrote:
"Fred" wrote in message

...

On Sat, 08 Aug 2009 10:57:33 +0100 (BST), "Dave Liquorice"
wrote:

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.

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.

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.

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

MBQ

"Man at B&Q" wrote in message
...
On Aug 10, 2:31 am, "BigWallop"
wrote:
"Fred" wrote in message

...

On Sat, 08 Aug 2009 10:57:33 +0100 (BST), "Dave Liquorice"
wrote:

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.

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.

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.

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.