I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

.... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please

Anna
--
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repair and conservation
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

"Anna Kettle" wrote in message
...
I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard

PLASTERBOARD???

:-)

- Will the blockboard/plasterboard wall be good and rigid

You're the expert!and will it

G

Mary

Anna Kettle wrote:
I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

never used one but they are a right bugger to install, allow at least a weekend
(fri sat sun) using two experienced people but, really, allow a week all in, for
building work, plumbing in and out, electrics etc. if you are doing it yourself.

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please

3 x 2 cls stud wall would be strong enough and if you complete the
wall with kingspan/marmox infill it certainly would be soundproof.
you could skin the walls with 4mm ply and decorate over it, would
be really strong.

hth

Anna Kettle wrote:
I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

Mock it up with a stool/boxes, to see if you can actually physically get
round the area.

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

IMO, this will be fine.
It will be marginally noisier, and slightly colder in use than a
'proper' wall, but...

Anna Kettle wrote:
I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

Have you taken into account door openings?


... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

The thing is, what's the blockboard going to be fixed to? - how is going to
stay upright?
- you could fix it to the ceiling timbers, but this would mean removing a
section of the ceiling, unless it's upstairs, then you could access via the
loft, if it's feasable...if you have fathomed a way of fixing the blockboard
upright, then yes, a 9mm PB will suffice, although you will need short
screws, 20mm if possible


I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please


it will be as soundproof as you can get for a 34mm wall, IE not very, but
considering the spaces involved, it's unlikely to be much of a problem,
unless you intend having parties in the steamroom.

On Sat, 02 Dec 2006 20:01:41 GMT, (Anna
Kettle) wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please

Anna

Go have a look at breeze blocks at your builders merchants. They are
large, very light and quite cheap.

A wall can be built quickly with them and it will be quite solid and
thin.

EricP wrote:
On Sat, 02 Dec 2006 20:01:41 GMT, (Anna
Kettle) wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking
of using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so
the shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the
sheds? Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please

Anna

Go have a look at breeze blocks at your builders merchants. They are
large, very light and quite cheap.

A wall can be built quickly with them and it will be quite solid and
thin.

They are 100mm wide Eric, she needs it to be 35mm max...studding also ends
up at around 100mm once skimmed

Anna Kettle wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please

Anna

couple of observations:
sheet ply is stronger than blockboard
ply is stronger than plasterboard, which would add sod all to the
stiffness of the wall.
Lining paper will make ply into a nice paintable surface, and is 1mm
thick.

You've got more or less no space for the usual sound deadening
materials there, but for better soundproofing you might be able to get
some kind of foamed rubber sheet a few mm thick in there.

I take it this wall lines up with a joist above such that a line of
many screws at whatever angle will fix it at the top. If not, whats the
fixing plan?


NT

Anna Kettle wrote:

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I would think about fixing some angle metal to the floor and ceiling and
screwing 12mm plywood to either side.
I think it will be better noise insulation than blockboard and plasterboard
and it will be a lot stronger.

Is there any need for fire resistance?

On Sat, 02 Dec 2006 20:01:41 GMT, (Anna
Kettle) wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please

Anna

Hi,

You could use two layers of 'Fermacell' cement board with a small gap
in between:

Like:

http://www.fermacell.co.uk/listSystemsByIds.php?ids=;66;

Maybe a thin lath in the middle of the 1.3m wall instead of a stud.

cheers,
Pete.

Anna Kettle wrote:
I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and
faced with a wall of the shower on the other side.


I'd use either 19mm ply or MDF. Both make reasonable small walls. MDF
needs no surface treatment..juts caulk where it meets stuff (leave a
little gap) as it shrinks a tad eventually. Then paint with emulsion
straight. Sand after first coat. Tile straight onto it.


I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it
be at least reasonable for sound dampening? The wall is only 1.3m
long. Would some other material like plywood be better? Comments
please

Sound rejection of any solid thin wall will not be great. My guess is
20-30dB versus 50-60 for a proper stud..
Anna

EricP wrote:
On Sat, 02 Dec 2006 20:01:41 GMT, (Anna
Kettle) wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as
well. After lots of entertainment as I drew the layout on a large
sheet of white paper and walked around to make sure that there is
enough space to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink
out of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and

Go have a look at breeze blocks at your builders merchants. They are
large, very light and quite cheap.

A wall can be built quickly with them and it will be quite solid and
thin.

I just burst out laughing imagining breeze blocks the size of lego.

Yes Mary! I have to admit to using plasterboard. Its in the new
extension so all the other walls are gypsum skim or plasterboard and I
haven't done any gypsum flatwork since I was at college which was a
long time ago so I dont trust myself to get it right first time and
plasterboard seemed the easier option, though having read some other
replies I might abandon the plasterboard

On Sat, 02 Dec 2006 20:18:07 GMT, "."
wrote:

never used one but they are a right bugger to install

If you have any handy hints for fitting this particular shower that
you can pass on to me then I'd appreciate them. I expect they are a
pig to fit but then I would expect it to take me a week to install a
normal shower and at least there is no tiling with this one! I must
try to find one of these showers to actually look at before I buy.
Partly to make sure its of good sturdy construction and partly to find
out where the water and electric feeds are. In such a confined space
its important that I do that at the planning stage. I suspect that the
main reason for choosing it is so I can singi along to thje radio as I
shower! (Expensive radio)

Ian and Phil: Yes I've done a mock up and it does all fit in,
including the doors though one has to be a sliding door but I think
its worth it for the added functionality. I could have sworn that
Screwfix did a 'pocket door' kit but it seems I am wrong and they have
let me down so shall have to go elsewhere for that bit

On Sat, 2 Dec 2006 22:33:21 -0000, "dennis@home"
wrote:
I would think about fixing some angle metal to the floor and ceiling and
screwing 12mm plywood to either side.
I think it will be better noise insulation than blockboard and plasterboard
and it will be a lot stronger.

Thats an interesting idea and it so happens that I have several
lengths of quality angle iron sitting in the shed looking for a use

Is there any need for fire resistance?

I hadnt thought about that. Are there any regulations I should be
complying with?

On 2 Dec 2006 13:37:03 -0800, wrote:

sheet ply is stronger than blockboard
ply is stronger than plasterboard, which would add sod all to the
stiffness of the wall.
Lining paper will make ply into a nice paintable surface, and is 1mm
thick.

Right, lining paper it is!

You've got more or less no space for the usual sound deadening
materials there, but for better soundproofing you might be able to get
some kind of foamed rubber sheet a few mm thick in there.

So I'm looking at angle iron holding a paper/ply/foam/ply sandwich.
Sounds good

I take it this wall lines up with a joist above such that a line of
many screws at whatever angle will fix it at the top. If not, whats the
fixing plan?

Yes fixing will be OK cos conveniently it all lines up nicely with the
joists. Oh the joy of new build

You could use two layers of 'Fermacell' cement board with a small gap
in between:

Fermacell sheets are 12.5mm so yes thats another option instead of
ply. Possibly more fireproof but more brittle?

On Sat, 02 Dec 2006 23:23:18 +0000, The Natural Philosopher
wrote:

I'd use either 19mm ply or MDF. Both make reasonable small walls. MDF
needs no surface treatment..juts caulk where it meets stuff (leave a
little gap) as it shrinks a tad eventually. Then paint with emulsion
straight. Sand after first coat. Tile straight onto it.

Or an MDF/foam/mdf sandwich with no lining paper needed

Sound rejection of any solid thin wall will not be great. My guess is
20-30dB versus 50-60 for a proper stud..

Worth it I think for the advantage of the extra loo

Anna
--
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repair and conservation
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

On Sat, 02 Dec 2006 20:01:41 +0000, Anna Kettle wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as well.
After lots of entertainment as I drew the layout on a large sheet of
white paper and walked around to make sure that there is enough space to
fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink out
of something much thinner than a normal stud wall. I'm thinking of using
22mm blockboard faced with 9mm plasterboard on one side and faced with a
wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it be
at least reasonable for sound dampening? The wall is only 1.3m long.
Would some other material like plywood be better? Comments please

Anna

===============================


Your target thickness of 31mm can be achieved as follows. A sandwich made
up from 6mm ply + 30mm PAR timber + 6mm ply. The 6mm ply layers to be
routered into the 30mm timber leaving a cavity of 18mm. The cavity to be
filled with 25mm thick polystyrene (Wickes etc) which will compress into
the 18mm cavity. This will make a very rigid panel with adequate sound
proofing.

This is a very similar construction to those 'Sapele' doors which were so
prevalent a few years ago and which are about 34mm thick. It might be
possible to use actual doors of this type to make your panel if your panel
size matches the size of a standard door.

The whole structure to be supported by angle brackets at top, bottom and
wall.

Cic.

--
================================
Testing UBUNTU Linux
Everything working so far
================================

On Sat, 02 Dec 2006 21:35:49 GMT, "Phil L"
wrote:

They are 100mm wide Eric, she needs it to be 35mm max...studding also ends
up at around 100mm once skimmed

My breeze walls upstairs are all the thickness of a socket box. Put
one in a wall and the back of the box is flush with the plaster in the
next room. (Much to my disgust when I did one.)

That's the sort of block I was thinking of. Sorry to cause confusion.

"Anna Kettle" wrote in message
...
Yes Mary! I have to admit to using plasterboard. Its in the new
extension so all the other walls are gypsum skim or plasterboard and I
haven't done any gypsum flatwork since I was at college which was a
long time ago so I dont trust myself to get it right first time and
plasterboard seemed the easier option, though having read some other
replies I might abandon the plasterboard

You see how highly I regard you!

Mary

Cicero wrote:
On Sat, 02 Dec 2006 20:01:41 +0000, Anna Kettle wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as well.
After lots of entertainment as I drew the layout on a large sheet of
white paper and walked around to make sure that there is enough space to
fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink out
of something much thinner than a normal stud wall. I'm thinking of using
22mm blockboard faced with 9mm plasterboard on one side and faced with a
wall of the shower on the other side.
snip

Your target thickness of 31mm can be achieved as follows. A sandwich made
up from 6mm ply + 30mm PAR timber + 6mm ply. The 6mm ply layers to be
routered into the 30mm timber leaving a cavity of 18mm. The cavity to be
filled with 25mm thick polystyrene (Wickes etc) which will compress into
the 18mm cavity. This will make a very rigid panel with adequate sound
proofing.

I really don't think it will compress.
At the least, you're going to get severe bowing, and unless the PAR
timbber is every 6", it's just going to pop off the fixings.

The trivial way to thickness polystyrene is to put down a couple of
battens, and run a hot wire between them, but a bow-saw, with the blade
at a right angle works just fine, though messily. (glue the polystyrene
down first, or be careful about it pulling up.

Cicero wrote:
On Sat, 02 Dec 2006 20:01:41 +0000, Anna Kettle wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as well.
After lots of entertainment as I drew the layout on a large sheet of
white paper and walked around to make sure that there is enough space to
fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink out
of something much thinner than a normal stud wall. I'm thinking of using
22mm blockboard faced with 9mm plasterboard on one side and faced with a
wall of the shower on the other side.

I'm planning for the shower to be "Steam cabin" Screwfix 83798 so the
shower wall will be a sheet of plastic

Two questions ...

- Has anyone used one of these steam cabins or seen them in the sheds?
Are they the bees knees or a heap of tat?

- Will the blockboard/plasterboard wall be good and rigid and will it be
at least reasonable for sound dampening? The wall is only 1.3m long.
Would some other material like plywood be better? Comments please

Anna

===============================


Your target thickness of 31mm can be achieved as follows. A sandwich made
up from 6mm ply + 30mm PAR timber + 6mm ply. The 6mm ply layers to be
routered into the 30mm timber leaving a cavity of 18mm. The cavity to be
filled with 25mm thick polystyrene (Wickes etc) which will compress into
the 18mm cavity. This will make a very rigid panel with adequate sound
proofing.

This is a very similar construction to those 'Sapele' doors which were so
prevalent a few years ago and which are about 34mm thick. It might be
possible to use actual doors of this type to make your panel if your panel
size matches the size of a standard door.

The whole structure to be supported by angle brackets at top, bottom and
wall.

Cic.

30mm framing will be lousy, and give almost no soundproofing. Yes it'll
work, but sheet material will give much more strength than framework
only, hence less sound transmission.

The sapele doors are cardboard interior with wood veneer finish, plus
strips of wood for the 2 side edges, and theyre a good idea if you're
ok with the finish and the lack of soundproofing. ISTR Ikea make tables
out of them.


NT

On Sun, 03 Dec 2006 12:48:16 +0000, Ian Stirling wrote:

Cicero wrote:
On Sat, 02 Dec 2006 20:01:41 +0000, Anna Kettle wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as well.
After lots of entertainment as I drew the layout on a large sheet of
white paper and walked around to make sure that there is enough space
to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink out
of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and faced
with a wall of the shower on the other side.
snip

Your target thickness of 31mm can be achieved as follows. A sandwich
made up from 6mm ply + 30mm PAR timber + 6mm ply. The 6mm ply layers to
be routered into the 30mm timber leaving a cavity of 18mm. The cavity to
be filled with 25mm thick polystyrene (Wickes etc) which will compress
into the 18mm cavity. This will make a very rigid panel with adequate
sound proofing.

I really don't think it will compress. At the least, you're going to get
severe bowing, and unless the PAR timbber is every 6", it's just going to
pop off the fixings.

The trivial way to thickness polystyrene is to put down a couple of
battens, and run a hot wire between them, but a bow-saw, with the blade at
a right angle works just fine, though messily. (glue the polystyrene down
first, or be careful about it pulling up.

================================
As you imply, compressing by 7mm might be difficult - a case of trial and
error really. Given that polystyrene is always messy to cut to size a
simple way to reduce thickness (if necessary) by a few millimetres is to
draw a large sharp saw across it as if using a spoke shave. Very messy but
effective.

Cic.

--
================================
Testing UBUNTU Linux
Everything working so far
================================

Cicero wrote:
On Sun, 03 Dec 2006 12:48:16 +0000, Ian Stirling wrote:

Cicero wrote:
On Sat, 02 Dec 2006 20:01:41 +0000, Anna Kettle wrote:

I have allocated a space for a shower-loo-sink room but what I'd
_really_ like is to squeeze a separate loo-sink into the space as well.
After lots of entertainment as I drew the layout on a large sheet of
white paper and walked around to make sure that there is enough space
to fit it all in, the answer is yes it will fit ...

... if I make the wall between the shower-loo-sink and the loo-sink out
of something much thinner than a normal stud wall. I'm thinking of
using 22mm blockboard faced with 9mm plasterboard on one side and faced
with a wall of the shower on the other side.
snip

Your target thickness of 31mm can be achieved as follows. A sandwich
made up from 6mm ply + 30mm PAR timber + 6mm ply. The 6mm ply layers to
be routered into the 30mm timber leaving a cavity of 18mm. The cavity to
be filled with 25mm thick polystyrene (Wickes etc) which will compress
into the 18mm cavity. This will make a very rigid panel with adequate
sound proofing.

I really don't think it will compress. At the least, you're going to get
severe bowing, and unless the PAR timbber is every 6", it's just going to
pop off the fixings.

The trivial way to thickness polystyrene is to put down a couple of
battens, and run a hot wire between them, but a bow-saw, with the blade at
a right angle works just fine, though messily. (glue the polystyrene down
first, or be careful about it pulling up.

As you imply, compressing by 7mm might be difficult - a case of trial and
error really. Given that polystyrene is always messy to cut to size a
simple way to reduce thickness (if necessary) by a few millimetres is to
draw a large sharp saw across it as if using a spoke shave. Very messy but
effective.

True, but in this case it's really wanted to be semi-structural, so
would need to contact either side, this is going to be moderately
difficult freehand, though I don't see why it wouldn't work with a
couple of battens to set the depth. probably better than the 'saw it'
solution, for a few reasons.