Hi,

We are having the loft converted to provide more living accomodation.
Box style with flat roof at the back and pitched roof at the front.
The sides are being done with celotex but parts of the pitched roof
are being done withwool and covered with plasterboard. There seems
to be a max of 3 to 4ins thickness of the wool. Should we be looking
to getting a greater thickness or asking for celotex instead? What
sort of vapour barier would be needed?

With regard to the flat roof - there are 6in joists with an angled
piece of woo atop each to provide a slight runoff angle. What sort of
gap or barrier do wee need here and would a double thickness of
celeotex be used here? (The stuff they have at the moment is 3ins
thick.

Sorry if this is basic but it is new to me and I want to get the best
job done now - the pitched roof in particular is north facing.

TIA

Francis wrote:
Hi,

We are having the loft converted to provide more living accomodation.
Box style with flat roof at the back and pitched roof at the front.
The sides are being done with celotex but parts of the pitched roof
are being done withwool and covered with plasterboard. There seems
to be a max of 3 to 4ins thickness of the wool. Should we be looking
to getting a greater thickness or asking for celotex instead? What
sort of vapour barier would be needed?

With regard to the flat roof - there are 6in joists with an angled
piece of woo atop each to provide a slight runoff angle. What sort of
gap or barrier do wee need here and would a double thickness of
celeotex be used here? (The stuff they have at the moment is 3ins
thick.

Sorry if this is basic but it is new to me and I want to get the best
job done now - the pitched roof in particular is north facing.

TIA

If the roof has a breathable membrane under the tiles, apparently you
don't need ventilation between celotex and tiles, so in theory you could
use double thickness.
Wool is cheaper and easier, especially if the joist spacings aren't
uniform. Whether this complies with building regs and/or your contract
with the builders is another matter. Query it and see what they say.

Francis wrote:

We are having the loft converted to provide more living accomodation.
Box style with flat roof at the back and pitched roof at the front.
The sides are being done with celotex but parts of the pitched roof
are being done withwool and covered with plasterboard. There seems
to be a max of 3 to 4ins thickness of the wool. Should we be looking
to getting a greater thickness or asking for celotex instead? What
sort of vapour barier would be needed?

What you do depends a bit on whether you have a breathable sarking under
the tiles. If you do then you can fully fill the gap between rafters.
Otherwise you need to leave a 50mm gap and arrange for soffit and ridge
ventilation.

If use just wool type insulation then a foil backed plasterboard would
be appropriate.

When I did mine I used 50mm foiled foam insulation between the rafters
(with 50mm gap behind) and 35mm under them, followed by the
plasterboard. That thickness was slightly in excess of what was required
to meet building regs at the time.

With regard to the flat roof - there are 6in joists with an angled
piece of woo atop each to provide a slight runoff angle. What sort of

Firrings being the posh term for the tapered bits.

gap or barrier do wee need here and would a double thickness of
celeotex be used here? (The stuff they have at the moment is 3ins
thick.

Is it on top of the firrings, or just in the gaps? The former is what
they call a "warm deck" construction - 75mm is about the minimum in this
case IIRC.

Might be of use:

http://www.internode.co.uk/loft/insulating.htm

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Sun, 25 Nov 2007 17:47:19 +0000, John Rumm
wrote:

Francis wrote:

We are having the loft converted to provide more living accomodation.
Box style with flat roof at the back and pitched roof at the front.
The sides are being done with celotex but parts of the pitched roof
are being done withwool and covered with plasterboard. There seems
to be a max of 3 to 4ins thickness of the wool. Should we be looking
to getting a greater thickness or asking for celotex instead? What
sort of vapour barier would be needed?

What you do depends a bit on whether you have a breathable sarking under
the tiles. If you do then you can fully fill the gap between rafters.
Otherwise you need to leave a 50mm gap and arrange for soffit and ridge
ventilation.

I am told it is "breathable membrane"

If use just wool type insulation then a foil backed plasterboard would
be appropriate.

When I did mine I used 50mm foiled foam insulation between the rafters
(with 50mm gap behind) and 35mm under them, followed by the
plasterboard. That thickness was slightly in excess of what was required
to meet building regs at the time.

With regard to the flat roof - there are 6in joists with an angled
piece of woo atop each to provide a slight runoff angle. What sort of

Firrings being the posh term for the tapered bits.

gap or barrier do wee need here and would a double thickness of
celeotex be used here? (The stuff they have at the moment is 3ins
thick.

Is it on top of the firrings, or just in the gaps? The former is what
they call a "warm deck" construction - 75mm is about the minimum in this
case IIRC.


No insulation has been put in here yet and the roof is on. So, IIUC,
a gap between the roof and the insulation is required, so enough room
for about 100mm of insulation directly above the ceiling ?

Thanks for the avice to you and Stuart.

Regards,

Francis

Francis wrote:

With regard to the flat roof - there are 6in joists with an angled
piece of woo atop each to provide a slight runoff angle. What sort of
Firrings being the posh term for the tapered bits.

gap or barrier do wee need here and would a double thickness of
celeotex be used here? (The stuff they have at the moment is 3ins
thick.
Is it on top of the firrings, or just in the gaps? The former is what
they call a "warm deck" construction - 75mm is about the minimum in this
case IIRC.


No insulation has been put in here yet and the roof is on. So, IIUC,
a gap between the roof and the insulation is required, so enough room
for about 100mm of insulation directly above the ceiling ?

That is a cold deck roof then. No requirement for a gap that I am aware
of since the space is probably not ventilated anyway. With these the gap
between the rafters is usually just stuffed with insulation.

Many of the insulation manufacturers have quite handy "how to" guides
that may make interesting reading. For example:

http://www.insulation.kingspan.com/uk/literature.htm


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Sun, 25 Nov 2007 12:13:45 +0000, a particular chimpanzee, Francis
randomly hit the keyboard and produced:

We are having the loft converted to provide more living accomodation.
Box style with flat roof at the back and pitched roof at the front.
The sides are being done with celotex but parts of the pitched roof
are being done withwool and covered with plasterboard. There seems
to be a max of 3 to 4ins thickness of the wool. Should we be looking
to getting a greater thickness or asking for celotex instead? What
sort of vapour barier would be needed?

This should have a Building Regulations application, and if it does,
insulation and ventilation of the roof are two of the things covered.

Mineral fibre is not a good insulant for the roof. You would need at
least 200mm between rafters, with all the attendant problems of how to
maintain the 50mm gap over. PIR (Celotex, Kingspan, Xtratherm, etc)
would be needed between and under the rafters (IIRC, 50mm & 70mm
respectively).

If it's a habitable room, then taping the joints of the PIR insulation
is usually enough. If it's a bathroom/showerroom, then a separate
polythene vapour barrier is usually recommended.

With regard to the flat roof - there are 6in joists with an angled
piece of woo atop each to provide a slight runoff angle. What sort of
gap or barrier do wee need here and would a double thickness of
celeotex be used here? (The stuff they have at the moment is 3ins
thick.

With the flat roof, you would probably need 140mm PIR between the
joists, with a minimum 50mm gap over at the lowest point (ignoring the
firrings or sloping piece of wood).

Sorry if this is basic but it is new to me and I want to get the best
job done now - the pitched roof in particular is north facing.

Get in touch with Building Control now before it's covered over and
you have to undo it all.
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
just how far from the pack have you strayed?"