How should this be constructed?

The first layer of Celotex fills the gapes between the rafters, and a
second layer leaves a flat surface. But how do you fix the plasterboard
on that? Do you try to remember where the rafters were, and use six inch
screws, or is it recommended to fit horizontal battens onto the rafters,
insulate the second layer between those, then fix to the battens?
--
Roland Perry

Roland Perry wrote:
How should this be constructed?

The first layer of Celotex fills the gapes between the rafters, and a
second layer leaves a flat surface. But how do you fix the plasterboard
on that? Do you try to remember where the rafters were, and use six inch
screws, or is it recommended to fit horizontal battens onto the rafters,
insulate the second layer between those, then fix to the battens?

Surely it's easy enough to mark the position of the rafters on the
bottom and top edges of each slab, when you fit the second layer? And
then transfer the marks to the plasterboard when you fit that? A
chalk-line might be handy, too...

David

In message , at 19:20:23 on
Wed, 17 May 2006, Lobster remarked:
The first layer of Celotex fills the gapes between the rafters, and
a second layer leaves a flat surface. But how do you fix the
plasterboard on that? Do you try to remember where the rafters were,
and use six inch screws, or is it recommended to fit horizontal
battens onto the rafters, insulate the second layer between those,
then fix to the battens?

Surely it's easy enough to mark the position of the rafters on the
bottom and top edges of each slab, when you fit the second layer? And
then transfer the marks to the plasterboard when you fit that? A
chalk-line might be handy, too...

Yes, although that leaves me with the six inch screws. But we have two
votes for that, so thanks for the advice.
--
Roland Perry

Roland Perry wrote:
In message , at 19:20:23 on
Wed, 17 May 2006, Lobster remarked:

The first layer of Celotex fills the gapes between the rafters, and
a second layer leaves a flat surface. But how do you fix the
plasterboard on that? Do you try to remember where the rafters were,
and use six inch screws, or is it recommended to fit horizontal
battens onto the rafters, insulate the second layer between those,
then fix to the battens?


Surely it's easy enough to mark the position of the rafters on the
bottom and top edges of each slab, when you fit the second layer? And
then transfer the marks to the plasterboard when you fit that? A
chalk-line might be handy, too...

Yes, although that leaves me with the six inch screws. But we have two
votes for that, so thanks for the advice.

Don't geddit - why 6"? Unless I'm missing something, you've only got 2"
celotex plus plasterboard actually over the rafters, so you'd probably
get away with 3"?

David

In message , at 06:35:04 on
Thu, 18 May 2006, Lobster remarked:
Surely it's easy enough to mark the position of the rafters on the
bottom and top edges of each slab, when you fit the second layer? And
then transfer the marks to the plasterboard when you fit that? A
chalk-line might be handy, too...
Yes, although that leaves me with the six inch screws. But we have
two votes for that, so thanks for the advice.

Don't geddit - why 6"?

OK, perhaps I'm exaggerating a bit. Five inches.

Unless I'm missing something, you've only got 2" celotex plus
plasterboard actually over the rafters, so you'd probably get away with
3"?

The rafters are 3", as is the celotex (two layers of). So if I used a
four inch screw, then about half an inch would be in the rafter.

Why six inches of celotex? That's what I've been told is required by da
building regs.
--
Roland Perry

Roland Perry wrote on 17/05/2006 :
Do you try to remember where the rafters were, and use six inch screws, or is
it recommended to fit horizontal battens onto the rafters, insulate the
second layer between those, then fix to the battens?

A nail at the top and bottom placed such that a piece of string run
between them would be roughly in the centre of the rafter - will take
care of that problem.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

Roland Perry wrote:
How should this be constructed?

The first layer of Celotex fills the gapes between the rafters, and a
second layer leaves a flat surface. But how do you fix the plasterboard
on that? Do you try to remember where the rafters were, and use six inch
screws, or is it recommended to fit horizontal battens onto the rafters,
insulate the second layer between those, then fix to the battens?

Why haven't you fixed PB to the joists, put thick insulation between,
and thin insulation over?

In message , at 23:45:02 on Wed, 17 May
2006, Chris Bacon remarked:
How should this be constructed?
The first layer of Celotex fills the gapes between the rafters, and
a second layer leaves a flat surface. But how do you fix the
plasterboard on that? Do you try to remember where the rafters were,
and use six inch screws, or is it recommended to fit horizontal
battens onto the rafters, insulate the second layer between those,
then fix to the battens?

Why haven't you fixed PB to the joists, put thick insulation between,
and thin insulation over?

I'm not sure I understand the sandwich there. Are you saying it should
go rafters-with-thick-insulation-between, then plasterboard, then thin
insulation, then what?
--
Roland Perry

Roland Perry wrote:
Chris Bacon remarked:
[snip all]
I'm not sure I understand the sandwich there. Are you saying it should
go rafters-with-thick-insulation-between, then plasterboard, then thin
insulation, then what?

I mis-read the question. Cider played a part, as did
general stupidity. Apologies. Ignore that post.

Roland Perry wrote:

How should this be constructed?

Do you have soffit and ridge ventilation? Or is the roof contructed with
a breathable sarking material?

In which case the usual technique is to leave a 2" air gap, then place
2" of insulation between the rafters (assuming they are 4" obviously),
followed by say 25 - 35mm over the top.

The first layer of Celotex fills the gapes between the rafters, and a
second layer leaves a flat surface. But how do you fix the plasterboard
on that? Do you try to remember where the rafters were, and use six inch

If you screw the celotex overcoat to the rafters then you can see the
screw heads as a guide.

screws, or is it recommended to fit horizontal battens onto the rafters,
insulate the second layer between those, then fix to the battens?

No need for battens - you will need longer screws - but *not* 6" unless
you want to push the tiles off the roof! I used 60mm drywall screws for
this.

See the second photo down:

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 |
\================================================= ================/

The message
from John Rumm contains these words:

No need for battens - you will need longer screws - but *not* 6" unless
you want to push the tiles off the roof! I used 60mm drywall screws for
this.

Seems to me the OP is intent on putting 4" of insulation between the
plasterboard and the rafters in addition to some between between the
rafters. Seems a bit like overkill to me unless he is living North of
the Arctic Circle.

--
Roger Chapman

In message , at 09:06:53
on Thu, 18 May 2006, Roger remarked:
No need for battens - you will need longer screws - but *not* 6" unless
you want to push the tiles off the roof! I used 60mm drywall screws for
this.

Seems to me the OP is intent on putting 4" of insulation between the
plasterboard and the rafters in addition to some between between the
rafters. Seems a bit like overkill to me unless he is living North of
the Arctic Circle.

Three inches, but otherwise correct
--
Roland Perry

Three inches, but otherwise correct

Be careful on layer sizes. The bottom layer must not be larger than the top
layer. i.e. if you have 75mm below the rafters, you need 75mm minimum
between the rafters to avoid interstitial condensation. However, you may
still need to maintain a 50mm air gap, which would require 125mm rafters
(i.e. 5").

If you happen to have breathable membrane sarking, then Kingspan allow you
to use no air gap at all according to their installation guide, allowing
75mm between and below the rafters only 75mm deep. I don't know if Celotex
claim the same.

Christian.

On Thu, 18 May 2006 17:06:29 +0100, "Christian McArdle"
wrote:

Three inches, but otherwise correct

Be careful on layer sizes. The bottom layer must not be larger than the top
layer. i.e. if you have 75mm below the rafters, you need 75mm minimum
between the rafters to avoid interstitial condensation.

Christian.


Where did you read that? I understood, and continue to hope, that
interstitial condensation is prevented by a vapour barrier which in
this case is the foil covering of the insulation.