Hi,

I'm using our converted attic a lot more now, and have noticed that
it's very cold in winter, and heats up in a few seconds when the sun
comes out. Clearly it's not very well insulated.

It's a slate roof with a layer of waterproof felt under the tiles (all
in good condition) supported on rafters that are 100mm thick and 600mm
apart. Ideally, I'd like to fit in as much insulation as possible,
and then cover it all with insulation-backed plasterboard.

But the question is, how much insulation is "as much as possible"? As
far as I can tell, the two options a

1. 50mm celotex wedged between the rafters, but leaving a 50mm air
gap behind for building regs as it won't be air-tight and condensation
behind the insulation needs to be ventilated away.

2. 100mm sprayed closed-cell polyurethane foam, onto the back of the
felt (NOT onto the tiles), thus removing the need for the air gap as
condensation cannot penetrate from the inside.

To me, option 2 sounds better as that's twice as much insulation as
option 1 (assuming it is not applied by cowboys and the roof is
sound). Am I right that it removes the need for the ventilation gap
and therefore can be sprayed to this thickness? So far I haven't been
able to confirm either way, other than through material provided
directly by foam contractors.

Thanks for your assistance,

Miles

2. 100mm sprayed closed-cell polyurethane foam, onto the back of the
felt (NOT onto the tiles), thus removing the need for the air gap as
condensation cannot penetrate from the inside.

but rain will seep in through some slates
and not be ventilated
and rot the timbers..

[g]

On May 17, 12:30*pm, "george (dicegeorge)"
wrote:
2. *100mm sprayed closed-cell polyurethane foam, onto the back of the
felt (NOT onto the tiles), thus removing the need for the air gap as
condensation cannot penetrate from the inside.

but rain will seep in through some slates
and not be ventilated
and rot the timbers..

[g]

Seconded. I;d be dubious about anything that removes ventilation and
possible sccess to the inside of the roof.

I suggest you put 50mm of celotex between the rafters, and another
25mm over the top, then plasterboard - or, as you say, insulation-
backed plasterboard. If you want deeper than that, attach 2 inch
battens to the rafters, so you can get 100mm between, and 25mm over
(which, by the way, is the depth that my local BC requires for a
habitable room in the roof. I'm not sure where building regs would
come in with your job - is the attic a habitable room already? If it
is, I wouldn't involve BC, myself. I'd still have that 2 inch air gap,
though.

Regards
Richard

On 17 May, 12:30, "george (dicegeorge)" wrote:
2. *100mm sprayed closed-cell polyurethane foam, onto the back of the
felt (NOT onto the tiles), thus removing the need for the air gap as
condensation cannot penetrate from the inside.

but rain will seep in through some slates
and not be ventilated
and rot the timbers.

Isn't the waterproof felt to prevent the water that gets through the
slates getting further into the roof?

I can't see any water marks on the joists or the floor so the roof is
definitely watertight.

Or am I missing something?

Miles

On May 17, 12:21*pm, wrote:
Hi,

I'm using our converted attic a lot more now, and have noticed that
it's very cold in winter, and heats up in a few seconds when the sun
comes out. *Clearly it's not very well insulated.

It's a slate roof with a layer of waterproof felt under the tiles (all
in good condition)

Which slate or tiles ?

On 17 May, 22:17, Kipper at sea wrote:
On May 17, 12:21*pm, wrote:

Hi,

I'm using our converted attic a lot more now, and have noticed that
it's very cold in winter, and heats up in a few seconds when the sun
comes out. *Clearly it's not very well insulated.

It's a slate roof with a layer of waterproof felt under the tiles (all
in good condition)

Which slate or tiles ?

Both - it's all in good condition, just need to up the insulation.

Miles

On Sun, 17 May 2009 04:21:45 -0700 (PDT),
wrote:

Hi,

I'm using our converted attic a lot more now, and have noticed that

Converted? how converted?

it's very cold in winter, and heats up in a few seconds when the sun
comes out. Clearly it's not very well insulated.

It's a slate roof with a layer of waterproof felt under the tiles (all
in good condition) supported on rafters that are 100mm thick and 600mm
apart. Ideally, I'd like to fit in as much insulation as possible,
and then cover it all with insulation-backed plasterboard.

But the question is, how much insulation is "as much as possible"? As
far as I can tell, the two options a

1. 50mm celotex wedged between the rafters, but leaving a 50mm air
gap behind for building regs as it won't be air-tight and condensation
behind the insulation needs to be ventilated away.

2. 100mm sprayed closed-cell polyurethane foam, onto the back of the
felt (NOT onto the tiles), thus removing the need for the air gap as
condensation cannot penetrate from the inside.

To me, option 2 sounds better as that's twice as much insulation as
option 1 (assuming it is not applied by cowboys and the roof is
sound). Am I right that it removes the need for the ventilation gap
and therefore can be sprayed to this thickness? So far I haven't been
able to confirm either way, other than through material provided
directly by foam contractors.

Thanks for your assistance,

Miles
--
http://www.freedeliveryuk.co.uk
http://www.holidayunder100.co.uk

On Sun, 17 May 2009 08:39:11 -0700 (PDT), a certain chimpanzee,
randomly hit a keyboard and produced:

Isn't the waterproof felt to prevent the water that gets through the
slates getting further into the roof?

I can't see any water marks on the joists or the floor so the roof is
definitely watertight.

Or am I missing something?

The felt is primarily a wind-barrier. It performs the secondary
function of a waterproof layer.

Vapour from the inside of the building will condense on cold surfaces
(and inside permeable elements such as joists). Normally this is not a
problem as ventilation will remove most of the moisture, but if the
rafters are encased in poly foam that prevent them breathing, the
moisture can lead to dry rot.
--
Hugo Nebula
"If no one on the Internet wants a piece of this,
just how far from the pack have you strayed?"

On May 18, 8:26*pm, Hugo Nebula abuse@localhost wrote:
On Sun, 17 May 2009 08:39:11 -0700 (PDT), a certain chimpanzee,
randomly hit a keyboard and produced:

Isn't the waterproof felt to prevent the water that gets through the
slates getting further into the roof?

I can't see any water marks on the joists or the floor so the roof is
definitely watertight.

Or am I missing something?

The felt is primarily a wind-barrier. It performs the secondary
function of a waterproof layer.

Vapour from the inside of the building will condense on cold surfaces
(and inside permeable elements such as joists). Normally this is not a
problem as ventilation will remove most of the moisture, but if the
rafters are encased in poly foam that prevent them breathing, the
moisture can lead to dry rot.
--
Hugo Nebula
*"If no one on the Internet wants a piece of this,
* just how far from the pack have you strayed?"

Precisely. And if you insulate between the joists, with foil-backed
celotex on the warm side, you provide a vapour barrier to prevent
condesnation on the rafters. If you have an air gap on the cold side,
then any moisture that gets in from the outside can evaporate again.
It's science, but it's not rocket science ;-)

Regards
Richard