I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings for 2
bedrooms (and a flat roofed landing). The pitched roofs has 4inch rafters,
and that flat roof 6 inch.

What depth of celotex should I be looking at, and more importantly what air
gap do I need to leave between the top side of the celotex and then roofing
felt?

There seem to be many different types of celotex, so how do I know what is
best suited for me?

Thanks,
Matt

Matthew Ames
wibbled on Monday 02 November 2009 19:57

I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings for
2
bedrooms (and a flat roofed landing). The pitched roofs has 4inch
rafters, and that flat roof 6 inch.

What depth of celotex should I be looking at, and more importantly what
air gap do I need to leave between the top side of the celotex and then
roofing felt?


2" or 50mm is the general guide.

There seem to be many different types of celotex, so how do I know what is
best suited for me?

The cheapest PIR foam - I got Ballytherm seconds for half the usual price.

--
Tim Watts

This space intentionally left blank...

"Tim W" wrote in message
...
2" or 50mm is the general guide.

Cool, so I should be looking for a thickness of about 50mm as well


The cheapest PIR foam - I got Ballytherm seconds for half the usual price.

Online, or from a local retailer?

Matthew Ames wrote:
I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings for 2
bedrooms (and a flat roofed landing). The pitched roofs has 4inch rafters,
and that flat roof 6 inch.

What depth of celotex should I be looking at, and more importantly what air
gap do I need to leave between the top side of the celotex and then roofing
felt?

There seem to be many different types of celotex, so how do I know what is
best suited for me?

I hope I am wrong but I have the impression that any work on a roof is
subject to building regulations that will require the roof to be to be
brought up to the current standard, which would require 65mm of PIR
insulation.

Matt A
wibbled on Monday 02 November 2009 21:12

"Tim W" wrote in message
...
2" or 50mm is the general guide.

Cool, so I should be looking for a thickness of about 50mm as well

That's the norm if you don't want to lose space.

But having a 2" air gap is meaningless unless it can vent out at the highest
point and vent in from the lowest point. Failure to do this may cause rot -
moreso than glass/rock/sheepswool as the latter can breath sideways.

Lower vents are usually a vent strip or repeated round vents in the soffit
board.

The upper vents are more of a pain - tile vents or ridge vents are a common
solution.

Both can be avoided with a breathable roofing "felt" like Tyvec, but unless
you have it already, that implies a re-tile job.

This is an issue that's causing me some headaches as I have a dormer
conversion in a full hipped roof, so the airways are not clear from base to
top - I'm having to find creative solutions...

Back to your point about depth - you could put another 25 or 50mm under the
rafters too if you can lose the space - that will up the insulation and
reduce cold bridging via the wood.


The cheapest PIR foam - I got Ballytherm seconds for half the usual
price.

Online, or from a local retailer?

I got mine off the mate of a supplier local to a another uk.d-i-y poster in
Essex (I'm in East Sussex) - the mate in question happens to do runs past
my village on a regular basis so is happy to drop off 20 odd sheets.

So - either local knowledge needed - these people can be hard to find, but
worth the looking.

Or, sometimes, the internet can turn up the goods at a fairly reasonable
price.

http://www.apinsulation.co.uk/prices.html
http://www.builditsecurepayment.co.u...nsulation.html
http://www.planetinsulation.co.uk/store/index.php
http://www.just-insulation.com/eshopPIR.html

I don't know what they're like - I just noted them down when I stumbled
across them...

Anyway, the magic word is "seconds" or "2nd grade". It means dinted,
imperfect, maybe a few mm off correct thickness, recovered surplus
materials from mega building jobs etc. The BCO might reject it for a new
build if he's being arsey, but it's fine for general work.

In practice, most people who have 2nd grade are happy with it. You can chop
damaged edges off for house work anyway - and PIR foam is PIR foam. It
doesn't really matter who makes it or if it's a bit bent - it still does
the job.

HTH

Tim

--
Tim Watts

This space intentionally left blank...

Roger Chapman
wibbled on Monday 02 November 2009 22:20

Matthew Ames wrote:
I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings
for 2
bedrooms (and a flat roofed landing). The pitched roofs has 4inch
rafters, and that flat roof 6 inch.

What depth of celotex should I be looking at, and more importantly what
air gap do I need to leave between the top side of the celotex and then
roofing felt?

There seem to be many different types of celotex, so how do I know what
is best suited for me?

I hope I am wrong but I have the impression that any work on a roof is
subject to building regulations that will require the roof to be to be
brought up to the current standard, which would require 65mm of PIR
insulation.

Let's not go there Roger ;-O

I do believe that you're right - but I think most BCOs would be happy if
people did something rather than nothing. I don't think anyone will ever
get prosecuted for failure to notify in this case. It's bloody stupid
anyway...

I told my BCO flat out that I couldn't bring it up to book standard, but I
thought I could make it twice as effective as before (it is currently 3"
glass wool - very crap). I hope to replace with 3" celotex and maybe 4" in
some segments...

He was perfectly happy with this and was more concerned with helping me with
useful suggestions about ventilation over it.

Cheers

Tim

--
Tim Watts

This space intentionally left blank...

In article ,
Tim W writes:
Matthew Ames
wibbled on Monday 02 November 2009 19:57

I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings for
2
bedrooms (and a flat roofed landing). The pitched roofs has 4inch
rafters, and that flat roof 6 inch.

What depth of celotex should I be looking at, and more importantly what
air gap do I need to leave between the top side of the celotex and then
roofing felt?


2" or 50mm is the general guide.

There seem to be many different types of celotex, so how do I know what is
best suited for me?

The cheapest PIR foam - I got Ballytherm seconds for half the usual price.

Make sure it's fire retardant though.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Andrew Gabriel
wibbled on Monday 02 November 2009 22:35

is best suited for me?

The cheapest PIR foam - I got Ballytherm seconds for half the usual
price.

Make sure it's fire retardant though.


Good point. No idea. I'll try a flame test.

But I *thought* that PIR and PUR foams were relatively non flammable, unlike
EPS which certainly does need a fire retardant?

--
Tim Watts

This space intentionally left blank...

"Roger Chapman" wrote in message
...
I hope I am wrong but I have the impression that any work on a roof is
subject to building regulations that will require the roof to be to be
brought up to the current standard, which would require 65mm of PIR
insulation.

I hope your wrong :-) Unless the government want to pay me to increase the
depth of my rafters by another 15mm, they have another thing coming.....

Matt A wrote:
"Tim W" wrote in message
...
2" or 50mm is the general guide.

Cool, so I should be looking for a thickness of about 50mm as well


The cheapest PIR foam - I got Ballytherm seconds for half the usual price.

Online, or from a local retailer?

I can recommend http://www.secondsandco.co.uk/ (deliver nationwide IIRC)

David

In article ,
Tim W writes:
Andrew Gabriel
wibbled on Monday 02 November 2009 22:35

is best suited for me?

The cheapest PIR foam - I got Ballytherm seconds for half the usual
price.

Make sure it's fire retardant though.


Good point. No idea. I'll try a flame test.

But I *thought* that PIR and PUR foams were relatively non flammable, unlike
EPS which certainly does need a fire retardant?

Both are highly flammable, although PIR slightly less so.
I thought it was all treated with fire retardant, but I bought a
sheet from a builders merchant that I found wasn't when I tested
it - it sustained and spread flame, albeit not very quickly.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Matthew Ames wrote:
I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings for 2
bedrooms (and a flat roofed landing). The pitched roofs has 4inch rafters,
and that flat roof 6 inch.

What depth of celotex should I be looking at, and more importantly what air
gap do I need to leave between the top side of the celotex and then roofing
felt?

There seem to be many different types of celotex, so how do I know what is
best suited for me?


I am not sure of the exact regs. I had 50mm in 6" rafters. I think you
can go to 75mm in 4" and leave a 1" airgap.


Thanks,
Matt

Tim W wrote:
Matt A
wibbled on Monday 02 November 2009 21:12

"Tim W" wrote in message
...
2" or 50mm is the general guide.
Cool, so I should be looking for a thickness of about 50mm as well

That's the norm if you don't want to lose space.

But having a 2" air gap is meaningless unless it can vent out at the highest
point and vent in from the lowest point. Failure to do this may cause rot -
moreso than glass/rock/sheepswool as the latter can breath sideways.

Lower vents are usually a vent strip or repeated round vents in the soffit
board.

The upper vents are more of a pain - tile vents or ridge vents are a common
solution.

Both can be avoided with a breathable roofing "felt" like Tyvec, but unless
you have it already, that implies a re-tile job.

This is an issue that's causing me some headaches as I have a dormer
conversion in a full hipped roof, so the airways are not clear from base to
top - I'm having to find creative solutions...


Ive got Tyvex AND roof vents :-)

Back to your point about depth - you could put another 25 or 50mm under the
rafters too if you can lose the space - that will up the insulation and
reduce cold bridging via the wood.


Damned good idea.

Andrew Gabriel wrote:
In article ,
Tim W writes:
Andrew Gabriel
wibbled on Monday 02 November 2009 22:35

is best suited for me?
The cheapest PIR foam - I got Ballytherm seconds for half the usual
price.
Make sure it's fire retardant though.

Good point. No idea. I'll try a flame test.

But I *thought* that PIR and PUR foams were relatively non flammable, unlike
EPS which certainly does need a fire retardant?

Both are highly flammable, although PIR slightly less so.
I thought it was all treated with fire retardant, but I bought a
sheet from a builders merchant that I found wasn't when I tested
it - it sustained and spread flame, albeit not very quickly.

celotex is polyisocyanurute foam and will not burn, though it chars
giving off pretty nasty fumes.

Andrew Gabriel
wibbled on Tuesday 03 November 2009 00:12


Both are highly flammable, although PIR slightly less so.
I thought it was all treated with fire retardant, but I bought a
sheet from a builders merchant that I found wasn't when I tested
it - it sustained and spread flame, albeit not very quickly.


Hmm. I'll definately try setting fire to mine - I'll be very upset if it
does burn, because, unless it was specifically a type made to go under
concrete floors, it would be unfit for purpose in any other use...

Thanks for the warning.

The problem is of course, that despite printing loads of crap like logos and
phone numbers on the foil face, neither Celotex nor Ballytherm seem to
print the part number of the particular board which makes identification
hard.

Ballytherm say this generically about their boards:

"Fire
Ballytherm Insulation remains stable at
temperatures up to 400ºC; at higher
temperatures a protective char forms
on the surface, slowing the spread of
flame. Ballytherm insulation may be
used safely behind non-flammable
materials such as plasterboard.
The products have been fully tested in
accordance with B.S. 476: Part 7: 1997
Surface of Flame and have achieved a
Class 1 Rating."

--
Tim Watts

This space intentionally left blank...

On 2 Nov, 22:25, Tim W wrote:

This is an issue that's causing me some headaches as I have a dormer
conversion in a full hipped roof, so the airways are not clear from base to
top - I'm having to find creative solutions...

Did you find a solution to this, by the way? I replied to your post
about it, though I didn't appreciate that it was a hipped roof.

Cheers
Richard

On Nov 2, 7:57*pm, "Matthew Ames" wrote:
I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings for 2
bedrooms (and a flat roofed landing). *The pitched roofs has 4inch rafters,
and that flat roof 6 inch.


I found the celotex website very helpful:

http://www.celotex.co.uk/Applications/Pitched-Roofs

If you register you can download the complete solution. I ended up
with a 50mm air gap, 70mm of celotex and then thinsulex
http://customaudiodesigns.co.uk/thinsulex/thinsulex.htm battened on
underneath. Building control were happy with it, in fact here is what
they actually approved:


Detail of works:
1. Remove lath and plaster ceiling (D)
2. Re-roof and add plasterboard ceiling: (re-slate, impermeable
membrane, 50mm air gap, rafters (currently 85mm deep at 360mm
centres), extend down by 35mm, 70mm Celotex between rafters,
Thinsulex, battens, plasterboard).


As i remember it, you could not get the required U value simply by
putting insulation bhetween the rafters becuase the rafters them
selves conduct too much heat,

I think you can also do away with the air gap if yo uuse a breathable
membrane, but the pitch of my roof was very low and I didnt; want to
do that.

The whole roof does not have to be brought up to modern regs, but if
you change it then the heat insulation does have to meet new regs
(according to my BCO).

Robert

RobertL
wibbled on Tuesday 03 November 2009 15:21

On Nov 2, 7:57*pm, "Matthew Ames" wrote:
I am looking at putting in some celotex insulation material between the
rafters in my house on the 2nd floor that provide the pitched ceilings
for 2 bedrooms (and a flat roofed landing). *The pitched roofs has 4inch
rafters, and that flat roof 6 inch.


I found the celotex website very helpful:

http://www.celotex.co.uk/Applications/Pitched-Roofs

If you register you can download the complete solution. I ended up
with a 50mm air gap, 70mm of celotex and then thinsulex
http://customaudiodesigns.co.uk/thinsulex/thinsulex.htm battened on
underneath. Building control were happy with it, in fact here is what
they actually approved:


Detail of works:
1. Remove lath and plaster ceiling (D)
2. Re-roof and add plasterboard ceiling: (re-slate, impermeable
membrane, 50mm air gap, rafters (currently 85mm deep at 360mm
centres), extend down by 35mm, 70mm Celotex between rafters,
Thinsulex, battens, plasterboard).


As i remember it, you could not get the required U value simply by
putting insulation bhetween the rafters becuase the rafters them
selves conduct too much heat,

I think you can also do away with the air gap if yo uuse a breathable
membrane, but the pitch of my roof was very low and I didnt; want to
do that.

The whole roof does not have to be brought up to modern regs, but if
you change it then the heat insulation does have to meet new regs
(according to my BCO).

Robert

Yes - I've seen that before.

It's Ok if you have a virgin roof, but not so easy if you have this:

http://photos.dionic.net/v/public/bu..._0003.jpg.html

The noggings are a problem as the block the vertical airflow - as do the
windows. So effectively I can't have a continuous inch between the wood and
2" over. The best I can do without butchering the wood is 1" between and
leaving a gap round the noggings, 1" over, leaving an air gap again, then
another inch or two over the lot. Now I'm not sure how effective that
arrangement will be as there will be colder spots amongst the well
insulated bits. Perhaps it will work well enough - need to think about it
more...

Cheers

Tim

--
Tim Watts

This space intentionally left blank...