insulating a loft with kingspan/celotex

[Stephen]

Hello,

It has been suggested here before, I think by the late Andy Hall, to
use celotex sheets to insulate your loft.

How is the best way to do this? I would seem to me that the easiest
way would be to butt the boards up to each other, above the joists;
that way you would not have to trim them to fit between the joists.
Also if the celotex was put between the joists, you would have some
heat loss through the joists as the wood would not insulate as well as
the celotex would. This way, there is a continuous surface of celotex.

But... what happens when you get into the eaves?

I tried pushing an off cut into the eaves but the sheet hits the
rafters, so there is a large gap between the rafters and the
eaves/sarking felt, which defeats the object if the heat can just
convect around the edge.

Are you supposed to cut out sections for the joists? I do not have a
ventilated soffit, though that said, I'm sure it's not airtight
either, would it cause problems if the celotex was butted right up to
the sarking?

Should I just use the traditional rolls of fibre glass instead?

Thanks,
Stephen.

[Another John]

In article <qmlp9c9mcudpuqsj2...@4ax.com>,

My inexpert, but experienced, views are:

- Put fibre glass between the joists.
- *And* put good thick Celotex across the joists.
- Don't worry about a gap at the eaves: you need ventilation through
a loft, or you will cause trouble.

Bonus: if you get good thick celotex, and walk carefully, you have
almost as good as boarded out your loft.

Cheers
J.

[Brian Gaff]

What about using bubblewrap to do the awkward areas where you don't walk. It
seems cheap almost too good to be true.
Brian

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[Nightjar]

On 09-Feb-17 9:03 PM, Stephen wrote:
> Hello,
>
> It has been suggested here before, I think by the late Andy Hall, to
> use celotex sheets to insulate your loft.
>

> How is the best way to do this?...

Use one of the methods detailed in the Celotex guide. Roof insulation
starts at page 44:

https://www.celotex.co.uk/assets/the-handy-guide_brochures_jan16.pdf


--
--

Colin Bignell

[Stephen]

On Thu, 09 Feb 2017 21:19:57 +0000, Another John <lal...@hotmail.com>
wrote:

>- *And* put good thick Celotex across the joists.

What size is considered thick?

Thanks.

[Stephen]

On Fri, 10 Feb 2017 09:03:22 +0000, Nightjar <c...@bignell.me.uk>
wrote:

>starts at page 44:
>
>https://www.celotex.co.uk/assets/the-handy-guide_brochures_jan16.pdf
>

Thanks, I never thought to see what the manufacturer said.

It seems most of it is about fitting celotex over the rafters rather
than the joists. The pictures are not very good. In fact the text says
to have the non-printed side facing out and the photos show the
printed side facing out! I never realised a little red ink would make
so much difference.

I get the impression that I was looking at doing this the wrong way;
rather than insulate over the joists, it seems I should insulate over
or between the rafters (or both) but leaving an air gap from the
eaves to the apex to ventilate the rafters.

It seems the traditional fibre glass roll still has a place between
the joists.

Thanks,
Stephen.

[Martin Brown]

I hate to be a party pooper but exactly how do you get large thick
celotex boards into your loft?

Loft access is typically something like a 3' diagonal if you are lucky.

--
Regards,
Martin Brown

[Martin Brown]

On 10/02/2017 08:40, Brian Gaff wrote:
> What about using bubblewrap to do the awkward areas where you don't walk. It
> seems cheap almost too good to be true.
> Brian

The aluminised plastic wrapped glass fibre is what I used. Much less
messy than the old stuff. Still need dust mask and gloves but relatively
easy to get it up there and deploy.

--
Regards,
Martin Brown

[RJH]

On 10/02/2017 09:35, Martin Brown wrote:
> On 10/02/2017 09:24, Stephen wrote:
>> On Thu, 09 Feb 2017 21:19:57 +0000, Another John <lal...@hotmail.com>
>> wrote:
>>
>>> - *And* put good thick Celotex across the joists.
>>

That's the method recommended in the Celotex guide - link elsewhere in
this thread.

>> What size is considered thick?
>

Thickness would depend on the insulation levels you'd like - I'd have
thought 50mm on top of existing rockwool would be enough before
diminishing returns start to set it.

> I hate to be a party pooper but exactly how do you get large thick
> celotex boards into your loft?
>
> Loft access is typically something like a 3' diagonal if you are lucky.
>

Cut lengthways, rejoin with tape?


--
Cheers, Rob

[RobertL]

On Friday, February 10, 2017 at 9:31:35 AM UTC, Stephen wrote:

> ...In fact the text says

> to have the non-printed side facing out and the photos show the
> printed side facing out! I never realised a little red ink would make
> so much difference.

AIUI the foil on one side is deliberately perforated; on the other side it is a an impermeable vapour barrier. I'm not sure which side has the printing on.

Robert

[harry]

I have done this.
It's not possible to accurately butt joint. The slightest gap defeats all your work
The best way is to buy the foam sheets the same thickness as the joists.
Cut around an inch smaller than the gap between the joists.
Put in place & fill the gap with expanding fixing foam.
Before the foam has gone off, put more sheets on top.
The expanding foam from lower gaps sticks to top layer.
(Leaves gaps on upper layer as before & fill with fixing foam)
You need bricks/concrete blocks to put on top to stop the sheets from being displaced as the foam expands.

The foam is twice the insulation value of mineral wool.

You can lay chipboard on top to provide walk way.

It is closed cell and waterproof.

You can get the insulating boards around half price with slight damages. (Easy to fix with the expanding foam)
Google:- insulating+boards+seconds

Don't bury electric wiring, especially heating related, it may overheat.
It's good to thread wiring that passes through the insulation through a bit of plastic conduit. (Keeps it cool and removeable.)

Well worth doing IMO.

[The Natural Philosopher]

2" min.

> Thanks.
>


--
Future generations will wonder in bemused amazement that the early
twenty-first century’s developed world went into hysterical panic over a
globally average temperature increase of a few tenths of a degree, and,
on the basis of gross exaggerations of highly uncertain computer
projections combined into implausible chains of inference, proceeded to
contemplate a rollback of the industrial age.

Richard Lindzen

[The Natural Philosopher]

On 10/02/17 09:31, Stephen wrote:
> On Fri, 10 Feb 2017 09:03:22 +0000, Nightjar <c...@bignell.me.uk>
> wrote:
>
>> starts at page 44:
>>
>> https://www.celotex.co.uk/assets/the-handy-guide_brochures_jan16.pdf
>>
>
> Thanks, I never thought to see what the manufacturer said.
>
> It seems most of it is about fitting celotex over the rafters rather
> than the joists. The pictures are not very good. In fact the text says
> to have the non-printed side facing out and the photos show the
> printed side facing out! I never realised a little red ink would make
> so much difference.
>
> I get the impression that I was looking at doing this the wrong way;
> rather than insulate over the joists, it seems I should insulate over
> or between the rafters (or both) but leaving an air gap from the
> eaves to the apex to ventilate the rafters.
>

Yes. IF you are doing a 'warm' roof, with stay 6" rafters, use 4"
celotex between and say 2" celotex over, with a 2" gap from eaves to
apex and roof vents.

tape the first layr to the rafters as a vapour barrier.

> It seems the traditional fibre glass roll still has a place between
> the joists.

Its OK. If that's where you want to insulate, esp. if boarded over.



>
> Thanks,
> Stephen.
>


--
“But what a weak barrier is truth when it stands in the way of an
hypothesis!”

Mary Wollstonecraft

[John Rumm]

On 09/02/2017 21:03, Stephen wrote:
> Hello,
>
> It has been suggested here before, I think by the late Andy Hall, to
> use celotex sheets to insulate your loft.
>
> How is the best way to do this? I would seem to me that the easiest
> way would be to butt the boards up to each other, above the joists;
> that way you would not have to trim them to fit between the joists.

Yup, you can then lay floorboards directly on the foam, and they will
take the weight.

> Also if the celotex was put between the joists, you would have some
> heat loss through the joists as the wood would not insulate as well as
> the celotex would. This way, there is a continuous surface of celotex.

use normal rockwool or similar between the joists.

> But... what happens when you get into the eaves?

Just rely on the rockwool etc.

> I tried pushing an off cut into the eaves but the sheet hits the
> rafters, so there is a large gap between the rafters and the
> eaves/sarking felt, which defeats the object if the heat can just
> convect around the edge.

With the insulation underfill that should be negligible.

> Are you supposed to cut out sections for the joists? I do not have a
> ventilated soffit, though that said, I'm sure it's not airtight
> either, would it cause problems if the celotex was butted right up to
> the sarking?

Generally as long as air can reach the roof timbers, they should be
fine. You may also have a breathable sarking depending on the age of the
property.

>
> Should I just use the traditional rolls of fibre glass instead?

As well, rather than instead. They are cheap and easy for between joist
installation, but far from ideal for anything deeper. The rigid boards
and then ideal for the next layer.


--
Cheers,

John.

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

[John Rumm]

On 10/02/2017 10:44, harry wrote:

> Don't bury electric wiring, especially heating related, it may overheat.

agreed

> It's good to thread wiring that passes through the insulation through a bit of plastic conduit. (Keeps it cool and removeable.)

That's nonsense.

It may be removable, but plastic (or metal) conduit is not going to make
the insulating effect of the insulation go away.

A flat T&E cable in >= 100mm of thermal insulation is treated as
installation method 103. Method 103 reduces the current carrying
capacity of a cable to HALF its clipped direct value. The addition of
conduit makes no difference.

http://wiki.diyfaq.org.uk/index.php/Cables#Installation_Methods
http://wiki.diyfaq.org.uk/index.php/Calculating_A_Cable_Size

[Roger Hayter]

John Rumm <see.my.s...@nowhere.null> wrote:

> On 10/02/2017 10:44, harry wrote:
>
> > Don't bury electric wiring, especially heating related, it may overheat.
>
> agreed
>
> > It's good to thread wiring that passes through the insulation through a
> bit of plastic conduit. (Keeps it cool and removeable.)
>
> That's nonsense.
>
> It may be removable, but plastic (or metal) conduit is not going to make
> the insulating effect of the insulation go away.
>
> A flat T&E cable in >= 100mm of thermal insulation is treated as
> installation method 103. Method 103 reduces the current carrying
> capacity of a cable to HALF its clipped direct value. The addition of
> conduit makes no difference.
>
> http://wiki.diyfaq.org.uk/index.php/Cables#Installation_Methods
> http://wiki.diyfaq.org.uk/index.php/Calculating_A_Cable_Size

How long does a piece of well-insulated conduit have to be before the
cable through it is fully derated? Common sense suggests that an inch
long piece of 20mm conduit will have negligible effect. What about six
inches, such as proposed *through* the loft insulation? I can't believe
that the equilibrium temperature rise at the centre is the same as in,
say, ten feet?

--

Roger Hayter

[RJH]

On 10/02/2017 10:44, harry wrote:

Is this really the case? So long as there's no air movement between the
warm and cold areas, I'd have thought the impact would be small. And
down to negligible if neatly cut board was joined with tape. Perhaps a
thin line of expanding foam at the join for good measure?

--
Cheers, Rob

[John Rumm]

On 10/02/2017 14:08, Roger Hayter wrote:
> John Rumm <see.my.s...@nowhere.null> wrote:
>
>> On 10/02/2017 10:44, harry wrote:
>>
>>> Don't bury electric wiring, especially heating related, it may overheat.
>>
>> agreed
>>
>>> It's good to thread wiring that passes through the insulation through a
>> bit of plastic conduit. (Keeps it cool and removeable.)
>>
>> That's nonsense.
>>
>> It may be removable, but plastic (or metal) conduit is not going to make
>> the insulating effect of the insulation go away.
>>
>> A flat T&E cable in >= 100mm of thermal insulation is treated as
>> installation method 103. Method 103 reduces the current carrying
>> capacity of a cable to HALF its clipped direct value. The addition of
>> conduit makes no difference.
>>
>> http://wiki.diyfaq.org.uk/index.php/Cables#Installation_Methods
>> http://wiki.diyfaq.org.uk/index.php/Calculating_A_Cable_Size
>
> How long does a piece of well-insulated conduit have to be before the
> cable through it is fully derated?

500mm

(note its not the conduit itself that is the problem - its when its in
insulation - with or without conduit)

> Common sense suggests that an inch
> long piece of 20mm conduit will have negligible effect.

Indeed, you can ignore anything up to 50mm.

> What about six
> inches, such as proposed *through* the loft insulation? I can't believe
> that the equilibrium temperature rise at the centre is the same as in,
> say, ten feet?

There is a sliding scale such that the de-rating factor increases with
the length that is insulated.

So from table 52.2 of the OSG you get factors of:

0.88 for >= 50mm,
0.78 for >= 100mm,
0.63 for >= 200mm,
0.51 for >= 400mm,
0.50 for >= 500mm

Note that there may be an additional factor to apply due to the ambient
temperature - since its common for many lofts to get very hot.

[Another John]

In article <o7k1hn$efg$1...@gioia.aioe.org>,

Good point Martin (that was me above btw). I've just been up the loft
to check: evidently, I sawed them in half, lengthways. I didn't tape
them back together: (1) a PITA and (2) I have rockwool between the
joists anyway.


I disagree with the point made in another post that being unable to
butt-joint tightly is a problem -- "The slightest gap defeats all your
work": a gross exaggeration.


To Stephen: I used the 50mm (2") thickness -- mainly because i didn't
want to spring for the 3" thickness.


J.

[Andrew]

On 10/02/2017 09:35, Martin Brown wrote:

I used Knauf loft boards which are 50mm thick EPS, the closed cell
varient which is pink. Back in 2009 they were being sold at a very
subsidised priced courtesy of the energy cos.

[Andrew]

On 10/02/2017 08:40, Brian Gaff wrote:

> What about using bubblewrap to do the awkward areas where you don't walk. It
> seems cheap almost too good to be true.
> Brian
>

Beware interstitial condensation if you use continous taped
celotex or bubble wrap.. A relative stuffed rockwool
willy nilly into the eaves of his link detached house
and the condensation got trapped around the truss ends
(Along the valley on top of the party wall)
in an unventilated space and did thousands of pounds of damage
requiring major timber replacement and strengthening.

[Andrew]

Myth..Celotex is closed cell, so will not allow the passage of water
vapour molecules.

[Roger Hayter]

Thanks, the derating goes up much faster than I would have thought.
Though I promise I would have looked it up if I was actually doing
anything like that.

In practice I think it is going to get much less hot in an open vertical
conduit than direct insulation though I don't suppose this helps evade
the regs. If it is really a nuisance complying with cable size, could
you put it in a bigger pipe?



--

Roger Hayter

[The Natural Philosopher]

Across the cells. But will between them


--
"Women actually are capable of being far more than the feminists will
let them."

[Nightjar]

On 10-Feb-17 10:44 AM, harry wrote:
...

> I have done this.
> It's not possible to accurately butt joint. The slightest gap defeats all your work
> The best way is to buy the foam sheets the same thickness as the joists.
> Cut around an inch smaller than the gap between the joists.

> Put in place & fill the gap with expanding fixing foam....

I found it effective to cut very slightly oversize and press firmly into
place. It compresses enough to close up any gaps. I also taped over all
joins with 6" wide aluminium tape.


--
--

Colin Bignell

[harry]

I found the stuff be only slightly compressible and brittle.
If you try to force it in, the edges spall away.