I am currently performing the mental planning of a single-storey kitchen extension which will have a pitched roof and ceiling. With reference to the following graphic showing Celotex insulation between rafters where and how should lighting and other ceiling-related cables should be run?

http://www.encon.co.uk/sites/default..._rafters_0.jpg

Specifically, am I right in thinking that they should be run on warm side of the insulation? If so, above or below the vapour barrier? Should you ensure that there is a small gap between the insulation and plasterboard or could this air gap undermine some of the insulating properties of the setup?

Presumably recessed downlighters would be out of the question given the heat/space issues without building a new ceiling suitably spaced from the structural ceiling/roof?

On a related note, what are the benefits of adopting an insulation between AND under the rafter approach?

http://www.encon.co.uk/sites/default..._rafters_1.jpg

Is it simply that more insulation can be used for a given rafter depth?

Mathew

On Wednesday, March 5, 2014 8:30:35 PM UTC, Mathew Newton wrote:

I am currently performing the mental planning of a single-storey kitchen extension which will have a pitched roof and ceiling. With reference to the following graphic showing Celotex insulation between rafters where and how should lighting and other ceiling-related cables should be run?
http://www.encon.co.uk/sites/default..._rafters_0.jpg
Specifically, am I right in thinking that they should be run on warm side of the insulation? If so, above or below the vapour barrier? Should you ensure that there is a small gap between the insulation and plasterboard or could this air gap undermine some of the insulating properties of the setup?

I'd lay the cables on top of the plasterboard.

Presumably recessed downlighters would be out of the question given the heat/space issues without building a new ceiling suitably spaced from the structural ceiling/roof?

Hopefully you'll come up with a better idea. Not all fashions are good.

On a related note, what are the benefits of adopting an insulation between AND under the rafter approach?
http://www.encon.co.uk/sites/default..._rafters_1.jpg
Is it simply that more insulation can be used for a given rafter depth?
Mathew

Better insulation. When only between rafters, the rafters still conduct thermally. OTOH you can stick cheap squishy stuff between them.


NT

On a related note, what are the benefits of adopting an insulation between
AND under the rafter approach?

http://www.encon.co.uk/sites/default..._rafters_1.jpg

Is it simply that more insulation can be used for a given rafter depth?

Mathew

I have that arrangement in part of my house.
Most rafters are only 100mm deep. So you get more insulation.
You can get the rigid foam with a crinkly plastic finish that can be
plastered direct.
(Joints need scrim)

I fixed mine with big (galvanised) nails, washers and canned foam.
Seems OK after three years.

On Thursday, March 6, 2014 2:08:45 AM UTC, wrote:

I'd lay the cables on top of the plasterboard.

That does seem to make sense. Indeed now that I've asked the question in public I can't help feel it's blindingly obvious!

Presumably recessed downlighters would be out of the question
Hopefully you'll come up with a better idea. Not all fashions are good.

He he. Quite. Bizarrely, I've often felt in other people's houses that that whilst I like the look of them in a kitchen environment they don't seem to always function particularly well, especially given I always end up sitting right underneath one of them feeling like I'm on Mastermind. There's no reason to believe my implementation would be any better so I'll scrap that idea.

On a related note, what are the benefits of adopting an insulation
between AND under the rafter approach?

Better insulation. When only between rafters, the rafters still conduct thermally. OTOH you can stick cheap squishy stuff between them.

I am still getting to grips with calcuating U-values so I might give that alternative a thought. I'd always assumed PIR boards were generally always better performers than mineral wool etc for a given thickness though?

Mathew

On 06/03/14 10:19, Mathew Newton wrote:

I am still getting to grips with calcuating U-values so I might give
that alternative a thought. I'd always assumed PIR boards were
generally always better performers than mineral wool etc for a given
thickness though?


You are correct - PIR foam (aka celotex etc) is about twice as effective
for a given thickness compared to most wool type products.

On Thursday, March 6, 2014 8:17:00 AM UTC, harry wrote:

On a related note, what are the benefits of adopting an insulation
between AND under the rafter approach?

I have that arrangement in part of my house. Most rafters are only 100mm deep.
So you get more insulation. You can get the rigid foam with a crinkly plastic finish that can be plastered direct. (Joints need scrim) I fixed mine with big (galvanised) nails, washers and canned foam. Seems OK after three years.

Thanks Harry. I must admit to being rather bewildered by the range of options available. Even within a manufacturer's range I'm finding it difficult to always discern the differences between some of their offerings or, if it's obvious, it doesn't always seem significant to justify both their existances!

On the subject of rafter depth this will be a new extension and so I am assuming I could specify 175mm rafters in order to accomodate the necessary insulation between them (say 150mm) and air gap (25mm) beneath the breathable felt.

Mathew

On Thursday, March 6, 2014 10:19:56 AM UTC, Mathew Newton wrote:
On Thursday, March 6, 2014 2:08:45 AM UTC, wrote:

Presumably recessed downlighters would be out of the question

Hopefully you'll come up with a better idea. Not all fashions are good.

He he. Quite. Bizarrely, I've often felt in other people's houses that that whilst I like the look of them in a kitchen environment they don't seem to always function particularly well, especially given I always end up sitting right underneath one of them feeling like I'm on Mastermind. There's no reason to believe my implementation would be any better so I'll scrap that idea.

If you like the look, you could have 5w halogen or 3w LED downlights with most of the lighting done by fluorescent uplighting.


On a related note, what are the benefits of adopting an insulation
between AND under the rafter approach?

Better insulation. When only between rafters, the rafters still conduct thermally. OTOH you can stick cheap squishy stuff between them.

I am still getting to grips with calcuating U-values so I might give that alternative a thought. I'd always assumed PIR boards were generally always better performers than mineral wool etc for a given thickness though?
Mathew

which alternative?
Yes, PIR is a much better performer, per given depth. And a poorer performer per cost.


NT

On Thursday, March 6, 2014 10:19:56 AM UTC, Mathew Newton wrote:
On Thursday, March 6, 2014 2:08:45 AM UTC, wrote:



I'd lay the cables on top of the plasterboard.



That does seem to make sense. Indeed now that I've asked the question in public I can't help feel it's blindingly obvious!



Presumably recessed downlighters would be out of the question

Hopefully you'll come up with a better idea. Not all fashions are good.



He he. Quite. Bizarrely, I've often felt in other people's houses that that whilst I like the look of them in a kitchen environment they don't seem to always function particularly well, especially given I always end up sitting right underneath one of them feeling like I'm on Mastermind. There's no reason to believe my implementation would be any better so I'll scrap that idea.



On a related note, what are the benefits of adopting an insulation

between AND under the rafter approach?



Better insulation. When only between rafters, the rafters still
conduct thermally. OTOH you can stick cheap squishy stuff between
them.

I am still getting to grips with calcuating U-values so I might give
that alternative a thought. I'd always assumed PIR boards were
generally always better performers than mineral wool etc for a given
thickness though?

PIR is better than rock-wool batts per mm, not as good per UKP.

*Personally*, I would stick the "over the rafters" insulation *above*
the rafters (so the rafters are in the warm, and hence dry). I'd
probably leave the rafters exposed too (but that's just me).

Else thread you also said:
On the subject of rafter depth this will be a new extension and so
I am assuming I could specify 175mm rafters in order to accomodate
the necessary insulation between them (say 150mm) and air gap (25mm)
beneath the breathable felt.

Why? Rafters are expensive. Don't make them bigger than you need
(unless you want them big for visual appearance). Much better to have
a solid layer of insulation above them.

On 06/03/2014 10:43, wrote:

which alternative? Yes, PIR is a much better performer, per given
depth. And a poorer performer per cost.


That depends on what figures you use..

The energy saving trust says that 135mm of glass wool is about the same
as 100 mm foam.

However they also claim that 75mm of foam with foil faces is about 30%
better than 110 mm of foam.

I don't trust either and I suspect the testing method makes a lot of
difference.

They don't give figures for fibreglass in those easy lay tubes with foil
on one side.

"Mathew Newton" wrote in message
...
On Thursday, March 6, 2014 8:17:00 AM UTC, harry wrote:

On a related note, what are the benefits of adopting an insulation
between AND under the rafter approach?

I have that arrangement in part of my house. Most rafters are only 100mm
deep.
So you get more insulation. You can get the rigid foam with a crinkly
plastic finish that can be plastered direct. (Joints need scrim) I fixed
mine with big (galvanised) nails, washers and canned foam. Seems OK after
three years.

Thanks Harry. I must admit to being rather bewildered by the range of
options available. Even within a manufacturer's range I'm finding it
difficult to always discern the differences between some of their offerings
or, if it's obvious, it doesn't always seem significant to justify both
their existances!

On the subject of rafter depth this will be a new extension and so I am
assuming I could specify 175mm rafters in order to accomodate the necessary
insulation between them (say 150mm) and air gap (25mm) beneath the
breathable felt.

Mathew

I have seen a few new buildings/roofs done that way.
I was insulating an existing roof.

Mind, there is an advantage with multiple layers and that is you can plaster
directly on to the finish of the insulation board rather than having to add
plaster board.
ie it will be big sheets on the inside if you see what I mean, few joints.

BTW, important there are absolutely no gaps. Even a tiny gap defeats the
whole project.
I cut my insulation undersize (by about 20mm) and filled the gaps with foam.
Trim excess off when set.
Slabs supported in place with temporary nails (removed when foam had set)

On Thursday, March 6, 2014 10:19:56 AM UTC, Mathew Newton wrote:

I am still getting to grips with calcuating U-values so I might give that alternative a thought. I'd always assumed PIR boards were generally always better performers than mineral wool etc for a given thickness though?

http://wiki.diyfaq.org.uk/index.php?title=Insulation
and related articles


NT

meow wrote:
Yes, PIR is a much better performer, per given depth. And a
poorer performer per cost.

That's why I get my by scavanging fly-tips

jgh

On Thursday, 6 March 2014 10:43:30 UTC, wrote:

I am still getting to grips with calcuating U-values so I might give that
alternative a thought. I'd always assumed PIR boards were generally always
better performers than mineral wool etc for a given thickness though?

which alternative?

Sorry, I meant 'cheap squishy stuff' (which I assumed to mean mineral wool type offerings).

On Thursday, 6 March 2014 13:28:34 UTC, Martin Bonner wrote:

Why? Rafters are expensive. Don't make them bigger than you need
(unless you want them big for visual appearance).

The intention was to have them covered so it was more to accomodate the necessary level of insulation to satisfy U-value requirements and it being between-the-rafters as I figured that would make things easier. Whilst I shall be getting a builder to build the shell I will be doing the inside, including insulation (or at least boarding once I've sorted the wiring), myself over a longer period.

Much better to have
a solid layer of insulation above them.

Ah, I hadn't considered that an option. Will go and find out more.

Mathew

On Wednesday, March 5, 2014 8:30:35 PM UTC, Mathew Newton wrote:
I am currently performing the mental planning of a single-storey kitchen extension which will have a pitched roof and ceiling. With reference to the following graphic showing Celotex insulation between rafters where and how should lighting and other ceiling-related cables should be run?


But surely there has to be some insulation below the rafters as well (for example battened-on thinsulex). You can't get the required U value just with insulation between the rafters.

Also, as it's a kitchen you need a Part-P electrician to do the wiring :-)

Robert

On 07/03/14 16:25, RobertL wrote:

Also, as it's a kitchen you need a Part-P electrician to do the wiring :-)

Not fully true...

Robert

On Friday, March 7, 2014 4:25:40 PM UTC, RobertL wrote:

But surely there has to be some insulation below the rafters as well (for
example battened-on thinsulex). You can't get the required U value just with
insulation between the rafters.

You can with 165mm Celotex and 175mm rafters (using 12.5mm plasterboard). This gives a U-value of 0.16. As mentioned in the thread though, the bigger rafters obviously cost more.

Also, as it's a kitchen you need a Part-P electrician to do the wiring

Indeed, but only as far as running the new circuits in i.e. a couple of radials for the oven/hob, and a ring for two sockets. Extending the latter, and sorting the lighting circuit, is something I'll do myself in due course.

Mathew

On 07/03/2014 16:25, RobertL wrote:

Also, as it's a kitchen you need a Part-P electrician to do the wiring :-)

A significant smiley there, I think :~)

FAOD Part P has never required that wiring has to be done by a member of
one of the Part P schemes. Scheme members can self-certify their work
and notify the local building control dept. via their scheme.

Others must (or rather, are supposed to) notify a building control body
by means of a building notice or a full-plans application, before
starting any notifiable work.

Since the revision that came into force on 6th April last year,
electrical work in a kitchen is no longer notifiable unless it involves
installation of a new circuit, or replacement of a consumer unit.

The fundamental requirement (P1) always applies though in a dwelling:
all work must be safe, whether it's notifiable or not.

--
Andy