Running lighting cables inside insulated roof/ceiling

[Mathew Newton]

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

tween_rafters_0.jpg

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

Presumably recessed downlighters would be out of the question given the hea t/space issues without building a new ceiling suitably spaced from the stru ctural ceiling/roof?

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

der_rafters_1.jpg

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

Mathew

[meow2222]

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?

between_rafters_0.jpg

of the insulation? If so, above or below the vapour barrier? Should you en sure that there is a small gap between the insulation and plasterboard or c ould this air gap undermine some of the insulating properties of the setup?

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

eat/space issues without building a new ceiling suitably spaced from the st ructural ceiling/roof?

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

n AND under the rafter approach?

under_rafters_1.jpg

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

NT

[harryagain]

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

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.

[Mathew Newton]

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

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 sittin g right underneath one of them feeling like I'm on Mastermind. There's no r eason to believe my implementation would be any better so I'll scrap that i dea.

hermally. OTOH you can stick cheap squishy stuff between them.

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

Mathew

[Tim Watts]

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

[Mathew Newton]

So you get more insulation. You can get the rigid foam with a crinkly plast ic finish that can be plastered direct. (Joints need scrim) I fixed mine wi th big (galvanised) nails, washers and canned foam. Seems OK after three ye ars.

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

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

Mathew

[meow2222]

at 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 sitt ing 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 m ost of the lighting done by fluorescent uplighting.

thermally. OTOH you can stick cheap squishy stuff between them.

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

NT

[Martin Bonner]

public I can't help feel it's blindingly obvious!

at 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 sitt ing 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.

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

*Pers> On the subject of rafter depth this will be a new extension and so

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.

[dennis]

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.

[harryagain]

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)

[meow2222]

and related articles

NT

[jgh]

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

jgh

[Mathew Newton]

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

[Mathew Newton]

The intention was to have them covered so it was more to accomodate the nec essary level of insulation to satisfy U-value requirements and it being bet ween-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, includ ing insulation (or at least boarding once I've sorted the wiring), myself o ver a longer period.

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

Mathew

[RobertL]

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 e xample battened-on thinsulex). You can't get the required U value just wit h insulation between the rafters.

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

Robert

[Tim Watts]

Not fully true...

[Mathew Newton]

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.

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

[Andy Wade]

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.