Position of Celotex in cold flat roof.

[JonB123]

I have a "cold" flat roof in which we are taking the ceiling down, so want to use the opportunity to renew the very thin original fibre glass insulation.
The roof has just been resurfaced, so not an option to fit insulated boards on top.
We have fire rated lv downlighters, so my question is, what is the best insulation option?
Joists are quite tall (200mm) so I had intended to fit 50 or 75mm celotex tight up to the underside of the roof boards between the joists, giving me a ventilation gap of 125 - 150mm between celotex and plasterboard, and a 25 - 50mm gap between celotex and top of downlighter.
BUT, should I actually have the vent gap above the celotex rather than below it?
I have had conflicting advice i.e:
1. That I need vent gap above celotex to prevent condensation on underside of boards.
2. That it is fine to fit celotex tight up to boards / joists, as then there shouldn't be any condensation.

If I lowered the celotex to provide a gap, it would interfere with the downlighters unless I start cutting holes in the vapour control layer which kind of defeats the object.

Or should I not bother with celotex and refill the gaps between the joists with a thicker layer of fibre glass type insulation, ensuring gaps round the downlighters and transformers where there would effectively be no insulation?
Sorry it's a long one, hope you can help.
Thanks.

 

[PrenticeBoyofDerry]

The insulation boards need the gap above them and ventilation there too.
They want to ideally lay on the plasterboard, tight into joist.
You downlights could need fire hoods on, unless they are IC rated.

 

[Nickam]

The insulation should go between the joists and you should leave a 50mm gap above the insulation. You should also ensure that the eaves have a continuous (or equivalent of in round vents) 10mm gap fitted with mesh vents. These vents should be fitted on opposing sides of the roof. There should be a vapour barrier on the warm side of the insulation as this prevents vapour condensing out within the insulation. Try to seal the vapour barrier as best as possible around the downlights. You should get Building Regulation approval in theory. To meet the current requirements will need a U-value of 0.25 w/m2K. This will require 150mm of insulation between the joists generally and the addition of 30mm gyproc thermal board for the ceiling. Some insulation has better thermal properties though. As an example, using Celotex XR3000 you should be able to use a reduced thickness of 130mm of insulation between the joists and standard 12.5mm plasterboard for the ceiling.
See:

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• Nickam
• 5 Mar 2010
• 1

Untitled

• Nickam
• 5 Mar 2010
• 1

Cheers,
Nick

 

[JonB123]

Perhaps I should add, that this is an unplanned (i.e. unbudgeted!) repair job following a small leak. I had considered just patching the plasterboard and leaving original insulation as is.
However as the ceiling has been knocked about quite a bit it is become reasonably cost effective to consider replacing the plaster boards and replastering.
I don't want to sound like I want to bodge this, but do I really need building regs approval to replaster a ceiling? The existing construction with no vents has worked fine for 40 years without a condensation issue, and I really don't want to spend more money on this than necessary (venting both sides of roof will be difficult).
It just seems sensible to up the level of insulation whilst the ceiling is down.
Also I have been advised by two tradesmen, a builder and a sparky (who has seen it installed quite a lot) that fitting celotex or similar insulation in contact with the roof boards is "normal".
Correct me if I'm wrong, but as celotex has a vapour barrier, if it is fitted tight between the joists won't it stop any warm moist air reaching the roof boards where it could condensate?
Hope you can see where I'm coming from on this.
Thanks.

 

[mointainwalker]

When you replace the ceiling, either use foil-backed pb or cover the top surface with a plastic-sheet i.e. vapour barrier.

The difficulty with down-lighters is that they need to be sealed to stop moist air being funnelled into the roof space, but if you do that you hit the other problem that downlighters need to be well ventilated otherwise they will fail frequently because of over-heating. No answer to that dilemma , I'm afraid.

You can re-use your old insulation whichever new type you decide to install

 

[ajrobb]

We have fire rated lv downlighters, so my question is, what is the best insulation option?

From your figures, the downlighters seem to stick up 100 mm above the plasterboard. You say they are fire rated so I GUESS that they are sealed and should not require much ventilation. I'd go the Spark's forum and ask advice about insulating around the lights.

1. That I need vent gap above celotex to prevent condensation on underside of boards.

This agrees with the advice I have seen - leave a 50 mm ventilation gap ABOVE the insulation.

2. That it is fine to fit celotex tight up to boards / joists, as then there shouldn't be any condensation.

It is common to install Celotex flush against the plasterboard. Then you don't need to worry about sealing any warm cavity to prevent cold drafts undoing any benefit of insulation.

If I lowered the celotex to provide a gap, it would interfere with the downlighters unless I start cutting holes in the vapour control layer which kind of defeats the object.

I don't think you have the option. You are going to have to cut holes in the insulation for the downlighters. Again, refer to sparks for advice on maintaining VPL.

Or should I not bother with celotex and refill the gaps between the joists with a thicker layer of fibre glass type insulation, ensuring gaps round the downlighters and transformers where there would effectively be no insulation?

Yes, you could fit 150 mm mineral wool between the joists leaving a 50 mm ventilated gap above. Ventilation of this gap might not be great, so you probably still need a VPL membrane next to the plasterboard.

 

[PrenticeBoyofDerry]

Are these Lights Low Voltage or Extra Low Voltage?
Extra Low Voltage lights are connected via transformers and Low Voltage is direct from the main supply.
You should avoid running cables directly through insulation, as it effects the current it can safely carry. If they are, they should be upgraded to a CSA that can safely carry the current, taking thermal insulating factors in to consideration. If that has not already been done.

 

[ajrobb]

Are these Lights Low Voltage or Extra Low Voltage?
Extra Low Voltage lights are connected via transformers and Low Voltage is direct from the main supply.
You should avoid running cables directly through insulation, as it effects the current it can safely carry. If they are, they should be upgraded to a CSA that can safely carry the current, taking thermal insulating factors in to consideration. If that has not already been done.

He says he has a transformer - so I guess ELV. But these are still inefficient. As he is changing the ceiling, it is an ideal opportunity to change to CFL lighting. I work on the assumption that three 11W CFL are equivalent to no more than two 50W halogen as I don't believe they look 4 or 5 times brighter.

 

[JonB123]

Thanks for responses.
In answer to last two posts, downlighters are extra low voltage, with 1 transormer / light. They are 90 min fire rated but they are not sealed i.e. they have vent holes in the top.
Clearly the downlighter and transformer should not be in contact with the insulation, to avoid overheating of the fittings not to mention damage to the insulation. I will check cleareance required.
The sparky made an allowance for the wiring potentially running through insulation.

It seems the problem I am running into is that I realy don't want to got to the difficulty and expense of installing ventilation, but if I improve the insulation (with celotex or fibre) I am more likely to have condensation issues as the area above the insulation will be colder than it was with just 1960's spec fibre glass.
Maybe I should just leave insulation as is?

 

[mointainwalker]

If you take the existing situation, by improving the insulation (witn vb) you diminish the possibility of condensation still further because less heat and less moisture is penetrating the loft.

If you install downlighters vented to the loft - either with current insulation or improved insulation - condensation problems are guaranteed.( or - having just re-read your post - have I made a fool of myself because your downlighters are already fitted with no problems ? )

Downlighters ( of any voltage) get very hot, hot air rises and if it can vent to the loft it will and the rising hot air in a confined space gives you a chimney effect which will suck up a lot of hot moist air which will condense in the loft.

Maybe you should post this question on the sparks forum - lots of helpful people over there and they must have seen the problem lots. Just a word of warning, they mostly don't like downlighters, considering them wasteful and ill-adapted for most domestic uses

 

[JonB123]

Hmm, downlighters are new addition (my sparky seems to like 'em, although I see no one here does!!) looks like I may be setting myself up with a problem.
I have realised one end of the roof is actually quite well ventilated, as it opens into a space with quite big soffit vents, so I am coming round to the idea of fitting vents at the other end, and having a proper gap above the insulation (whatever type I use).
This is single storey with very visible fascia boards so they need to be reasonably attractive vents but sure I can find something.

 

[mointainwalker]

Well, let us know how you finall ydecide to fit the downlighters, will you ?

 

[ajrobb]

IF you get rid of your downlighters, there is an effective ways of installing Celotex to maximise the insulation for the thickness bought. I've been banging on about this a lot lately, so I apologise for going over it again!

Reflective cavities are essentially free insulation. A vented cavity (above Celotex) has an R value of about 0.47 (actually a wall cavity R value from Proctor's "Reflective Technology" brochure). A sealed cavity (at least 19 mm) between the Celotex and the plasterboard has an R value of about 0.64 (same source). Together, they are like an extra 25 mm of Celotex for free.

Using GA3100 with 2 cavities, you get U value of about 0.18 between the joists. This is not bad, considering 250 mm of mineral wool has a U value of about 0.16. 150 mm of mineral wool with a 50 mm non-reflective vented cavity has a U value of about 0.25 but is a LOT cheaper than Celotex. The actual U values will be increased by thermal bridging of the joists but decreased by the other roof/ceiling layers.

 

[Nickam]

I would then suggest you fill to the top of the joists with fibre wool because:

You should NOT fill to the top of the joists with insulation as this is still a cold roof construction and as such requires that the area above the insulation be ventilated. You have a temperature gradient across the insulation from warm side to cold side. Any moisture laden air which does get into this area then isn't ventilated away and this allows any moisture vapour to condense out and create a situation where the timbers rot as the timbers then are trapped between the insulation. A warm roof construction has the insulation wholly above the timbers which are all in the warm side. That's why it's called a warm roof construction.

 

[mointainwalker]

@ Nickam

Please re-read post.

I said JOISTs not RAFTERS . So the whole of the loft above the top level of the JOISTS is still cold.

Agree ?