Flat roof insulation

[Twickers]

Hi

I have an extension which is my kitchen, it has a flat roof which has been covered in EPDM, having had all the light wiring done I'm just about to insulate using Celotex or Xtratherm 1200x2400x100mm (3 boards required) between the joists, once this has being done I was going to put up 1200x3050x12 featheredge plasterboard (again 3 required) skrim, fill and paint.

Is there anything else I need to do i.e. allow gaps top and bottom of the Celotex or can I push it up to the underlying boards, do I need a vapour barrier etc or can I just put it up as described, is 100mm sufficient/too thick?

I have down lights going in once completed so need to clear them height wise.

Any help/advice greatly appreciated.

 

[Deluks]

Is there insulation under the epdm (over the joists?)

How deep are the joists?

 

[Twickers]

Is there insulation under the epdm (over the joists?)

How deep are the joists?

There's 18mm OSB3 between the joists and epdm
The joists are 180mm deep running up to 200mm at the rear/wall.

Have attached drawing showing measurements/possible application.

Untitled

• Twickers
• 18 Aug 2010
• 1

 

[Deluks]

You almost have a warm roof. You really should be insulating above or below the joists as well, as these act as a bridge, drawing heat to the outside.

Off the top of my head I think 150mm is minimum requirement for flat roofs. You are going to have to cut around the downlights no doubt.
Which type of lights are you using, if it's the transformer ones that get hot then you may suffer premature lamp or TF failure due to heat build up. Your electrician will be be able to advise here though.

these might come in handy.

Your insulation also needs to come down at the edges to meet the wall insulation.

If the roof isn't vented you might as well go full fill, and eliminate any air void which could cause problems. Insulated plasterboard over the bottom of the joists, and fudge the downlights in, possibly cutting the holes before putting the PB up and cutting away excess insulating foam around the holes so they will clip in normally.

 

[Twickers]

You almost have a warm roof. You really should be insulating above or below the joists as well, as these act as a bridge, drawing heat to the outside.

Off the top of my head I think 150mm is minimum requirement for flat roofs. You are going to have to cut around the downlights no doubt.
Which type of lights are you using, if it's the transformer ones that get hot then you may suffer premature lamp or TF failure due to heat build up. Your electrician will be be able to advise here though.

these might come in handy.

Your insulation also needs to come down at the edges to meet the wall insulation.

If the roof isn't vented you might as well go full fill, and eliminate any air void which could cause problems. Insulated plasterboard over the bottom of the joists, and fudge the downlights in, possibly cutting the holes before putting the PB up and cutting away excess insulating foam around the holes so they will clip in normally.

That does seem very excessive and a total waste of money if full filling, no offence, especially if using a solid insulation board like Celotex/Xtratherm that is foiled both sides and apparently proven to be better than fibre insulation.

This is a retro fit, as it's an old extension and the adjoining toilet just has fibre insulation and plasterboard so wouldn't contemplate full filling with Celotex et al.

I see where your coming from with the insulated plasterboard (Vapourshield Plasterboard) which I could plasterboard over again with the 10ft featheredge.

 

[Deluks]

No, the problem is one of air and moisture. If you haven't got any ventilation running under that roof then any air in there will be trapped. If any moist air (warm air holds moisture, warm air also rises) were to get in there (through the holes for the downlighters?) it could condense on the timbers, being that the timbers are not insulated from the outside and thus could become cold during the winter. Over a period of time your joists would absorb this moisture and become damp, leading to mould and eventually rot.

This wouldn't have been a problem if you'd insulated under the edpm membrane, but you weren't to know. If the roof was installed professionally then they should have known better and insulated over the joists. But that's roofers for you.

You could use polystyrene sheets as part of your infill (cheaper), probably about 75mm would be the maximum to go for, up against the osb, then the celotex. Celotex insulates much better than EPS, so best to have a reasonable depth of the pricey stuff in there.

I know it's more dough but it will pay for itself when you find that the room can be heated using just the warmth of your farts.


I take it you already have the plasterboard? The insulated stuff I mentioned is actually PB with a layer of celly or k'span stuck to it. The foil backed stuff provides vapour barrier only, not insulation.

That said you will need a vapour barrier. So as well as my other recommendations, you should fit a minimum of 25mm celotex under the joists, tape up all the joins with aluminium tape, then screw your plasterboard to this with 90mm screws.

You will lose 25mm headroom but better than having the joists shadow through the plasterboard after going through several cycles of expansion/contraction due to temperature change.

 

[Twickers]

Okay, done some searches and found relevant products to use as follows;

75mm Jablite Jabfloor70 pushed up tight to OSB3 then
100mm Celotex XR3100z up to that, joints sealed with foil backed tape, with voids around the downlights
Leaves a gap of 5mm (gutter) to 25mm (wall) to bottom of joists, as these are angled for run-off i.e. not totally flat
25mm Kingspan TP10 fixed to the joists, joints sealed with foil backed tape then 12.5mm featheredge plasterboard on top of that, skrimmed, skimmed and painted.

I did look at Kingspan K18 boards, 25+12.5mm plasterboard but it works out stupid money, over double the price of the above.

Diagram attached to show.

Untitled

• Twickers
• 18 Aug 2010
• 1

 

[Deluks]

Much better solution.
You should really get your lighting plan sorted out before insulating as some downlighters need a lot of space around them to stop them from burning out, you don't want to find this is the case and end up having to remove all the insulation above the fitting.

If you can swap the downlight bulbs for the best LED equivalent then there will be no heat given off at all. Plus will save a packet on leccy costs, despite the initial very high investment (£15 - £30 a bulb!) and you may need extra to compensate for lower output.

 

[Twickers]

Much better solution.
You should really get your lighting plan sorted out before insulating as some downlighters need a lot of space around them to stop them from burning out, you don't want to find this is the case and end up having to remove all the insulation above the fitting.

If you can swap the downlight bulbs for the best LED equivalent then there will be no heat given off at all. Plus will save a packet on leccy costs, despite the initial very high investment (£15 - £30 a bulb!) and you may need extra to compensate for lower output.

Thanks

I did look at LEDs but they are quite pathetic when you need bright light especially when preparing food etc plus wanted to have them on dimmers so they can be on the lowest light for ambiance or walk throughs, all planned though with wires ready to be connected.

Just need to raid the piggy for the double expenditure on insulation although all worked out should only be £180ish.

 

[Deluks]

Insulation is an investment, ok the returns diminish the thicker you go but in the long run it will save you money on bills.

Look into alternative's to running hot bulbs, I'm not too au fait when it comes to electrics but I know that poorly vented hot downlighters = bad news.

Mood and ambient lighting should be a seperate installation IMO. Use under cabinet lights for food prep, and maybe plinth lighting for ambience.