Roofing insulation advice needed

[seneca]

Hello,
I've moved into a house with an old flat roof extension which has a bathroom in it. I've stripped out most of the room and removed the plasterboard ceiling. It has exposed the underside of the roof which did not look well insulated. There were a few rolls of old fibreglass type quilt and I removed those too so I have a completely empty space. I am about get quotes for new plastering of the bathroom and thought I would get new insulation put up there before it. Could someone please advise me on what the best form of insulation is, how it should be installed, where ventilation space should be and anything else I could do to improve the existing roof. I can't afford to have the thing rebuilt, but need a new bathroom in it. The top joist beam is 3" deep and the larger cross joist is 6". The sides of the roof have wood panels covering them and air flows through the gaps around it. Here's some pics

What are these kinds of roof called: warm or cold decked? Are any extra precautions needed, as it is a bathroom that will be more humid.

 

[PrenticeBoyofDerry]

I would leave the top timbers free of insulation, this will allow for ventilation. I would then insulate the remaining 6" depth fully with 150mm earth wool.
Then before boarding fit a vapour check barrier to the joists
or use foil back plasterboards.

I assume the electrical cable is lighting only, if not then they would be best left/clipped outside the insulated areas.

A warm roof is where the insulation is above the deck of the roof
A cold roof is where the insulation is below the deck of the roof.
Yours would be the cold version, if you intend to insulate the void of the deck.

 

[seneca]

Is earthwool the best thing to use? One plaster said it would be better to cut 150mm rigid kingspan PIR board (not sure of what exactly this is??) Should the foil backed plasterboard touch the insulation? Are all foil backed boards the same, or are there specific ones for the ceilings and walls of a bathroom. As it will only be done once, I'd prefer to pay extra, knowing I've done all I've can to improve the new bathroom (short of having the extension rebuilt!)

 

[PrenticeBoyofDerry]

You can use any insulation you like, I prefer earthwool to PIR!
It is ok for insulation to be in contact with foil backers, just make sure you have the ventilation above the insulation at at least 50mm.
The duplex board (foil back) is suitable for both walls and ceilings, where a vapour check is required.

 

[catlad]

I would go with rigid pir as its a bathroom and I would use insulated plasterboard
which has a vapour barrier included. Don't be tempted to put those small spotlights in the ceiling as they leak heat and moisture through.

 

[PrenticeBoyofDerry]

Don't be tempted to put those small spotlights in the ceiling as they leak heat and moisture through.

They also don't like being covered or installed within thermal insulation and must be given a ventilation gap, which then compromises the heat loss factors, you have already tried to deal with!

Of coures you can always use loft guards(difficult in inaccessible area to fit) or downlights that have integrated protection. But that would be difficult to install, using rigid PIR boards!

 

[seneca]

Thanks everyone for the help and advice. It looks not too difficult. I'm a reasonably competent DIYer, and so am considering doing it all and then just leaving the plasterboards for a professional to skim. I'm looking to draw up a list of things I'll need, so would appreciate help on a few points
1) 120mm thick insulation board leaves around 30mm on the 150mm lower joist. Should I leave a gap before the plasterboard, as some recommend, or use 150mm thick insulation and have the plasterboard touching it.
http://www.wickes.co.uk/Celotex-XR4000-Insulation-Board-1200X2400X120mm/p/113799
2) Which plasterboard should I get: the vapor panel with foil backing?
http://www.wickes.co.uk/Knauf-Vapour-Panel-Square-Edge-2400x1200x12-5mm/p/220230
OR
The moisture panel with no vapourshield foil?
http://www.wickes.co.uk/Knauf-Moisture-Panel-Tapered-Edge-2400x1200x12-5mm/p/190540

If foil backed vapour panel or the non foil backed moisture panel, do I still need a polythene vapour barrier between the board and the joist?
http://www.wickes.co.uk/Wickes-Green-Polythene-Vapour-Barrier-2-5-x-20m/p/153230

 

[catlad]

Celotex PL4000 on the underside of the ceiling with whatever you choose between the joists with no gaps, and because its a bathroom I would use a vapour barrier as well.

 

[seneca]

Hello catlad, you've put another option to me. Do you think my idea of :
- celotex 120mm between the 150mm joists (leaving 30mm free at the top) and leaving the joists above clear, is ok?
- vapor panel (with foil-backing vaporshield) on joists with no gap
is ok? Do you suggest I put the celotex PL4000 underneath and between the plasterboard and joist? Is that or extra insulation o some other reason? I have not come across that yet, but would do it if it helped.
hanks

 

[cjard]

I'd put the 120mm rigid board between the joists (cut it so the sides are angled - widest point at the top and a snug fit to hold the board up (wedged between joists), then use expanding foam to fill the rest of the angled gap. Don't forget to leave boards standing off from each other too, and fill that gap with foam - rigid boards are great insulators but there isn't much point in using them if the cold air from one side can get through a gap/blow round to the warm side. In this regard wools are better because theyre squishy - you can easily get a 650mm wide strip of wool in a 600mm gap between joists and it fills the space up

After your inter-joist space is insulated, fit a vapour control layer (something like tyvek airguard), then fit insulation backed plasterboard with long drywall screws. Be as accurate as possible (laser/chalkline) when screwing through the VCL so your board achieves a good pinch around the hole the screw makes. The VCL is to stop bathroom moisture (water vapour in the bathroom air) making its way into the timbers

It's probably cheaper to buy e.g. 25mm of PIR in sheet form and bond it to the plasterboards yourself than it is to buy factory bonded board - your time vs your money i guess.. bear in mind that moisture resistant plasterboard (as should be used in a bathroom) bonded to insulation isn't often used and may carry a price premium as a result. You could, additionally, not bother bonding the plasterboard to the insulation but then if youre using 600mm centres and no noggins you should really be using 15mm board (according to the manufacturers)

 

[seneca]

Ok. Thanks. I'm going to go with the 120mm between the joists. Then 25mm below joists, vaporshield sheet, moisture board and then skim. Sound ok? (simplified form , but then I'm simple!)

 

[cjard]

Yep. As a tip for holding the vapor sheet up, get ahold of some underfloor heating staples, big plastic u shaped clips they are, with barbs sticking out.. Pushed through the sheet with a thumb, into the insulation and they will hold the sheet up while you fit boards

 

[cjard]

Or you can staple the sheet to the joists and fit the 25mm after