Insulating a flat and pitched rooves

20 Nov 2010
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United Kingdom
I've recently taken on a bit of a project, and learning a few new techniques as I go, so any assistance greatly appreciated.
I have a 3.6x 1.7x2.4 bathroom with a slightly pitched flat roof. I've ripped the ceiling out and the bathroom is going to be replaced.
My rafters look like about 3 inch high and the roof is Stirling boarded. Looks like it has been replaced recently, but not the rafters which are no brilliant but ok.
I want to know what I should do to insulate the roof. Can I put celotex right up against the board tight or should I lay it under the rafters. I intend to make up the 160mm required. Do I need to add a vapour membrane or allow ventilation?
What should I use to attach the celotex? Bearing in mind any screws are likely to be 160mm long? Or 160 minus the 3inch if I can go up against the board?
When applying the plasterboard onto the celotex, can I just screw to the celotex or do I need to screw through to the rafters?
I also have a kitchen extension uninsulsted with a pitched roof. This is felt and asbestos cement tiles. Again, can I just insulate up to the felt or do I need to leave an air gap?
Thanks in advance!
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If you can spare the headroom in the bathroom I'd look at maybe levelling the joists (for aesthetics only) and then bang some insulated plasterboard straight onto them. Or make the joists a bit deeper (fix some 4 x 2 to them?) and put the insulation between joists (leaving 50mm clear above the insulation).

With the kitchen (I've got a similar job facing me fairly soon) yes you'll need to leave an airgap- same board would do the job.

Both spaces will generate a lot of water vapour so you'll need to make sure ventilation is up to scratch (the insulated plasterboard will block vapour giving you condensation issues otherwise).

But those are my (not a building professional) thoughts- I'll keep an eye on this and see what other more experienced persons come up with!
Yes, I'm sure the gap is needed on the tiled roof, but I if I board straight over the rafters and leave a gap on on the flat roof in the bathroom, then that gap will not be ventilated anyway will it, as there will be no flow anyway. Why I was thinking of a vapour barrier.
In both room rooms there are extractors, probably due to the high levels of vapour produced as you say.
All this vapour control stuff, condensation etc- makes life very hard work!

I think the theory is that if you use insulated boards or vapour control layer on the joists then there won't be lots of warm waterladen air in the gap.

Unless you've got sealed rooves there will be fresh air getting to the ends of the rafters- the 50mm gap will allow enough circulation of that air to remove any small amounts of condensation above the insulation. A bit of water coming and going isn't a big problem- if the timber (or insulation) is permanently damp it'll not last very long
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So is the sliver barrier on the celotex sufficient as a vapour barrier or do you need to fit one in addition?
What about the single skin brick walls? Can these be battened to say 25mm and insulated in the same way?
Re the vapour barrier bit- I've just spent about 30 minutes on Google trying to determine the answer to that one! All these insulated boards use closed cell insulation so you'd think they would be vapour barrier as well but they don't explicitly say so (except for Marmox, which might be a bit thin for your purposes). Lots of the Celotex wall boards are shown as suitable for dot and dab which implies to me they would go straight on the wall (not much point dotting and dabbing to a sheet of polythene!)

The walls- depends on which sheets you use. Some (again Marmox I've been looking at) advise against leaving any clearance to the walls, others aren't specific (I haven't registered on Celotexs' website so can't see their data sheets).

My kitchen is very very small (only 2000 wide) so I'm not going to be able to fit as much insulation as Part L would require (since to do so would reduce the room area by more than the figure deemed as 'reasonable'. Plus I hate stud walls.
Got this off the celotex site
Insulation between rafters

Make sure there is enough rafter depth to
accommodate not only the thickness of the Celotex
insulation, but also a minimum 20mm drape space
for the breathable membrane.
▶ For optimum thermal performance, the unprinted
foil surface should face the air cavity.
▶ Fix battens to the inside face of the rafters, to ensure
that the drape space is maintained.
▶ Alternatively, counter battens can be fi xed over
the breathable membrane to provide a channel for
moisture run off . The whole depth of the rafter can
then be fi lled with insulation.
▶ All details are to be in accordance with the
membrane manufacturer’s details.
▶ Measure the space to be fi lled between the inside
face of the rafter prior to cutting the board.
▶ Use the Celotex Insulation Saw to cut the boards
at a slight angle, making the board width slightly
oversized on one surface to achieve a ‘friction fi t’.
▶ Push the boards into the void between the rafters until
they are tight up to the battens or the membrane,
ensuring that lateral joints are closely butted.
▶ Tightly fi t the insulation to the ridge plate and carry
over and tightly butt the wall plate at eaves.
▶ A vapour control layer (VCL) should be installed to
the underside of the rafters. A polythene sheet of
higher vapour resistance is recommended for high
humidity areas such as kitchens or bathrooms.
▶ Complete the internal fi nish with plasterboard.

It doesnt mention anything about airgap. it looks like the VCL is added as the last layer. Not sure what it means by the 'drape' space.
It also looks like the Celotex cannot be load bearing, that is carry the weight of the plasterboard. If you dont have sufficent clearance for the thickness you require, you need to add the insulation as a second lay and another set of rafters going to opposite way... Expense!
Interesting- ta. Note it only mentions breathable roof membrane though- if you have old school felt things might be a bit different. And VCL on the rafters (so the insulation is in the damp bit)- counter intuitive to me but they make the stuff so they should know!

Doesn't surprise me that you can't fix to the stuff and yes it is flipping pricey- again with my walls I was finding studwork to 150mm and rockwool costs (materials) less than half what Celotex etc comes to. Need to check my U value sums though...but the roof (when I get to it) will be getting 250mm rockwool on the joists (not big enough to live in so no point going for rafter insulation) at about £3/sq metre from Wickes.
Link to celotex guides.

I reckon in the case of the flat roof it might be worth going for the VCL, then pre insulated plasterboard, although mega $ it would be way quicker, and much lighter.

Installing insulation on the walls looks like a similar job, or building up a sort of a stud wall with battens.

Installing the shower/fittings might be slightly more tricky as clearly they must be held either against the stud or straight onto brick.

VCL is probably important in each case as it keeps the condensation away from the insuation and brickwork, though not sure how this would work around the shower.

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