Insulating dormer-type roof?

Eldest daughter has finally got her way and the job of redecorating her room has been bounced to the top of the Round Tuit List. Not before time TBH!
Our first floor is built into the roof of the house, (c. 1912; dormer style?) so that a chunk of the ceiling is sloped on both sides, courtesy of the overlying pitched roof. Her room is particularly cold in the winter, hardly surprising since over that area of ceiling, all there is between the sky and the air in the room is one layer of lath and plaster plus one layer of rooftiles - yes really. So it's high time that Something Must Be Done.
There's no room to slot anything down the gap from above, and I know that a proper job would involve removing the lath and plaster, fitting a layer of Kingspan with a 50mm air gap etc, which would involve extending the rafters inwards to make them deep enough - all in all, a lot of work and mess which I really don't feel I have time for. So I'm looking for the next-best solution, which wouldn't involve ripping out the lath and plaster and which would minimise the loss of volume of the room. Something like attaching a new layer of insulated plasterboard to the existing slopey ceiling? What is there out there? Suggestions please...
Thanks David
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Nobody got any thoughts on my query below?! I'm surprised that this one of the very few queries posted here over the past week not to get any responses.... would be very grateful for any input.
Thanks David
Lobster wrote:

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Lobster wrote:

Depends if you want to compy with regs, or have a short-term solution that long-term may damage the roof. You could blow insulation into the gap, but that would block airflow and cause rot eventually. One layer of insulated plasterboard would be next to useless. The thinnest is perhaps the tri-iso stuff, but this doesn't work properly (if it ever does !) without air gaps. Probably no answers as what you ask cannot be done ! Simon.
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sm_jamieson wrote:

Thanks. Objective is to make the room a bit more comfortable and the heating bills a bit lower (without having to rip the place apart ideally). I'm certainly not going to attempt to fill the existing air gap for just thr reasons you say.
One layer

I guess this is the crux of the matter then - would it be really that bad (considering what's there at the moment?!) What do others think? What actually is this stuff anyway, never having seen it - what insulation does it have?
David
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On 21 Jul,

A 1" or more layer of Kingspan K17? or is it K18 would probably make a big difference. Not as much as the full job, but if there is /no/ insulation at all in the roof and walls, it would make a considerable difference.
Downloas the calculator from the superbeam website and do the calcs, you may be suprised.
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snipped-for-privacy@privacy.net wrote:

Have been wondering some more about this. Maybe the answer *is* to take the bull by the horns and do the proper job; ie ripping off the lath & plaster and fitting Kingspan between the rafters....
So this would be my scenario (from the Kingspan website): http://www.insulateonline.com/index1.htm?pitched7.htm~main
Slight problem is that my rafters are only 75mm deep; therefore to maintain the requisite 50mm airgap between tiles and insulation, that means I can get only a paltry 25mm thickness of Kingspan between the rafters before it protrudes below. I would have attached a further layer of 1" insulated plasterboard below, across the rafters.
Options that I can see are:
1. Fit 50mm insulation between the rafters, leaving only 25mm airgap (how much would that matter? Not interested in conforming to Building Regs just for the sake of it). 25mm insulated p/board below rafters, so 75mm Kingspan in total.
2. As 1, but first fix 25mm-deep furring to bottoms of all rafters, thereby generating extra 25mm airgap. Extra faff, and I'm wondering just how rigid this assembly would be; also means I lose an inch more of the room.
3. As 1. but fit only 25mm insulation between rafters, thereby maintaining 50mm airgap.
4. Any other options I've missed?
Would appreciate input on the relative merits of these!
Thanks David
2.
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On Sat, 22 Jul 2006 15:42:42 GMT, Lobster

Cutting cellotex boards reduces their heat resistance as it reacts to air at the cuts. 25mm is hardly worth it anyway so I'd put 50mm below the rafters. Probably best to remove the lath n plaster 1st so as to reduce ceiling height loss.
I've previously put 60mm below rafters. Its tricky to hold it in place as nails just pull through so I hammerd nails into offcuts of timber and then hammered them through the insualtion. When they are all in situ you tape up the joins and generaly the friction between the boards and the strength of the aluminium tape holds them up as you remove the temporary nails to attach the PB.
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marvelus wrote:

I did all my new build with 60mm between joists only. It jams in enough to tape up if cut carefully..you can apply looser 'pre taped' panels in awkward spaces..you could use bits of 'foam in a can' to hold it as well..I think that, with insulated board over the joists would make for a very snug solution. So option 1. is my recommendation.
One tip that I didn't follow. Seal to *airtight* with the tape or expanding foam. Only when I detected tiny little icy draughts round a window frame and caulked them up did I realise just how much difference air movement through the structure can make.
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The Natural Philosopher wrote:

Well, I've decided to go for it and pull it all down - God help me; given that I am it seems daft not not to fill up the inter-rafter gap with Celotex...

So you reckon I can get away with a 25mm air gap? I must say, providing I'm careful and don't accidentally block it up anywhere (is that the reason for the regs specifying 50mm?) it would make life much easier (and my room bigger!)
Thanks David
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Lobster wrote:

I think I had 4" deep rafters..but yes, for a small area 25mm is probably more than enough. I think the building regs are very over generous in regard for ventilation on loft spaces.

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What about that tin foil bubble wrap stuff I've seen? not sure how good it is but I thought it was supposed to be as good as the thicker more traditional insulation.. I've got the same problem with my bungalow, and would like a simple solution....
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Harris wrote:

Sadly there isn't one.
You need to insert someting into an inaccesible gap. You have to make it accessible.
That means either stripping out whats below, or whats above.
Sure any insulant will make a difference, even thin layers, but there is a reason why celotex is used. Nothing beats it. And nothing is , whilst an exceptional insulator, hard to package in the right depths ;-)
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