Best way to insulate flat roof dormer from the inside (convert warm roof design)

[Stoozy]

Hi All - looking for some advice. Decided to redecorate bedroom which has a dormer window in but failed miserably attempting to scrap off wallpaper on dormer part of the ceiling.  So.. decided to re-sheet with plaster board and after pulling off the old found out that there was zero, zilch, nada insulation in the roof (not totally surprised as the room was always a bit chilly).  So started some research on best way to insulate it and it looks like the correct way is to leave a 50mm gap ventilated gap, then insulation of your choice, then a vapour barrier, then the the plasterboard.  Sounds great but I have a problem with the ventilating part as it looks like I would have to ventilate between each joist which I can do but would be a pain.  The joists are 120mm deep and the area is 1.6m x 3.2m.  So my question is can I spray on expanding foam instead and cover the whole area and so convert it to a warm roof design?

I've attached a photo.

Ta in advance.

[clockmanFR]

If this was one of my problems?

1. Drop ceiling and make a suspended frame for the plasterboard to attach, normal insulating betwen frame and the original.
2. Strip/ Raise roof and timber work, and allow extra space for good insulation.

I think anything else will be a short term bodge???

[JohnS]

Is the existing space ventilated?

If it is not, you would need to ventilate it.   Clockman's first option would not work without ventilation.

[MR GUS]

What's wrong with 70mm foil / foil pir insulation like celotex between each joist? (which leaves 50mm air gap)
simple job!

depending whats on the outside cut-outs for a few trickle vents?

[Stoozy]

Thanks for the replies. 

I should have said there is currently no ventilation and this has caused no problems to date presumably because there is no insulation and so no hot to cold transition for condensation to form on.

I still like the idea of filling the whole void with polyurethane but it does look like more of a faff (if that is how you spell it) and I am nor sure if I would get condensation forming somewhere that it shouldn't.

 I was hoping I could do something that wouldn't entail putting in ventilation.  On reflection however I think I will do this as I can get to the soffits fairly easily and fit these - http://www.screwfix.com/p/circular-soffit-vent-white-pack-of-10/68936.  I would then do as Mr Gus suggested and friction fit 70mm celotex between the joists, tape the edges with aluminium tape and then fit 12.5 mm plasterboard with a combined vapour barrier.  Or maybe even plasterboard with a combined insulation and vapour barrier, I can live with loosing an additional 30mm or so.


So what do you think. Sound alright?

[MR GUS]

What is the state of the roof above? ..is it a flatty?

if it's not in very good nick(& I'm no expert here believe me) you could use composite pre-felted insulation boards, finished off with a layer of roofing felt , OR (for good condition flat roof) insulating boards topped off with lightweight slabs..

Just a thought, but i'd refer to Biff & others in the first instance, as this was gleaned from a library book which may not be that new! ,options however are always good.

[Stoozy]

Hi MR GUS - yes the roof is a flatty but is in good condition.  It is also one half of a dormer that is attached to my neighbour i.e I am in a semi.  So I think it would complicate it to have a different height than my neighbours half and probably isn't required.  I reckon I'll do as I said last post and ventilate between the joists and insulate between the joists leaving a 50mm gap.

I need to get this done ASAP as I can't get on to my next project until this is done..  Installing shiny new thermal store with solar and linking up my stove and gas boiler.  Now that's going to be more fun than mucking about with flat roofs.

Thanks again

[biff]

50mm of kingspan or celotex as gus said,inserted neatly between the joists,leaving a gap of 50mm above and 25mm below to the foil backed plasterboard. this will leave the room toasty roasty.it can be done in a couple of hours.
         you will feel the difference before you start clading the celing with the foilbacked plasterboard,you will be sweating putting it up.
                                                                                                 biff

[Heinz]

50mm of kingspan or celotex as gus said,inserted neatly between the joists,leaving a gap of 50mm above and 25mm below to the foil backed plasterboard.

What's the 25mm gap between the plasterboard and kingspan for?

Heinz

[biff]

its another thermal barrier heinz, making contact between the surfaces breaches the thermal qualities.
                                                                                                  biff

[stannn]

Biff
I'm surprised that you think that only 50mm thickness of Kingspan will make it 'toasty roasty' in a bedroom especially on the coldest days of the year. My bedrooms have the rafters as part of the ceiling and they are less than 4" deep. So the glassfibre insulation is only 3" thick and it performs badly. I can't see how to improve matters without a new roof and that is a huge project because our plasterboard/decor is attached to the rafters.
Stan

[EccentricAnomaly]

What's the 25mm gap between the plasterboard and kingspan for?

Expanding on Biff's reply: the point of the foil backing is to reduce the radiative transfer of heat (as long-wave infrared). If you put the two surfaces in direct contact most of the transfer will be by conduction¹; separating them means that it has to be by radiation (which the foil will reduce) and convection. Even a tiny gap would help² but from a practical point of view a wider gap is easier to set up reliably.

¹ Which the foil would tend to make worse rather than better as it would help transfer heat "sideways" from areas not in contact to areas in contact.

² In fact, might be better (or at least more efficient use of the space available) because convection could be less organised. Like double glazing not getting much more insulating once the gap goes beyond a certain amount.

[biff]

hello stann,
           that particular combination i described above is very efficent,even 50mm of kingspan is brilliant but put the whole lot together as described and you have a very well insulated job. the kingspan needs to be well fitted with no gaps so a bit pf patience is required and proper use of the tape.keeping the 25mm gap between the two foils is the first line of defence then the kingspan really blocks the heat in.
                                                                                      biff

[Stoozy]

Hi All - thanks for the replies guys.  Checked a few more of the joists and they actual go down from 120mm to 100mm so there is a slope on the flat roof.  Also opened up the sloping part of the room (see photo below) and the joists are all 100mm (4 inch).  I would also like to insulate this part as well.  So not sure what to do.. go with 50mm gap and 50mm kingspan and then an air gap ranging from nothing to 20mm and then plasterboard with vapour barrier or go with 50mm gap, use 40mm kingspan (if you can get it) and have an air gap ranging from 10mm to 30mm and then the plasterboard with vapour barrier.  I could also use plasterboard with insulation bonded to it as well.

Anymore thoughts would be appreciated.

Thanks

PS I am hoping to buy the stuff tomorrow afternoon and get started tomorrow night so any ideas before then would be great.

[Stoozy]

I was being a bit thick on that last post I obviously hadn't put my brain in gear .  I can do as Biff suggests and just nail on 25mm battons to the existing joists to maintain an air gap between the plasterboard and kingspan. So it would be -  roof - 50mm to 70mm ventilated gap - 50mm kingspan - 25mm air gap - plasterboard with vapour barrier.  I have a question on this though - wouldn't there be a chance of condensation forming in the 25mm gap as there is no ventilation?