Flat roofed timber framed extension. best way to insulate?

[Iwantsolarpv]

Any advice on how best to insulate our extension would be greatly appreciated.
 
This is a 1960's extension. It is single skin brick with a timber frame, I started to remove the pecan wood covering ( which got a lick of emulsion a few years ago when we could no longer bear the brown! There is  a thin layer of fibreglass insulation between the wooden frame and what appear to be a chipboard behind it. This sounds hollow behind it when tapped. The depth to the chipboard is 100mm. Would celotex or similar be best? Is it best to fill it completely and then add ordinary plasterboard? Do I need a vapour barrier? Photos below






The flat roof is a 10degree slope towards the house and after removing the plaster board between the beams there appears to be strange looking boards (see photos) with bitumen seeping through. The rafters are about 100mm in depth Again would celotex or similar be best? Do I fill it or should I leave any gaps?  Or would I be better to board directly on to the rafter with some insulated plaster board?

[Fintray]

1960's, might be best if you got that checked for asbestos as that could be asbestos insulating board you have there.

[biff]

hi folks,
       its been a few years since i saw that type of roofing, all it is, is straw mixed with cement and covered in tar.it was supposed to be a quick fix for flat roofers in the 60s but as far as i was concerned it was a hearbreak. if it is in good condition ,just leave it alone,it is also supposed to be a form of insulation but i always tried to stay clear of it, it is not good to walk on it, not so much that it will collapse but your feet will leave hollows which will collect the rain,
    meanwhile your idea of celotex or kingspan is an exellent one,between the studs but on no account bridge the cavity, it will bring the water in and degrade the woodwork.i would also avoid the use of a vapour barrier.
                                                                good luck,,,,biff

[JohnS]

If it is 50 years old, it might be time to rip it all off and create a new warm or inverted roof.

John

[biff]

johns,
        i have not seen many of these inverted roofs,,however if a thing works well then there is no reason to disturb it.
                                                                                                          biff

[Iwantsolarpv]

johns,
        i have not seen many of these inverted roofs,,however if a thing works well then there is no reason to disturb it.
                                                                                                          biff

Thanks Biff  i googled straw board and came up with stramit board, which I guess is what you are talking about. So phew no asbestos!
the roof is fine the felt is good and we have had no issues with water ingress. I would be worried about roof alterations as don't know what foundations are like with it being single skin. So would rather stick to insulating it. Would it be as well to place 50mm celotex right up to the stramit board and then place the plaster board directly onto it or should I leave any gaps.

Re: wall. Should I fill the whole 100mm void with celotex or leave a gap between celotex and plasterboard?

[biff]

hi iwant,
         leave a 25mm gap between the bottom of the strawboard and the celotex and just clad the whole ceiling flush with foil backed plasterboard,you will find that it is brighter and better acustics.it should take 75mm celotex between the celing joists.make sure all the seams are snug and tight.
      same on the walls leave a gap and 75mm of celotex or kingspan is extremely efective.clad flush with 12mmfoil backed plasterboard with tapered edges,leave a 3mm key gap between the sheets,screw everything home with a 35mm black starpointed screw in boxs of 200, they take filler and become invisible, it should all work out fine,
                                                                   biff

[Iwantsolarpv]

hi iwant,
         leave a 25mm gap between the bottom of the strawboard and the celotex and just clad the whole ceiling flush with foil backed plasterboard,you will find that it is brighter and better acustics.it should take 75mm celotex between the celing joists.make sure all the seams are snug and tight.
      same on the walls leave a gap and 75mm of celotex or kingspan is extremely efective.clad flush with 12mmfoil backed plasterboard with tapered edges,leave a 3mm key gap between the sheets,screw everything home with a 35mm black starpointed screw in boxs of 200, they take filler and become invisible, it should all work out fine,
                                                                   biff

Thanks, much appreciated. Wouldn't have a clue otherwise. Will post pictures as the job moves on.
Shaun

[JohnS]

Inverted roof are where the construction, from bottom up is deck, waterproofing, insulation.  The insulation keeps the deck and waterproofing warm and hence less liable to thermal expansion and contraction.  The most suitable insulation is polystyrene and Dow do something suitable

http://building.dow.com/eu/mdk/mk/applications/flatroof/index.htm

Another way is to have waterproofing both above and below the insulation.

[biff]

this business of calling a flat roof with its membrane covered in poly and gravel an inverted roof ie kind of daft and intended to confuse and make the spec blokes look clever.
  its over 25 years ago since i did a roof with esso butyl.it was midwinter and the roof in question had been damaged in a storm, we ripped the old flat roof off and threw on new decking,and area 13ft x 24ft, it was rush rush and we had no time or weather to stick the sheet,we threw a layer of ordinary felt down on the ply decking to take the shock out of the joints and then threw the butyl sheet down on top of that,(i rember it was damd heavy for the two of us)we walked about in slippers one holding down the polly sheets while the other put pebbles on the polly sheets to keep them from blowing away.it was hard graft and the weather was atrocious, but we got it done,the fillet round the top of the facia was finished with the supplied s/s trim,trapping the edge of the sheet.
  the job was a resounding sucess, no leaks and the decor and furniture in the house were saved, now all these years later i discover that the guys who draw up the specs are calling it an inverted roof,,the reason i used the polly on top of the sheet was that i needed something to keep the gravel from puncturing the sheet and "2 of polly was the only thing that would have carried our weight while we weighed it down with pebbles.the fact that it insulated the roof sucessfull was a bonus.
i would have thought that an inverted roof was a roof that peaked downwards rather and upwards and indeed the people who designed such a calamity refered to it as an inverted roof.over 40yrs i have been dodging and shuffling round arcitechts,trying to ignore their cockups and asking them to correct their plans,one thing i did notice was that the higher up the ladder they were,the more liable they were to invent new words for old bones.i did not mind the in least when the tried to get one up on the juniors,just as long as they kept the latin out of it.
         anyhow,, now i know what an inverted roof is,,
                                                                biff

[Iwantsolarpv]

Inverted roof are where the construction, from bottom up is deck, waterproofing, insulation.  The insulation keeps the deck and waterproofing warm and hence less liable to thermal expansion and contraction.  The most suitable insulation is polystyrene and Dow do something suitable

http://building.dow.com/eu/mdk/mk/applications/flatroof/index.htm

Another way is to have waterproofing both above and below the insulation.

Aaah, thanks John will bear this in mind when it does give up the ghost!

[wookey]

Iwantsolar: I'd be wary of just putting in a load of insulation below an existing flat roof. It's very easy to create a design where all the water vapour migrates behind the insulation and makes your timber soggy. Eventually that can be disastrous. That's why warm or inverted roofs are preferred these days, and ventilation is recommended/required for cold roofs (which is what you are proposing if I understand correctly). There has been a lot of discussion of this sort of thing on the Green building forum. You may find reading some threads there illuminating. The vesma u-value calculator can do simple-minded condensation sums, but it just uses the British Standard sums which are generally acknowledged to be unrealistic. However they are pessimistic so if it says it'll be OK, then you should be OK. http://www.vesma.com/tutorial/uvalue01/uvalue01.htm
For a properly multi-year analysis download the free version of WUFI and check with that. Then you can be sure.