flat warmroof insulation position question

[razorman]

Hi guys
ive built my extension to roof level which will be a flat warm roof design.
100mm brick outer leaf, 100mm cavity drithetm 32 full filled, 100mm aerated block internal leaf.
Joists rest on 100mm internal wall leaf and innerleaf blockwork continues between joists and above.
External brick skin to same height as internal leaf so as to envelope joists and insulation inside of build.
9mm wbp board fixed to top of joists.
Vapour barrier.
130mm celotex slabs to 9mm wbp board.
18mm t&g osb3 on top of celotex.
waterproof membrane (grp proposed) to finish.

Question 1. Initially I envisaged celotex to fit flush within the internal aerated blockwork top inner edge build and osb to go across top of Insulation and over inner and outer cavity skins. Would this give me a significant cold bridge where osb rests on the themal blocks and also causing possible condensation problems with a dewpoint?

Question 2. Would it be better to lower internal blockwork so as to accomodate celotex to then go over the too of it and continue to inner face of outer leaf brickwork?

Question 3. Continuing with q2 proposal would it be best to end celotex on top of internal skin and butting with the cavity drytherm raised up to the celotex top edge?

BC had advised not to place celotex next to inside of external wall unless either leaving a 25mm gap or adding a dpm between.
Im worried leaving a 25mm gap would allow a void for interstital condensation?
He also advised I wouldnt need a vapour membrane at warm side of ceiling under the celotex which really has confused me.

Im happy ending the celotex on top of internal blockwork where it will be in butting junction with the drytherm cavity insulation just wondering if they will interact negativley regards condensation?

Any advise will be appreciated so I can get this roof finished hopefully before christmas to keep her indoors happy!


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[razorman]

Surprised no replies!!
Anyone recommend another forum for advice/ answers to these questions??

 

[freddiemercurystwin]

Hold your bloody horses! **** sake!

 

[razorman]

Hi FMT.
lol contact made!

Yes im in a wee bit of a hurry need to get my grp roof done before winter really sets in.

im feeling the way to go is to butt the 130mm celotex to the inside of the external brick skin so as the top of the celotex is level with the top of the 50x100mm plate ive fixed on top of exterbal brickwork for facia fixing.
There will be a vpm in place below insulation on the warm side folding up around and over celotex etc.
this means the cavity drytherm batts will butt up undernath the celotex and vpl.

Really not sure if im gonna cause a dewpoint in the wrong place but hopefully not!

Really could do with some advise as ive lost trust with the bc a little bit.

 

[freddiemercurystwin]

The 18mm OSB on top just runs through and sits on a wall plate on top of your outer skin.

Stop your roof insulation in line with the outer face of the inner skin.

Run your Dritherm up so that it abuts the underside of your 18mm OSB, thus meeting the Celotex.

Vapour barrier underneath the Celotex. I don’t know why he’s saying don’t bother, there is even LABC guidance recommending their fitment, its good practice to do so especially considering the low cost.

 

[razorman]

Thanks FMT for this advise its well appreciiated!
Yep osb over the top to leading edge of plate, that will be good for affixing any battening for grp drip trim adjustments.

Im thinking of adding some butyl self healing tape along area run of joists above or maybe below vapour barrier so as to minimise hole sizes made by board fixings. Something like this maybe?
http://building.dow.com/na/en/products/specialty/wmstraightflashing.htm
The vapour membrane ill use is a dpm 1200 gauge which with experience minimise tearing.
Concerning the v membrane would you recommend wrapping it around edges and lap over top of celotex thereby creating a barrier between dritherm batts and the celotex edge?

Thanks again.

 

[freddiemercurystwin]

Use your tape f you want, though IMO its overkill and I doubt any normal extension Pro would bother.

Lap your VP down against the outer face of the inner skin, if you feel the need. frankly you can just trim it where it meets the Dritherm.

You're over thinking it all a bit.

 

[razorman]

I know I know im in the class of "do it right do it once"!

Which is exactlywhy im a bit paranoid about trying to go one step beyond and causing more probs etc.

At least im more assured of which way im going, with which Im grateful to you FMT!
I can now progress with getying the celotex in solid.

Bricks arriving for final part of job for widening doors tomorrow (late decision of the other halfs) then I can get one of the internal walls knocked through and get the thing a little more progessed to its finality.

Thanks again for ur advice.
Rob.