garage conversion and the flat roof

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Help!!! i am planning to convert our attached garage into another bedroom and have just met with BC. We have met a major stumbling block!!! Because the garage is attached on 3 sides we are unable to get any through ventilation for the cold deck. I think the only way around this is to bite the bullet and fit a new warm deck flat roof. Apart from cost this isn't straight forward as the garage is one of a pair, so great care will have to be taken on the junction with next door. Plus i am also restricted by how much i can raise the existing roof level due to 2 small borrowed lights on the utility rm extension which abuts the back of the garage.
Any ideas? how much will i be looking at? can you combine the required insulation on top and between the joists? any ideas on providing the required ventilation without replacing a weathertight flat roof and possibly upsetting the neighbours who i don't know very well as we only moved in 10 weeks ago. Thanks
 
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If you remove all space between the joist from underneath by packing with insulation it will not require ventilation. A layer of rigid board across the joists after is belt and braces.
 
confused! By insulating below the deck i am forming a cold roof. Everything i have seen points to ventilation being a requirement in a cold roof. If it was as easy as filling the void and then plasterboarding the underside surely BC would have said this. If you can point me in the way of some techinical literature to back this up then i will be very gratful as this will solve all my problems
 
Sometimes BC will allow no insulation on top sometimes they won't. With no insulation on top and the bottom of the joists exposed either in the void between the insulation and plasterboard or even with the void fully filled with insulation condensation can occur against the bottom of timbers or the plasterboard can discolour where fixed to the joists as they are bridging direct to the outside.

Normally BC allow filling the voids and a thin continuous layer above say 40mm. This is a hybrid roof, a cross between a cold and warm roof but generally enough to get past any cold-bridging issues.
 
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Freddie.so you mean I would still have to insulate above the roof, but could get away with 40mm, by filling the voids below.sounds better as it would fit underneath the windows.still an expense I could do without, but better than what I was expecting
 
I agree with Fred although it does depend upon the BCO and their reasoning.

On authority actually let us fill entirely below a recently re-felted roof, without having to ventilate.

Another BCO said he would not pass this type of arrangement as a 'warm' roof and would ask for additional vent's.
 
Freddie.so you mean I would still have to insulate above the roof, but could get away with 40mm, by filling the voids below.sounds better as it would fit underneath the windows.still an expense I could do without, but better than what I was expecting
As mentioned it depends on the BCO you are lumbered with, you'll have to try and persuade him if he's awkward.
 
As I see it the most important part of a warm roof is an adequate vapour barrier which stops the moist warm air getting to the timber within the roof space and causing rot.

If you completely fill the space with insulation then you need an air tight layer on the underside before the plaster board goes on. :D
 
If you remove all space between the joist from underneath by packing with insulation it will not require ventilation. A layer of rigid board across the joists after is belt and braces.
Would be very interested to read from any of the insulation manufacturers that completely filling the joist voids on a cold deck flat roof without cross flow top ventilation is now acceptable to building control.
Many years ago, when U value for flat roofs was 0.35W/m2k or higher you could sometimes get away with this all as Approved Document C item 6.14, but as the current U value is now 0.18W/m2k, cross ventilation is now required all as AD C item 6.11.
With regard to hybrid warm roofs only one manufacturer. Kingspan with there PUR insulation is acceptable to building control technical note 10. This consists of 100mm TR31 on top of deck and 30mm TP10 between joists However different LAs have different views as to whether there should be a vented air space above the TP10. If you wish to use a single ply covering as the carpet then you need an additional 12mm ply on top of the TR31.
Regards to warm roofs. We only ever use Celotex PIR, but as both the PIR and PUR manufacturers achieve a lamda of either 0.021 or 0.022W/mK an average of 120mm of either PIR or PUR insulation will be required. There is no big problem doing a warm deck along side a cold deck. It just needs a bit of care at the join. Will make no mention of PWA at this point.
If you could post a picture of rear abutment, there may be a way of doing abutment ventilator and keeping cold deck.
Regards oldun
 

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