Cold roof / warm roof - does it matter??

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Hi all,

I have a 22' x 14' flat roofed extension that i am currently renovating.

When i initially removed all the ceiling plasterboard, the roof void was filled with old insulation and appeared to have minimal ventilation., although no damp / condensation was evident.

As i build back up, i was intending to fit 100mm celotex insulation between the 8" joists - therefore leaving an additonal 100mm airspace above the insulation and creating a 'cold' roof??

If i add some additonal ventilation to the soffits / fascias is this construction going to be ok?

Thanks....
 
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You can fill it with 200mm Rockwool or Dritherm which will have the same performance as Celotex and be cheaper to do (plus Rockwool = better sound insulation), and have no air gap. Fit foil-back plasterboard as a vapour check

But there will also be furrings , so really if using quilt the whole gap should be filled tight up to the underside of the deck

Otherwise, your proposal is OK, but you will need to foil tape the joists to have a continuous vapour-check (or use foil back plasterboard)
 
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Thanks for your advise.

Seeing as i've already purchased the 100mm celotex i need to pursue the cold roof option

What do you mean by 'foil tape the joists'?

Also can i place the round vents in the fascia board, as the soffit is not wide enough?

Cheers
 
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You need to prevent moist air from the building from going up into the ceiling, then joists, and then condensing on the underside of the [cold] deck. So you either get some aluminium tape and tape the gaps and joists between the insulation, fit a polythene membrane, or foil backed plasterboard. Tape is the best option

If these vents are in the fascia, wont driving rain get through them? If not , then yes you can fit them there.

If the soffit is not wide, then take it out and fit a mesh or perforated plastic strip if that is easier
 
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Great - thanks for your help

So whats the advantage / disadvantages between a warm or cold roof?
 
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Mainly, cold roofs are more prone to condensation problems to the underside of the deck. There are several issues generally relating to the air void and/or maintaining air flow, and preventing damp air getting up through the ceiling.

A warm roof tends to be be less troublesome and helps reduce thermal movement of the deck (creaking). But insulation may need to be sacrificed and replaced when the cover does. Leaks may be harder to find too.

There are other pro's and cons for given situations, but a warm roof is normally a better option for our climate
 
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Could i use a combination of the 100mm celotex that i already have + 100mm of standard loft type insulation ontop (under the deck) and therefore have no air gap and create a warm roof?

The less trouble the better......
 
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does the insulation have to go over the top of the joists for a warm roof, or can the insulation be packed between the joists?
 
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You should probably have started a new thread.

Anyway, your question: either, but if insulating between then you will need an extra layer over or under. As long as the joists are insulated against the outside or inside then they won't be able to conduct heat from inside to out.
 

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