Flat roof under-insulated?

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30 Apr 2019
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Hi,
I've recently had a flat roof (warm) installed by a local contractor, who used 100mm Kingspan over the main deck, followed by torch-on felt. BC now advise that the insulation is too thin: 100mm gives 0.22W/m2K, requires 0.18W/m2K. Is that reasonable? What is my recourse? can anything besides stripping the roof back to the deck and adding insulation be done? A quick googling reveals that 130mm should be OK: can 30mm be added under the main deck (between the rafters), or does this then stop being a warm roof?

Thanks.
 
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If you consider it?

Why not [if you can afford it] fixing something thicker than the minimum 30. mm required? this will "future proof you?"
Yes, they will accept this.
As an aside? would the additional thickness be jammed hard up to the decks underside or will there have to be a vent gap between top of the insulation and the underside of the deck? just wondering??
 
I make it 120mm insulation for a warm roof, although that might be influenced by the type of fixings.

You will need thicker than the difference, if making it in between rafters -as they create an element of bridging.

Strictly speaking this would be a hybrid roof which celetex etc dont recognise.......
 
will there have to be a vent gap between top of the insulation and the underside of the deck?
This is what worries me: if you add insulation below, then the deck isn't as 'warm' as the room, so could collect condensation, requiring ventilation, like a cold roof. Not sure how likely that is with 100mm of solid insulation above though.
 

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