Flat roof under-insulated?

[redcoatonline]

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.

 

[durhamplumber]

100mm gives 0.22W/m2K, requires 0.18W/m2K. Is

.22 is bigger than .18!......

 

[JohnD]

a bigger number, for heat loss, is a bad thing.

 

[durhamplumber]

a bigger number, for heat loss, is a bad thing.

He never mentioned what the figures were for..04 w/m2 is miniscule

 

[redcoatonline]

Building regs L1B (https://www.gov.im/media/1346190/approved-document-l1b-2010.pdf) section 5, flat roof. Requirement is 0.18W/m2K, and yes, higher is worse, so must come in below 0.18.

 

[noseall]

can 30mm be added under the main deck (between the rafters)

Yes, they will accept this.

 

[KenGMac]

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??

 

[motorbiking]

Have they accounted for plasterboard?

 

[Notch7]

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.......

 

[noseall]

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?

Jammed up tight. Zero air gap.

 

[redcoatonline]

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.

 

[noseall]

This is what worries me

Don't worry.

Fix the internal layer tight against the deck and get it as snug as you can. Then fold some foil tape into the corners. Finally, fix Duplex boards to the ceiling.

 

[durhamplumber]

Building regs L1B (https://www.gov.im/media/1346190/approved-document-l1b-2010.pdf) section 5, flat roof. Requirement is 0.18W/m2K, and yes, higher is worse, so must come in below 0.18.

What about 5.9.......lots of ifs that say higher values are acceptable.

 

[ban-all-sheds]

He never mentioned what the figures were for

Yes he did - W/m²K is a thermal transmittance measure...

 

[ban-all-sheds]

What about 5.9

The bit that applies to the upgrading of retained thermal elements, not to the installation of new roofs, you mean?

What about it?