Flat cold roof to flat warm roof

[LordFrederickIII]

Part of my 1800s house a flat roof. Whilst the loft is insulated, the flat roof isn't, and the bedroom quickly gets cold.

The roof itself is in good condition. I've only had one quote so far and I want to check what the builder said was correct. Essentially to leave the roof as is, build a new structure on top, insulate new structure with Kingspan and fibreglass on top. There was something that went between the insulation and fibreglass but can't recall what.

Is that correct? With having such an old house I'm always conscious of the fact that it needs to breathe to prevent condensation.

 

[tony1851]

If you want to insulate the roof, what your builder has suggested (ie a warm roof) is the better way.
With the insulation on top, the roof structure stays warm and condensation will not form in the voids between the joists.
The thing between the insulation and roof finish is probably thin plywood or OSB. The only disadvantage of a warm roof is that the extra thickness necessitates a deeper fascia.

 

[LordFrederickIII]

Thanks for the reply.

What size Kingspan k7 should be used? I'm thinking of 100mm but is that overkill?

 

[freddiemercurystwin]

Not overkilll at all.

 

[stevewestern]

That's good news for me too !
I am buying a house with an extension that gets cold so want to increase the roof insulation with minimal upheaval so will just add a second roof above the existing - Wonderful !

 

[tony1851]

I'm thinking of 100mm but is that overkill?

As advised, that may be a bit 'underkill'. Depending on the actual structure of the roof,to comply with modern regs for flat-roof insulation, I think you are looking at around 125mm Kingspan.