I recently moved into an original bungalow with a four sided pitched roof, with a recent rear extension with a flat roof. From within the original loft I can see between the joists of the new extension and can see the upper surface of the plasterboard ceiling. Given that the original is a cold pitched roof and the extension is a warm flat roof should they be thermaly insulated from each other, i.e. should I introduce thermal insulation so that I can no longer see into the new warm flat roof?

 

The original loft is well ventilated from the eaves on three sides with additional ventilation in all four pitches and on all ridges and hips. The old roof was stripped and new tiles, 'felt' etc laid and was subject to building control, so modern materials, practices and standards were used.

 

If the extension is a warm roof then you definitely don’t want your cold roof ventilation going under the insulation of the new bit.

warm roof part should be blocked off to prevent cold air entering but you need to maintain the ventilation on cold roof - with 3 other sides there shouldn’t be any problem.

I cannot be quite sure of what I am seeing here. I can see a black membrane under the OSB (which I take to be the bottom of the 'Warm Roof Sandwich') but I cannot see the detail of what happens downwards of that. It looks like there could be a gap between the roof 'sandwich and (???) a false ceiling below (???). Given what I cannot see in the pic could you sort of 'talk us through it'? Cheers. Edit: As @markc says above "you definitely don’t want your cold roof ventilation going under the insulation of the new bit". From what little I can see it looks like this might be the case. I hope not.

Edited December 18, 20241 yr by Redbeard
Incl @markc comment

Thanks both markc and Redbeard.

What you are looking at in the photo are the sloping rafters of the old roof with insulation over the original joists, with the osb and black membrane with tiles above being the lower part of the original, pitched roof, and the ends of the new flat roof joists and the top surface of the plasterboard ceiling of the new roof below.

Why do you say I definitely don't want the cold roof ventilation going into the warm roof? What are the consequences of this? I can see that some heat might escape so that could have heating cost implications. Is that it, or might damp prove to be an issue over time, the extension was completed late this year.