New Room Insulation

[GrahamLinton]

We have had a new roof on our lean to. They have left it without insulation for us to install but I need to know when fitting the insulation inbetween the rafters do we fit it flush with the underside of the marine ply or leave a gap and fir it closer to the T&G we're finishing the ceiling with?
Thanks

 

[Deluks]

It would have been a good idea to insulate before the new roof went on.
As it stands, you are probably going to make it a 'cold roof'
This will require a 50mm gap between insulation and underside of roof deck, plus ventilation at opposing ends in line with the joists. And a vapour barrier.

(This is all assuming that the room is airtight and heated. If it's not, then not much point insulating)

 

[GrahamLinton]

Only the tiles and plywood are on the roof, it's all done but it is by no means finished.
The new roof is over a lean to. I have attached a photo.
There's 14" between rafters and they are 7cm deep so allowing for a 5cm gap would only allow 2cm for insulation. I am then finishing the roof with either T&G or plasterboard

 

[Deluks]

Os the lean to open to the rest of the house, I can see a light switch?

From your initial post I assumed flat roof, I'm not sure why the marine ply is under the tiles. Are there any membranes in this roof construction?

 

[GrahamLinton]

The tiles on the lean to are some kind of plastic which is resembling slate there is then a fabric layer whether that’s waterproof layer or not I don’t know and then the marine ply. It is a slanted roof and it’s attached to the house with patio doors to the left and the kitchen is off to the right so it needs insulating as it will be very cold. Winter as it is at the moment.

 

[Deluks]

Its got a slope on it (©J.Clarkson),
what headroom would you have if you installed flat joists running front to back?

 

[GrahamLinton]

Couldn't be done as the kitchen goes straight into the lean to with no door. I will post some photos inside which may explain it better. I am there this week so will post some later.

 

[foxhole]

Are you putting in a level ceiling ?

 

[GrahamLinton]

Are you putting in a level ceiling ?

No.

 

[foxhole]

No.

Why not . ? Badly designed lean to?

 

[flameport]

Couldn't be done as the kitchen goes straight into the lean to with no door.

What you have got won't comply with building regulations, and with such shallow roof supports any insulation would be a token effort at best, and do approximately nothing to reduce heat loss.

 

[Dereekoo]

What you have got won't comply with building regulations, and with such shallow roof supports any insulation would be a token effort at best, and do approximately nothing to reduce heat loss.% reduction

Not quite right, with no insulation U value would be in the region of 2.5 with 50mm celotex this would reduce to around 0.5 so about a 80% reduction in heat loss through the roof although the calculation is based on basic heat loss formulae and how it compares to floor/heat walls heat loss is something else to be considered