Insulating between roof rafters - confused!

[andrewparker]

Hi chaps, I'm in the process of planning the conversion of my roof space into a bedroom, however I'm confused as to the best way to provide adequate insulation.

The house is a large Victorian mid terrace. The attic space has always had a staircase to it, and has floor boards running the entire length, so it is to all intents and purposes a functioning room, however beneath the plaster boarded roof there is no insulation. The rafters are quite narrow, probably about 100mm, so I don't have a huge amount of space to play with, and head height is critical so I can't afford to increase this.

I will be removing the plasterboard, but what is the best, and most economical way to effectively insulate the room? There is also no moisture barrier, and the roof is not felted.

I take it that insulating between the floor joists is not necessary?

 

[regsmyth]

I think I would use Kingspan (or Celotex or similar) between the rafters, but leaving a ventilation gap between the top of the boards and the roof felt.
Then I would add a second layer (say 25mm) across the underside of the rafters, then polythene, then plasterboard.

You need to avoid breaks between the roof insulation and the tops of the external walls, and this can be quite tricky.

Often the loft conversion involves building vertical walls below the mid point or purlin. You insulation would need to go on the outside of these walls, and then continue along the ceiling of the room below to the external wall.

Hope this helps.

Although this solution is neither cheap nor quick.

 

[andrewparker]

I think I would use Kingspan (or Celotex or similar) between the rafters, but leaving a ventilation gap between the top of the boards and the roof felt.
Then I would add a second layer (say 25mm) across the underside of the rafters, then polythene, then plasterboard.

You need to avoid breaks between the roof insulation and the tops of the external walls, and this can be quite tricky.

Often the loft conversion involves building vertical walls below the mid point or purlin. You insulation would need to go on the outside of these walls, and then continue along the ceiling of the room below to the external wall.

Hope this helps.

Although this solution is neither cheap nor quick.

Many thanks for your reply, it is certainly food for thought.

The second layer that you suggest is a worry - head height is critical and I really need to maximise what we have.

I completely understand what you mean about insulating from the vertical walls to external walls - I actually hadn't thought of that.

So would 50mm Kingspan or Celotex between the rafters suffice, with polythene ran across the underside of the rafters, and plasterboard on top of that? Are there any differences in the types of Kingspan that is available, or any you recommend?

The other product that caught my eye was the new Knauf Earthwool flexible insulation slabs -

http://www.building-supplies-online.co.uk/knauf-earthwool-flexible-insulation-slab-4704-p.asp

 

[catlad]

Measure the amount of kingspan you can get in between the rafters without touching the batons, you do not need a vapour barrier and buy insulated plaster boards to stop cold bridging from the rafters it comes in different thicknesses so you choose.