Loft conversion insulation question

[Cabbyfan]

Hi,

Looking for advice on best way to insulate an existing dormer loft conversion. Pictures below should explain the issue, there is no insulation between the rafters and very old thin layer between the ceiling joists.

I'm planning on ripping out the existing internal dormer internals and adding on a new side dormer to the hop roof. Part of this means I also want to properly insulate all the space under the roof and also between the ceiling joists (for noise and heat).

Comments welcome !

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[Alastairreid]

You should have a 50mm airgap with 100mm insulation between and approx 30mm underdraw.

You will need to thicken the rafters.

 

[ban-all-sheds]

He's showing "85mm or more" of insulation, so with the same basic idea he could get to 100mm.

Why does it all need to go between the rafters? What's wrong with some between and some over? (Or under, depending on your POV).

 

[Cabbyfan]

Thanks for the replies.

The drawing is a sectional view with 40mm or so of insulation between the rafters then a second layer fixed spanning fixed under the rafters. I can't get the thickness I would like simply between the rafters.

I get the point about thickening the rafters but I'm not sure the time or expense is worth it when I could just overboard with say 50 or 100mm kingspan or similar ?

I'm also looking for confirmation to the location of the vapour barrier as I think it's in the wrong place on the sketch.

 

[catlad]

Putting the insulation on the rafters takes up space, that and not being easy to fix it to the rafters when it gets to a 100mm, if you buy some foil tape and all the joints then that will serve as a vapour barrier.

 

[ban-all-sheds]

I'm also looking for confirmation to the location of the vapour barrier as I think it's in the wrong place on the sketch.

Should be on the warm side of all of it.

Foil-backed insulation and taped joints will do the job.