Eaves space/knee wall insulation

[CH_18]

Hi all,

 

The attached detail shows 2 options how to insulate the roof space of a room in roof. Our architect had specified the left hand option, 100mm between rafters of sloping roof and knee wall covered with a 50mm overlay then plasterboard. We would like to use the eaves space for storage/access so the right hand side option of insulating the roof line all the way down to the wall plate is my preferred way. Just seems way more energy efficient and easier for airtightness etc. So my plan was to put 100mm PIR in between rafters ideally from wall plate to ridge, overlay with 50mm celotex top to bottom and the install VCL and plasterboard. So my insulation and VCL envelope will follow the roof line.

 

My questions are, if I insulate the roofline as preferred,

 

Do I need to insulate the knee wall at all?

Do I need to insulate the floor below the eaves space?

Can I leave the eaves space with no plasterboard, and just plasterboard the room?

 

thanks in advance

[Buzz]

Do I need to insulate the knee wall at all?

Do I need to insulate the floor below the eaves space?

Can I leave the eaves space with no plasterboard, and just plasterboard the room?

simple answer is 

No

No

yes

has this been built yet ? Or still in the design stage 

[CH_18]

Already built mate, currently 1st fixing 👍

 

Thanks for the reply, I'm finding hard to find info and understand what becomes a cold bridge, to my mind, the knee wall timbers could still be a cold bridge? Or with insulation in the rafters and under neath, is the cold bridge at the knee wall eliminated? 

 

Where does cold bridging stop? I've got stud walls screwed to some rafters, are they cold bridges? The caberfloor is screwed to the attic trusses, which is sat on the wall plate, are they cold bridges? Very complicated.

[Buzz]

I assume all of the rooms underneath are part of the main house , no integral garage or similar 

 

[Redbeard]

11 hours ago, CH_18 said:

I'm finding hard to find info and understand what becomes a cold bridge, to my mind, the knee wall timbers could still be a cold bridge?

 

How is the knee wall fitted? If it was already there, then after carefully establishing that it was non-load-bearing I would have taken it out, insulated the entire sloping ceiling and then fixed the 'knee wall' (cupboard-front) through it to the (carefully-marked!) rafters. If you did that the wall is entirely 'on the warm side' of the insulation, with insulation above and a habitable room below, and there can be no thermal bridge, as I understand your description.

[ProDave]

If you want to use the eaves space for storage, then there is no question, insulate the entire roof following the roof line all the way.

 

The eaves space then becomes part of the room and is inside the insulated and hopefully air tight layer.

 

Air tightness will be a tedious detail to get right having to accurately cut and then seal (air tightness tape) around every single one of the knee posts.

[joe90]

57 minutes ago, ProDave said:

Air tightness will be a tedious detail to get right having to accurately cut and then seal (air tightness tape) around every single one of the knee posts.

And insulate the doors and make them airtight. No, @ProDave is right, whole roof is the only (easiest) way.