Superquilt - Above or below PIR for insulating loft room?

Hello -

Am in the process of upgrading the insulation in an old loft room.

Rafters are just 100mm deep, so will need to use a combo to achieve 0.18 u value

The headroom is reasonably low so would rather not build up the rafters for deeper PIR if possible.

Was thinking about 75mm of PIR (allowing 25mm droop gat for the breather membrane) with YBS superquilt or similar over the top, counterbattened.

Am no expert on the thermal dynamics - but wouldnt the counterbattening of the superquilt compress it and thus make is subject to thermal bridging?

Have also seen a vid when the quilt goes first as it were (i.e. between the rafters, under the PIR board)

So rather than the below, the quilt would go under the breather membrane, with the PIR on top?

Any advice on how best to achieve required u-values without dropping the cieling height would be more welcome!

NB (Rafters are spaced 400mm)

Cheers

Looking at those numbers, 70mm of PIR is 3.18 and 64mm of “superquilt” including its two required air gaps is 2.5. If you forget about the “superquilt” and add more PIR you’ll get better insulation in the same depth.
PIR between the rafters, then a second layer over.

Forget the Superquilt - it's snake oil. You need the air gap both sides for it to work, as a large part of its insulation value is in the reflectivity, which doesn't work if it is hard up against another material.
If you have 100 rafters, put 75 PIR between the rafters, and 25 or a little more below (with taped joints) and then the plasterboard.
There's no rigid requirement to meet the U-value if it would compromise headroom and so reduce uesable space; the only requirement is that the insulation thickness should be 'reasonable' in the circumstances.
If doing it under Building Control, private inspectors are usually more realistic on what can reasonably be achieved than LABC people.

Cheers Tony -

Yes - i'd heard that opinions on a pure 'quilt' solution vary - TBH, we're just going to try to find a solution that the BCO will sign off on, as this is a situation draggin on from this (almost two year old) thread here: https://www.diynot.com/diy/threads/...xisting-non-compliant-loft-conversion.502974/

The long and short is the BCO would not sign off on the roof (& thus out main bulid project) without evidence of upgrade to the thermal elements (our lack of understanding)

....but wants a seperate application for the re-roofing...

... and as we had inherited an unauthorised loft conversion from 40 years ago (missed on our purchase survey docs) - which ideally we would have had reguralized in conjunction if possible.

So i'm trying to grapple these three to get our completion certificate for the work that was actually completed some time ago!

To be frank - my conversations with the BCO have been extremely frustrating (as a layman) he will not advise on what is acceptable - but put the onus on us demonstrating compliance feel like have been going round in circles.

I now have a pretty good grasp of L1b conservation of fuel and power, but still seem to be getting nowhere with this fella.

any advice most welcome!

tony1851 said:

If you have 100 rafters, put 75 PIR between the rafters,

Click to expand...

50mm ventilated air space over insulation

Leofric said:

50mm ventilated air space over insulation

Click to expand...

Is this the case if the roof covering is breathable?

we had this installed when we had the roof re-covered a couple of years back: https://www.roofingsuperstore.co.uk...breather-membrane-felt-underlay-50m-x-1m.html

seem to be getting conflicting advice on whether the insulation needs no air gap, a 25mm gap or 50mm?

above site claims: 'Tyvek Supro does not need any soffit, tile or ridge ventilation'

yet when i've spoken to one of the PIR insulation companies they've said leave a gap

If its of any relavance, this area of loft is above a 'dry' part of the below house (not kitchen or bathroom) & would be a temporarily used chill out den for the kids.

the below shows how this area connects to the recently finished loft area.

Keen to get weaving over the Xmas break with getting this insulated.

feels like a bit of a waste to batten out by 50mm to allow for a gap of its not needed, but likewise I dont want to pop in a load of celotex to the gaps and teh constantly worry that its going to be causing condensation.

You don’t appear to have any flat ceiling space to encroach on by increasing rafter size?