How do I upgrade the thermal element of my roof to meet Regs ? Help!

[Neverfinished]

Whilst plans to re-roof my Victorian mid-terrace home back to natural slate isn’t too daunting to do myself, but meeting the regs in upgrading my roof’s thermal element effectively seems to necessitate much expert knowledge – and Building Control state . . I must improve roof insulation (to U = 0.15) as part of this re-roofing project!

 

The manner in which 130 year old properties like mine were built seems to constrain the options for improving this particular insulation element up to current standards:- 

Essentially, there’s always been a 2^nd floor attic space ‘under’ the pitched roof, making a 4th bedroom, but which by its virtue/presence leads to-

1)     a low-ish pitched attic bedroom ceiling (only 1.72 metres height at the room entrance) -  making it impractical to add additional insulation below ceiling level;

2)     75mm-only depth rafters above the ceiling, limiting insulation within this space;

3)     roof tiles (currently 70’s concrete ones) immediately over the rafters & which run through & across on to neighbour’s adjoining terrace as if our 2 roofs are one, probably limiting options at this level too.

 

Its basically a cold roof, currently with limited amounts of mineral wool under the rafters. Also have a NW facing dorma. Continues below -->

My property inc.dorma,left, my roof adjoins neighbour to the right, @ mid-centre of chimney

 

Section dagram :-

 shows- habited area in green, stud dwarf walls in red.

 

Notes: (A) I see one immediate way for some u-value improvement – by ‘over insulating’ outersides of the dwarf walls, ie the 2 eaves (storage areas), compensating for deficits elsewhere. (but likely still insufficient).

(B) And/or could I could ask for a dispensation from the regs in light of the already restricted ceiling height inside the room?

Also to mention, I’m not in a conservation area, & its not a listed property.

 

I’d also like to prioritise the use of wood fibre/hemp/rockwool rather than PIR, in the upgraded element if possible, being concerned for condensation risk, decrement delay & reducing road noise. All without breaking the bank.

 

In an ideal world all could be achieved by my raising the roof a few inches (perhaps 5”?) then placing mineral wool &/or wood fibre above + within the rafters, ie making a hybrid roof. BUT! – I’d need ‘the mother of all’ bonding gutters where I adjoin my neighbour’s roof [BC officer says it could be done, in lead, ie with a 5” step-up adjoining neighbouring roof] – Is this even pheasible?

Also, would I need Planning permission for this 5”uplift ?

Raising the roof/adding room for insulation to me seems the only way to get to the U-value requirement.

– anyone on the forum know answers to these ideas? . . would be most helpful !

 

I could pay for advice from an architect/technologist but scared of getting a dud!

 

Thanks.

 

[Redbeard]

I fear wood-fibre etc. *will* 'break the bank', at least compared to the costs of PIR. Don't get me wrong, it is exactly what I propose to use in the refurb of my rooms-in-the-roof, but mine is a steep-pitched roof with plenty of headroom. My 'sandwich' is proposed to be:

 

20mm rigid WF (lambda 0.044W/mK) between 75mm rafters leaving the recommended 50mm (well, 55 here, but Gov't guidance says 50) ventilation path. 225mm flexi WF (lambda 0.039W/mK) in Larsen Trusses, and 20mm WF again as plaster carrier.

 

This sounds like it would take a lot of space which you have not got, and it *will* cost plenty. For me this is the right thing to do and I will shut my eyes and scrimp on other things to achieve it.

 

Unless you use PIR (and I am certainly not trying to persuade you to) even your +5" on top will not get you a compliant U value.

 

There are some get-out clauses in the Regs. See pp 25 and 26 of 'Part L': https://assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf

 

And try to get a word with a Building Control officer. Depending on staffing levels they can be hard to pin down, but as well as ensuring compliance with the regs they also have to be realists.

 

I think if you are trying to get an area-weighted U value of 0.15 with such limited space in or under the rafters you might find that you are trying to get more insulation in the eaves voids than they will physically contain!

 

Fairly certain Planning permission would be required to lift the roof but, importantly, if you are using WF or similar, 5" will not be enough - see my lay-up referred to above. Also, as Warm roofs are typically done with rigid material, the weight of rigid WF @ 140-180kg/m3 would, I think, break your roof! Flexi is, I think, about 55kg/m3.

 

I am running out of stamina tonight but have a look at this and fire back further Q's and I'll try to help if I can.

Edited May 21 by Redbeard

[Iceverge]

You want to avoid moving the tiles upwards I'm guessing too much. In any case I'd prefer to have the ventilation up there and full fill the rafters. 

 

How about this. 

 

Tiles

Tile battens

Counter battens 

Breather membrane 

Osb

75mm mineral wool between rafters 

75mm counter batten at say 800cc with mineral wool. 

Airtighess membrane 

75mm service cavity with mineral wool. 

Plasterboard and skim. 

 

Should work out to about 0.2W/m2K 

 

Lots of common material so little waste. 

 

 

[Iceverge]

If you get the green light to raise the roof you can stick 100mm PIR above it, a taped OSB deck 75mm mineral wool between the rafters and you're golden. 

[Redbeard]

@Neverfinished, could you please post the dwg again with dimensions, so we can see what the pinch-points are? I am particularly unsure of how the 1.72m headroom 'kicks in'. Is it because the stair comes up under the sloping roof? Is there so much more headroom in the room itself that you could fit insulation under the rafters? Your statement below suggests not:

On 21/05/2025 at 15:32, Neverfinished said:

impractical to add additional insulation below ceiling level;

 

[Neverfinished]

Many thanks@Redbeard your continuing interest in my roof+insulation upgrade. To answer yr Q's of last night hope the following helps:

1st pic is a 'section thru' my current roof as in my prior post but now with measurements. (It does include info too about the insulation currently in there but this likely ripped out during upgrade).

And here's a a pic taken from the entry point @ top of stairs:

 

 

 

 

In light of yr suggestions I've made a bit of progress this week & am drafting this right know, so will post this too forthwith

[Neverfinished]

Many thanks @Redbeard, also @Iceverge for your constructive suggestions. These have helped me focus harder on the project, the only major refurb of my 36 yrs in the house - to put slates back on this 1897 terrace after previous owner opted for a cheapo cement tile job - & with no insulation. And its the upgrading of the roof thermal element which will require technical ingenuity & thank you for your already helpful advises etc.

Currently I'm awaiting BC considering dispensation allow me a lesser U value in roof thermal element, in light of a low attic ceiling, as in reality it prevents my insulating under it, although, the only other place I could then put insulation is within the 75mm rafters (minus a 50mm air gap !).

Instead therefore, I DO HAVE TO seriously consider raising the roof & fitting upto say 5" of insulation there (& making a warm/hybrid roof). Thus planning permission would be sought & I may have to bear with neighbours objections & some significant uplift in project cost. But this increasingly looks the only realistic option, + would make for a thermally more efficient house.

And, I've begun looking closely at the type of 'bonding gutter' I'll need at junction with neighbours roof (one side). A chat with a 'lead fabricator' / supplier has recommended a box gutter instead.

Photos below are:-

1, my current roof 'junction' (with neighbour); this runs down centre line of chimney.

 

2, This a perhaps suitable box-gutter design??

I'd much appreciate any views/suggestions from anyone on this aspect of the build. I'd employ a leadworker for such work but any independant views on this detail from forum members welcome, before I commit. Infact I can't find any photos of a box-gutter install on a roof like mine - only pics of where 2 (opposite) pitches meet together. Did the 'lead-worker' mis-advise me?

Any thoughts please, appreci\ted !

[Redbeard]

Glad to have been  of help.

 

OK, so you have maybe decided to go for the +125mm on the roof (subj to Planning approval). But before you do that...

 

Can you add a dimension for floor to horizontal ceiling (I could draw it and measure it but (a) I am lazy and (b) I have lost my scale rule!). If it would not make things too crazy tight I have in mind a proposal to Bldg Control. For the width of the doorway 50mm PIR between rafters (leaves a 25mm vent gap which I regard as sub-optimal but as you are re-roofing with slate, get a really good membrane and you should be OK for that small area). That gives you a 'tunnel' to enter the room and you don't crack your head. 

 

For the rest, 25mm PIR between rafters and 75 below, fixed at wide centres with insulation collars/screws and tightly taped as VCL. Plasterboard fixed directly below (which required you to remember where the rafters are under 75 PIR. For a loss of a further 25mm space, x25 x 50 battens at right angles to rafters over the PIR and adjust centres to suit your chosen boards (I can no longer wrangle 1200 x 2400 so I use 900 x 1800.

 

A very rough calc (unadjusted for the bridging of the rafters) for the main area with the 25 + 75 'sandwich' gives a U value of around 0.22W/m2K. You could propose that to BC as a compromise and quote the section from the Regs in my previous post. Note that even that reduced amount would give you about  9-10 times the insulation value of the un-insulated roof. They *could* ask you to have a SAP calc done to prove that doing it to the full std would have a payback of more than 15 yrs but I have only been required to do that once. 

 

Edit: If you decide to go for the 'raise the roof' option be very clear on the design of the box gutter and the box it sits in. I would (could) be concerned about condensation on the bottom and sides of the metal box , wetting the timber box. Maybe consider building the 'under-box' out of Compacfoam or XPS? I would also get a roofer to express an opinion of the 'tails' on the box gutter. I would worry they are potentially too short/not profiled sufficiently.

Edited May 27 by Redbeard
Add box gutter comment

[Neverfinished]

Thanks yet again @Redbeard for your latest suggestions & indeed I hadn't thought of this 'tunnel', & if I decide I can 'live with it' + convince B.C. this could be a way to avoid expense of raising the roof / insulation height.

I attach the diagram again but with various additional measurements.

Of course I could also add substatial amounts more insulation to those voids in both eaves, to (hopefully) create a much improved U-value & to compensate for deficiency elsewhere (though perhaps never able to get sufficient in) as you yourself mentioned a while back, as well as the 75 + 25 PIR you mention in yr prior post. If anyone knows how to calculate a u-value for these 2 'triangular voids' in Ubacus etc (filling up with rockwool perhaps) could be helpful.

Your 'tips' too about a SAP & a greater than 15yr payback period 'excuse' noted; then also your tips re materials for the box gutter construction sound helpful, thanks. This box gutter would be very much for a specialist to do, if I go that direction.

 

[Redbeard]

...and something I forgot to add about the box gutter. It is not just a condensation risk but also a whacking great thermal bridge, even if you mitigate it.

 

Re area-weighted U values:

 

Very roughly: 300 on the flat in eaves and 300 on walls, @lambda 0.044W/mK (U = 0.15W/m2K approx (no allowance made for 'base case' U value, but equally no negative allowance made for bridging thro joists/studs, so assume they cancel out.

 

From https://www.pbctoday.co.uk/news/building-control-news/glazed-home-extensions/93045/

 

Calculate the overall area-weighted U-Value

This calculation can help to show that the area-weighted U-Value of all the elements (walls, floors, roof windows etc) in the extension is not more than that of a similar extension having the same size and shape, but which complies with the minimum U-Value standards for all the walls, floors, roof, and the opening areas as set out in the Approved Document.

The area-weighted U-value is given by the following expression:

{(U1 x A1) + (U2 x A2) + (U3 x A3) + …)}

{(A1 + A2 + A3 + …)}

Where: U = the U-Value of the element e.g.: wall U1, roof U2, floor U3 etc.

A = the total area of the element (m2) e.g.: wall A1, roof A2, floor A3 etc.