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 7 hours ago 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.