Is Building Control really required, small insulation task?

[Neverfinished]

Hi all,

I'd like to ask a 'general' question about adding internal insulation to ONLY the 2 external-facing walls of a bedroom in my victorian terrace, with regard to what Building Control are likely to require of me, etc.

These 2 walls amount to precisely 49.1% of the total 4-walls area of the room. I read somewhere that if an upgrade is below 50% then B.C.don't need to be involved, and thus I wouldn't need to meet U-value requirements etc!

Or, do things need to be looked at more holistically & the whole room taken into consideration - for instance - the ceiling was, some years ago re- boarded (but not yet skimmed!) so would B.C. add this in as part of my upgrade (& thus becomes >50%)? I've no plan to add more insulation to the ceiling but there's an existing 200mm of wool atop in the loft space, brought up to Regs & without having to apply to BC !

 

Also, to ask opinions of folk on here about whether its worth all the 'faff' of obtaining B.C. consent for these smallish, almost DIY projects - For myself I'm keen to comply since, when I eventually sell (10+ yrs hence) I believe a search is made, for the sale documents, to ascertain if home improvements/works legitimately comply with Regs. But it seems to me the public commonly are quite blasé about the whole matter, + some tradesmen 'forget' to apply on behalf of client, on small jobs at least. This 'delinquent' attitude seems quite common to me. Am I right this comes back to bite the house-owner eventually?

 

Any comments much appreciated.

 

 

 

[JohnMo]

If it's not a building of historical interest (listed) and you are doing insulation internally and externally, not sure why anyone needs to be involved. Photograph everything good for future reference.

 

Only thing other than the above, if the wall is currently breathable keep it that way, so add mineral wool wood fibre etc not PIR.

13 minutes ago, Neverfinished said:

existing 200mm

I would be adding another 150mm on top of that.

[Redbeard]

'Renovation of a thermal element' is 'adding or replacing a layer'. The 50% is *of the relevant thermal element in the room you are in*, so if there are 2 external walls and you IWI them both, you have done 100%, so definitely BC approval required. Theoretically if you did 49%, 49% and 2% in separate tranches you could arguably do it 'outside the Regs'. However when questions are asked when you come to sell the purchaser or their solicitor may not understand the 'letter of the law' under which you carried out the works, and may ask for an indemnity of a Regularisation Certificate. Your walls and ceiling to unheated space would be treated separately if done at separate times, but for counting purposes the 2 external walls are 100% of the wall area and the ceiling is 100% of the ceiling area.

 

I always get BCO approval even for IWI of 1 external wall since, if and when I sell, I would want to be able to wave the completion cert in front of the person who was trying to find reasons to reduce the price.

 

200mm of wool will not reach the 0.16 U value for compliance (Edit: It might, just, if the exg 200mm was 'extra special' sheep's wool with a (lower-than-mainstream-sheep's wool) lambda of 0.035W/mK) but if it was done at a time when 200mm did 'cut the mustard' then as you are not adding a layer or replacing a layer (the pl'bd) you do not *have* to add anything. I would if I were replacing the ceiling (and of course arguably at that stage you have to stump up the £180 or whatever (may have changed since last I did one) for a Building Notice).

Edited December 31, 2025 by Redbeard
Clarific'n re roof U value

[Roger440]

3 hours ago, Neverfinished said:

Hi all,

I'd like to ask a 'general' question about adding internal insulation to ONLY the 2 external-facing walls of a bedroom in my victorian terrace, with regard to what Building Control are likely to require of me, etc.

These 2 walls amount to precisely 49.1% of the total 4-walls area of the room. I read somewhere that if an upgrade is below 50% then B.C.don't need to be involved, and thus I wouldn't need to meet U-value requirements etc!

Or, do things need to be looked at more holistically & the whole room taken into consideration - for instance - the ceiling was, some years ago re- boarded (but not yet skimmed!) so would B.C. add this in as part of my upgrade (& thus becomes >50%)? I've no plan to add more insulation to the ceiling but there's an existing 200mm of wool atop in the loft space, brought up to Regs & without having to apply to BC !

 

Also, to ask opinions of folk on here about whether its worth all the 'faff' of obtaining B.C. consent for these smallish, almost DIY projects - For myself I'm keen to comply since, when I eventually sell (10+ yrs hence) I believe a search is made, for the sale documents, to ascertain if home improvements/works legitimately comply with Regs. But it seems to me the public commonly are quite blasé about the whole matter, + some tradesmen 'forget' to apply on behalf of client, on small jobs at least. This 'delinquent' attitude seems quite common to me. Am I right this comes back to bite the house-owner eventually?

 

Any comments much appreciated.

 

 

 

 

Or you could not bother and just crack on and fit the insulation.

 

How will anyone ever know?

 

As always, aside from cost, what will the BCO add to this simple excercise? Bear in mind, they may not agree with your proposal and you may need to do more/something different to comply. Once they know work is happening, they know. You cannot have them "unknow".

 

 

 

 

[Neverfinished]

Thanks to you guys, as above - @JohnMo @Redbeard & @Roger440 for your views & input &, on balance, I think I'll bite the bullet & pay B.C. for a building notice; I'm eager to have a record of improvements done for when I ultimately sell the house. You Redbeard offered some clarity to my oversight - as of course its only those 2  exterior facing walls that'd really require upgrade in IWI (+ ceiling ideally too).

 I'll just add a little more of the story of this (small) project, to keep you in the loop - I originally registered with BC in 2020 to upgrade insulation of these 2 out-facing walls but only ever got as far as fitting 60mm wood fibre to brickwork, the intended 45+mm of hemp lime plastering being too hard to complete on my own  - & I even stupidly let the building notice expire too.

Pic. below shows current 'state' of the project; 

So I'll re-apply (+pay) BC again !, complete the 'insulated plaster' work etc over the wood fibre base, with some help this time, to obtain a completion 'certificate'.

My house is a victorian mid-terrace, in red brick + lime mortar, in a conservation area but not listed.  In my original application I emphasised how I've retained the breathable fabric for vapour permeability etc across the house; BC officer who viewed 'site' at the outset seemed accommodating to the historical limitations to improving its u-value, so I'm hoping to get the same consideration again this time, & complete, without actually attaining the now even tighter u-value.

You guys perhaps already know how Approved Doc. L states some interesting 'accommodations', for example :- Vol 1, para 0.9 . . "the work should comply with standards in this approved document to the extent that it is reasonably practicable"

para 0.10 says: "traditional dwellings should be improved only if doing so will not cause long-term deterioration of the building’s fabric or fittings . ."

Here's for hoping BC will consider my new application sympathetically again now!    Cheers

[mjc55]

If you started on the BN work then why would you need to re-apply.  It's only if you haven't started that it expires (AIUI)

[Neverfinished]

29 minutes ago, mjc55 said:

If you started on the BN work then why would you need to re-apply.  It's only if you haven't started that it expires (AIUI)

 

Many thanks for that info mjc55; I came onto the forum now as plan getting the project going again in the next month, & these replies will assist my discussions with BC, as . . they wrote saying application cancelled because I hadn't responded to their enquiry/s 3 years into my application! Stupid of me really! So I will try to retrieve something. Cheers

[Redbeard]

So BC did visit. Have they agreed in principle your 60mm WF and hemp/lime? Did you state a U value? (I know you do not have to in a BN).

 

The trouble, to my eyes, with the clause which says:

"Historic and traditional dwellings
0.10 The energy efficiency of historic and traditional dwellings should be improved only if doing so will
not cause long-term deterioration of the building’s fabric or fittings. In particular, this applies to
historic and traditional buildings with a vapour permeable construction that both absorbs moisture
and readily allows moisture to evaporate. Examples include those built with wattle and daub, cob
or stone and constructions using lime render or mortar." (p.3 of Part L)

 

...is that it is open to debate as to what thickness of WF EWI will "cause long-term deterioration of the building’s fabric or fittings". Some say 'Let WUFI tell you', (which could mean up to 100mm on a solid 225 brick wall) and others say never go above 60mm. I always try to get WUFI to prove that 100 is OK, but the BCO does not *have* to agree. My local BCOs always have, but it may be more hit-and-miss elsewhere.

 

If they quibble you could substitute Diathonite for your hemp/lime.

Edited 11 hours ago by Redbeard

[Roger440]

3 hours ago, Redbeard said:

So BC did visit. Have they agreed in principle your 60mm WF and hemp/lime? Did you state a U value? (I know you do not have to in a BN).

 

The trouble, to my eyes, with the clause which says:

"Historic and traditional dwellings
0.10 The energy efficiency of historic and traditional dwellings should be improved only if doing so will
not cause long-term deterioration of the building’s fabric or fittings. In particular, this applies to
historic and traditional buildings with a vapour permeable construction that both absorbs moisture
and readily allows moisture to evaporate. Examples include those built with wattle and daub, cob
or stone and constructions using lime render or mortar." (p.3 of Part L)

 

...is that it is open to debate as to what thickness of WF EWI will "cause long-term deterioration of the building’s fabric or fittings". Some say 'Let WUFI tell you', (which could mean up to 100mm on a solid 225 brick wall) and others say never go above 60mm. I always try to get WUFI to prove that 100 is OK, but the BCO does not *have* to agree. My local BCOs always have, but it may be more hit-and-miss elsewhere.

 

If they quibble you could substitute Diathonite for your hemp/lime.

 

It is the lack of clarity, and ultimately, the judgement of the BCO, that i would not involve then unless its unavoidable.

 

If BCO doesnt agree, tough on you. There is no "appeal" process even if they are wrong.

 

On some of the old house groups, theres been a couple of people running into quite some difficulty with BCO's that seem to be unwilling to relax standards based on it being an older building etc etc. Which the clause that Redbeard highlights specifically allows for. But if the BCO says no, then its no. To the point that one person couldnt insulate his roof to current standard, the BCO just said replace the entire roof structure so you can. All while roof is stripped.

 

Sadly, as the old school BCO's retire and the new breed of box tickers comes in, i cant see it getting any better.

 

OP, hope yours goes smoothly.