Internal wall insulation upgrade during refurbishment (Part L)

[Mos]

Hi 

 

I am currently extending and renovating our end of terrace house. The external wall construction is made of solid brick (2 leaf) with no cavity. The party wall is made from 4-leaf solid brick with no cavity. As I have removed the old internal render, this has triggered the requirement to thermally upgrade the external walls as per Part L. I have two issus raised by a very difficult building control officer, below:

1. PARTY WALL: the officer seems to be convinced that the party wall requires upgrade to meet the 0.3 u-value target stated for external walls. However Approved Document L, section 4, table 4.2 (Limiting U-values for new thermal elements) states a u-value of 0 for party walls in NEW thermal elements. It does not refer to party wall u-value requirements in renovated walls. However in the same document, section 4, table 4.3 (Limiting U-values for renovated or retained elements)... this table lists u-value recommendations to existing thermal elements. A thermal element, as defined in page 76 is: “a wall, floor or roof… which separates a thermally conditioned part of the building from the external environment (including the ground)”. It assigns a u-value of 0.30 W/m²K to external wall upgrades. A party wall u-value for renovated or retained elements is not listed, and a party wall does not represent a thermal element by definition according to Approved Document L (as it is a heated space between two dwellings). Therefore, there is no stated requirement that exists in law to thermally upgrade existing party walls during renovation.  However, the BCO seems adamant and as yet has not been able to provide evidence to refute this.

 

2. EXTERNAL WALLS: Part L states that a u-value of 0.3 should be achieved when upgrading external walls in renovated properties. This would require using 62.5mm insulated plasterboard. However this would come at a hefty cost to us. As such, I have used the following clause in Part L to justify using a thinner insulated plasterboard (37.5mm):

 

Approved Document L, (vol 1, 2021 with 2023 amendments) section 4, states the following (page 26):

“4.13 If achieving the u-value in Table 4.3, column (b) either:

1.     is not technically or functionally feasible or

2.     would not achieve a simple payback of 15 years or less

then the element should be upgraded to the lowest u-value that both:

1.     is technically and functionally feasible and

2.     can achieve a simple payback not exceeding 15 years.

Generally, a thermal element once upgraded should not have a u-value greater than 0.7W/(m2·K).”

 

I have provided the BCO with a calculation that shows using 62.5mm insulated plasterboard achieves a u-value of 0.27 BUT it would take approx 19 years to pay back the cost on material and installation. As such, we have gone ahead and applied internal wall insulation with 37.5mm which achieves a u-value of 0.67 (within the 0.7 backstop mentioned in the document)... which is the most practicable and economically justifiable option for us.  Again, she is very stubborn on this and says she will need to go back to the document and dig up evidence to show why this would be non compliant. As yet, she has not provided this information and I now feel she is trying to frustrate the process. 

 

I would be grateful for any advice and thoughts regarding the above. We have not yet applied any drylining to the party wall as I do not know what the outcome with the BCO will be, but I have already insulated the external walls with 37.5mm plasterboard and do not want to remove this as it would cost a fortune. 

 

Thanks

Mike

[Redbeard]

Welcome,

 

Until I read your last line I had a couple of thoughts as to what you might do, but the fact that you have already got 37.5mm board on (and for clarification, that's 25mm PIR, isn't it?) could weaken your bargaining position. 

 

The one experience I had with trying to argue the 'not-a-15-year payback' thing was that BCO insisted on a SAP assessment which, even 17 years ago, was not cheap. In that particular case it was complicated much further by the SAP assessor managing to 'prove' that it *would* give a 15-year payback (it wouldn't), leaving us with no option but to increase the depth of insulation!

 

As regards the party wall, whether you can make the 'it's free heat from next-door' argument (and whether they can too!) may depend on the BCO's opinion. On the other hand it is not an external wall so I would ask the BCO to point out the specific section of the regs which requires party walls to be insulated as per external walls. As a 'negotiating stance' you may be minded to offer 'cloaking of a potential bridge' by returning the IWI onto the party wall for, say, 1m at each return. That will help your heat-loss a bit. Don't forget to cut back the plasterboard off the external wall IWI otherwise you have just built in another, albeit smaller, thermal bridge.

Edited October 7 by Redbeard

[ADLIan]

You interpretation on the refurb/upgrade of party walls is correct. There is no mention of 'party walls' in the text and table 4.3 relating to upgrading of existing walls (unlike new build). See also the definition of a 'thermal element' in Approved Doc L. In any case the heat loss from your solid party wall is 0.00 (ZERO) W/m²K (see Table 2.1). The heat loss mechanism for party cavity walls is all related to air movement rather than the temperature difference across the wall, which in theory, is zero.

 

Unfortunately the 15 year payback calculation is more complex and is based on a complete SAP assessment of the whole dwelling - a comparison of 'existing' and with the 'upgrade(s)'. The Approved Doc has more detail on the complexity of this assessment. Perhaps a sledgehammer to crack a nut here and may be easier just to aim for the U-values in Appr Doc L1

[torre]

There is an exception if adding the insulation would reduce the floor area by 5% or more, so it's worth calculating the before and after areas. You might also be able to argue the condensation risk is too high. 

Pretty sure the BCO is plain wrong about insulating a solid party wall in a renovation, maybe confused with a new build.

[Mos]

Adding further insulation would not reduce the floor area by greater than 5% unfortunately. So I only have the simple payback clause to hang on to. Surely this clause is in Part L to enable some economic flexibility with homeowners like myself ... when I raised the fact that 62.5mm plasterboard would exceed the 15 year payback to the BCO, she asked me to send her calculations, which I did. It sounded like she was not familiar with this clause, and when I prompted her for feedback nearly a week later she replied "Insulation is insufficient. I need access to all documents to show where your interpretation is not correct." I literally copy and pasted extracts from Part L to support my case regarding the payback clause. Still waiting for her reply... to me it looks like she is deliberately trying to "prove me wrong" despite me citing word for word the document to her. 

 

I've now instructed an energy performance consultant to try to communicate with the BCO on my behalf to understand (1) what she wants exactly and (2) if a letter outlining the overall thermal enhancement would suffice. I may also get an quantitive surveyor to validate my simply payback calculation (which I got using chat GPT, which has been quite good at helping me).

 

SAPs are not a formal requirement for renovated properties, they are only a requirement in new builds. They are not listed as a requirement in Part L. She has requested one, but according to the energy consultant, this is incorrect as a SAP on an old dwelling will always flag up issues and they are only appropriately applied to new builds. 

 

If anyone has further advice, particularly regarding the simple payback clause, I would much appreciate. I am in a desperate position and being inaccurately forced to add another layer of insulation over the existing would cost me thousands. 

 

Thanks

[Mos]

FYI to add... the 15 year payback rule is not based on a 'whole dwelling' calculation so it should not require a SAP-type assessment. Part L specifically says that this relates ONLY to the material and labour cost (ex VAT) of upgrading the thermal element in question... in my case only to the external wall upgrade. 

Here is the calc for anyone interested:
 

62.5mm insulated plasterboard.

Material and labour cost of element: £11,160 ex. VAT
Fabric: solid brick, 215 mm (2 leaves), uninsulated; typical U-value from SAP tables ≈ 2.20 W/m²·K
U-value after wall upgrade: 0.27 W/m²·K
Improvement (ΔU): 1.93 W/m²·K
Current gas energy pence/kWh (as per BEIS): 6.3p/kWH
kWh saved/yr: ≈ 9,264 kWh/yr
£ saved/yr @ 6.3p = 9,264 × £0.063 ≈ £584/yr

>19 years to pay back initial costs.

[Mike]

I agree that insulating the party wall isn't required. However, when insulating an external wall it is good practice to run the insulation for around 1m along any interesting walls if it's practical to so so, as that significantly cuts the heat loss at the junction, for minimal extra cost. For example, on a party wall, that often means running the insulation back to a chimney breast.

[ADLIan]

@Mos I understand and agree with you that a simple benefit vs cost is possible. Unfortunately you are wrong here. Appr Doc L does not work like that and you need to read the guidance on the 15 year payback calculation in Appendix A of AD L. Among other points SAP IS used for the calculation and must be done by a suitably qualified person. Looks like you have a strict BCO who will do everything by the book, perhaps a case of picking your battles.

[Mos]

I've read the appendix on the simple payback method. The above calculation does use a SAP-based method. This is what Part L appendix says on simple payback method:
 

• The marginal additional cost is the additional cost (materials and labour) of incorporating, for example, additional insulation – not the whole cost of the work.

• The cost of implementing the measure should be based on prices current at the date when the application is made to the building control body and be confirmed in a report signed by a suitably qualified person.

• The annual energy savings should be estimated using the Standard Assessment Procedure.

 

I just need to get an energy assessor or quantum surveyor to sign the calc off. 

 

[ADLIan]

You will probably need both an energy assessor and a QS. The energy assessor can undertake the SAP assessment but may not be ‘qualified’ for the costing calculation, this where the QS steps in. Best get agreement from your BCO  on the definition of ‘suitably qualified’ and who can be used.

[Mos]

Okay thank you