U value for thermal element modification (external wall render)

[Harryca123]

Hello,

I have a late 70s built semi detached house that has pebble dash above the ground floor window and porch up to the soffits. The pebble dashing is starting to crack and we've had some water ingress to the porch. In light of this we're looking at getting the front elevation rendered in an Eco Rend product (will probably go for a similar colour to the pebble dashing (beige) in light of planning and not making a significant change to the appearance).

My question is with regards to the building regs approval required for modification of a thermal element. Since its over 50% of the thermal element, we'll need to submit a notice, but I'm wondering about the U-values. I've used an online calculator and put in, to the best of my knowledge, the materials used in the construction of the house, and it states that the U-value is 0.39 (see image), which exceeds both the threshold and the improved value in approved document L. Does this seem about right? And if so, can I just submit this with my building notice and go ahead with the work assuming there will be no issues later?

Thanks in advance, and please point out anything I'm missing.

Harry.

 

[^woody^]

The idea of L1b is that you improve the insulation of the wall - eg consider adding insulation.

Are you saying that the house/wall insulation does not need improving?

The calculation would normally be of the payback period to either justify (or not) adding certain insulation, and not a calculation of thermal values.

 

[Harryca123]

The idea of L1b is that you improve the insulation of the wall - eg consider adding insulation.

Are you saying that the house/wall insulation does not need improving?

The calculation would normally be of the payback period to either justify (or not) adding certain insulation, and not a calculation of thermal values.

I'd say the idea of L1b is to ensure certain standards of insulation are met, and only to improve where they are not met.

I'm saying that my calculations and best estimates using online tools show that my wall already exceeds the standards set out to be achieved in L1b, and I'm asking the forum whether this seems correct. I'm hoping that people with experience of this will be able to say something like.. yes, a cavity wall with blown fibre insulation will exceed the threshold in the approved document and you won't need to insulate when you render your wall.. or.. No, your calcs are wrong and you need to look at it again.

 

[^woody^]

Your calcs are correct but your interpretation of what is required is flawed

 

[Harryca123]

Your calcs are correct but your interpretation of what is required is flawed

Ok, do you care to expand on that?

The way I see it, L1b states if you renovate more than 50% of the area of a thermal element you are required to meet a certain standard of thermal conductivity. When renovating a retained thermal element that has a U-value greater than column a in table 4.3 you are required to meet the standard set out in column b of table 4.3.
The current U-value of my wall (to my calculation as shown above, which you've stated is correct) is 0.39, which is lower than the 0.7 threshold value. Incidentally, it is also lower than the improved value (column b) of 0.55. Therefore, if undertaking the work to renovate this particular thermal element, no additional work is required to improve the U-value since it already meets the requirements.

The ONLY mention of payback is when a thermal element cannot be brought up to the standards listed in table 4.3, which is completely irrelevant in my example.

 

[^woody^]

Speak with your building control team to clarify how they will interpret 4.3

Table 4.3 deals with walls to be insulated ie to be renovated to the new thermal standard by adding insulation - either to the cavity or to the face of the wall.

As your wall already has CWI, then strictly 0.30 u-value should apply because you would consider adding insulation to the face

From Table 4.3

5. This applies only to a wall that is suitable for cavity insulation. Where this is not the case, it should be treated as ‘wall – internal or external insulation’.

The payback concept is better explained in a previous version of L1

* table 3 is table 4.3 in the new version

 

[Harryca123]

Speak with your building control team to clarify how they will interpret 4.3

Table 4.3 deals with walls to be insulated ie to be renovated to the new thermal standard by adding insulation - either to the cavity or to the face of the wall.

As your wall already has CWI, then strictly 0.30 u-value should apply because you would consider adding insulation to the face

From Table 4.3

5. This applies only to a wall that is suitable for cavity insulation. Where this is not the case, it should be treated as ‘wall – internal or external insulation’.

The payback concept is better explained in a previous version of L1

View attachment 371748
* table 3 is table 4.3 in the new version

I will speak to my local BC team and see what they say.

I'm fairly sure though, that table 4.3 doesn't deal with walls *to be* insulated, it deals with retained thermal elements (i.e. existing walls) that are being renovated. Rendering or adding new material (such as cladding) to a wall constitutes 'renovating a thermal element' - insulating does not mean renovating, (but it does include it if that's the job you were doing - in this case I'm not). The only time insulation is required by regulations is when the retained thermal element does not meet the threshold set out in column a, and it therefore needs to be insulated and brought up to the value set out in column b.

My wall is a cavity wall, therefore it is suitable for CWI and does not need to be treated as a 'wall - internal or external insulation', therefore the u-value of 0.3 is completely irrelevant in my case. The value above that, of 0.55 for cavity walls is also irrelevant, since my U-value already meets the threshold value of 0.7 for a retained thermal element that is being renovated.

Payback still has no relevance in this example because I'm not trying to make any exceptions to the rules set out in L1b.