U-value elements

[DevilDamo]

Which ones do you use for a domestic extension?

AD L1B

http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADL1B_2006.pdf

Page 17 (Limiting U-value standards table, Column A)...

Wall (0.35)
Floor (0.25)
Roof (0.25)
Windows, etc... (2.2)

( Note: Page 20 asks for windows, etc... to be 1.8 )

Page 22 (Standards for thermal elements table, Column A)...

Wall (0.30)
Pitched roof - Insulation at ceiling level (0.16)
Pitched roof - Insulation at rafter level (0.20)
Flat roof or roof with integral insulation (0.20)
Floors (0.22)

I have always gone with the table on page 17 and haven't had any problems in the past, so how and where would the table on page 22 come into force?

 

[freddiemercurystwin]

As I understand it, if you are building an extension from scratch with new elements you would use page 17.

If you upgrade elements (such as a floor or wall in a garage conversion or existing outhouse conversion then you should technically use page 22.

Then again I could be wrong!

I have had it picked up by BC once, in a lofty as I recall. Since then I have always used page 22 for all extensions whether converted structure or new build. I do not consider the cost differences between achieving the two standards significant.

 

[DevilDamo]

Yeh I see what you mean. I may have to get clarification from the LA.

I kind of read if that if the better u-value's (Page 22) can be achieved, then use them. If not, the u-values on Page 17 will suffice (apart from new windows).

 

[jeds]

Table 1 gives U-values to be used when calculating a SAP. Otherwise for extensions you should use Table 4.

Column A is for any newly built elements. Column B is if you need to rebuild any existing elements.

 

[mikric]

P22, para 50 ie table 4, column a, as said above by Jeds

 

[DevilDamo]

Just going back to this...

Table 1 gives U-values to be used when calculating a SAP. Otherwise for extensions you should use Table 4.

I have used the Table 1 values for a new house and when I passed my drawings/specification to the SAP assessor, he told me to not do it that way because the SAP wouldn't comply if using those values. The way around it is to provide a SAP assessor with some preliminary planning drawings and from those, he can work out u-values/insulation thicknesses.

As homes are becoming more energy efficient, why wouldn't designers aim for better (i.e. lower) u-vales as per Table 4, than those in Table 1?

 

[jeds]

They're not prescribed U-values, they are just minimums. The SAP should be designed around whatever U-value makes the SAP comply. That's what the SAP is for - to allow flexibility.

 

[DevilDamo]

Yeh and I'm saying if you use those minimums when designing a new house, the SAP calculations will not comply hence why an assessor is to take the whole house and then they tell you what insulation to put where and the u-values achieved.

 

[freddiemercurystwin]

Yeh and I'm saying if you use those minimums when designing a new house, the SAP calculations will not comply hence why an assessor is to take the whole house and then they tell you what insulation to put where and the u-values achieved.

I've never had a SAP Assessor tell me what insulation to use. Unless the spec's I've given him lead to a failed test of course, then he would recommend where to beef things up, gotta say that happens very rarely though.

 

[jeds]

What your assessor says makes no sense. The purpose of Table 1 is to allow the calcuation of a SAP which compares the performance of an existing house with notional extension against the same house with an actual extension. The difference between the two must be reconciled so that the more flexible design does not reduce the overall performance.

So, lets just say that you design an extension against an existing property which is allready way above the minimum performance. In that case you could use the limiting values off of table 1 and it would easily comply.

As an example I've just looked at one I did in December and the floor and roof was as table 1 and the walls were 0.26 - which is only really a fraction off the table 1 limiting value. And that was against a 20 year old house. Put the same extension against an three year old house and 0.30 would have easily been sufficient.