Compliance with L1B building reg for loft conversion

[RonBowater]

I'm handling the building reg submission for an extension to my daughter's house consisting of both ground floor extension and loft conversion. I can easily show that we comply with L1B for the ground floor extension using the area-weighted approach. However, I am having some difficulty in seeing how the same approach can be used for the loft conversion, and as there is quite a large dormer window which is maybe 50% of the floor area of the converted loft, it will be hard to balance this with improved insulation elsewhere.

Can anyone offer me any words of wisdom on how to apply the L1B regs to a loft conversion and whether maybe there are any special considerations for loft conversions, please ? For example, the U values of the conversion may be mcuh better than the roof of the existing house. How is this taken into account ? What tricks can I use to gain compliance ?

Any advice most welcome.
Thanks
Ron Bowater

 

[freddiemercurystwin]

For all loftys or extensions I only ever show overall U values of construction types ie wall, roof, floor etc. Always seems to get passed ok. If glazing areas are particularly large I get SAP calcs done.

 

[mikric]

SAP calcs would be the only way to go, but you need the relevant accreditation to produce them.
Beat me FMT

 

[DavidWP]

In the past I have used the 'whole house' approach to prove the new work provides no more heat loss than the existing building. I don't think this is still strictly permissible under the latest L regs a BCO accepted the arguament last month.

 

[jeds]

You need a SAP calculation which shows the whole extended house with a standard size windows (i.e. ones that are within the 25% limit) and then the same sap re-calulcated with the over-sized windows. The overall SAP rating of the second calculation must be the same or better than the first one. Of course the window will worsen the calc so you will need to compensate for that somewhere else to bring the rating up. e.g. fit a bit of extra insluation in the roof.

That's how it should be done. An alternative is to say to the inspector; I'm going to put extra insluation in the roof to compensate, is that allright? If s/he is not having too bad a day s/he might say ok. If not you'll have to get a calc.

 

[DevilDamo]

In the past, I have had a "Heat Loss Trade Off" calculation carried out where I'm limited as to how much insulation I can provide to the floor/walls and therfore having to compensate elsewhere.