what is the u value needed for my house extension walls?

[DIYedboy]

I'm having an extension across the back of my 1930's semi in 2016. It'll have a flat roof with kingspan across the top and two roof windows, suspended timber floor with 100mm kingspan inbetween and a 9foot Bifold door...................what wall U value am I supposed to meet? I tried looking in that part L , but it just blew my brains out with too much info that wasn't user friendly

 

[tony1851]

0.28, but that figure on its own is meaningless - you may have to go better (ie lower) or alternatively need not be as good.

It depends on factors such as; area of glazing, U-values of other elements, efficiency of heating system etc etc.

Agree that Approved Document L is the most user-unfriendly document.

 

[^woody^]

There should be calculators on the Kingspan web site. Otherwise there definatly are on the Celotex site - and there is also an app for Celotex.

 

[DIYedboy]

Thanks lads, I intend the walls to be solid 215mm aircrete blocks rendered externally with kingspan type insulation on the inside face. I think this must be the most thermally efficient way to build a wall because the insulation is on the INSIDE.................what do you reckon?

 

[Nakajo]

With solid wall construction, unless your rooms are very large, I think it is easier to achieve higher thermal efficiency with external wall insulation. You may be heating a larger volume, but it's easier to provide thicker insulation without losing space - and the mass of the wall acts as a heat sink.

 

[DIYedboy]

The room will be about 25ft x 25ft.................why use all that extra energy heating something when you don't have to? I will get a faster warm up cos my walls won't be acting as heat sponges

 

[Nakajo]

Are you asking or telling?

 

[DIYedboy]

I'm doing neither, this is an open forum for you to make any useful comment you wish to, it is my opinion that using the heating system to heat up your walls so they will act like radiators would only ever be effective if one lived in the house 24/7. If the occupant goes off to work for 10+ hours a day (travelling time plus working day) having warm walls is pointless.