u-values

trying to achieve 0.15 in roof with 300mm high density fibreglass,0.15 in the walls with cavity insulation and drylining is there any advantage to 0.15 in the floor if i'm not going with underfloor heating?it would need 125mm of insulation but my builder tells me 100mm would be still very good and that the extra price of the 125mm wouldnt warrant it.he's not buying ,just laying it by the way so he should have no issues just his opinion

the current regs are:
0.16 roof (on flat)
0.2 roof (sloped)
0.27 walls
0.25 floor

therefore you are trying to increase
roof by 0.01
walls by 0.12
floors by 0.1

as you can see you are disproportionally upgrading the walls and floors in comparison to the roof... also remember that most heat is lost through the roof (thus the reason for the required low u value)

I think you would be better off trying for:
0.1 - 0.11 in the roof
0.15-0.17 in walls
0.16-0.18 in floor

cheers syd, what would it take to reach those values in the roof,400mm mineral wool?and if it took 125mm in floor to get 0.15 would 100mm approx get the 0.17 mark

you may reach 0.1 in the ceiling by

120mm rockwool (lambda value 0.042)
120mm rockwool (lambda value 0.042)
150mm rockwool (lambda value 0.042) @ 91% between
150 x 44 ceiling joists @ 9%
12.5mm plasterboard with skim finish.

so thats 390mm rockwool

The u value of the floor depends on the area to perimeter of the floor so i cant tell you exactly what u value you would get. however i would always recommend 2 x 60 sheets over 1 x 100 sheet. This is because there may be more cold bridges when 1 sheet is used as the concrete can push boards apart on pouring

thanks again syd,what would you recommend for reducing cold bridge at inner leaf/rising wall level?the foamglas block i hear about is very expensive ,would be around 2500k,is there any other method.can quinn lite foundation blocks be used for the last row on the rising walls if standard dense blocks and conc. 1st floor is used above.
if they can would it be of any benifet?

i see where you are going with this..... youre going a bit outside the realms of public internet forum advice... onto very specialised areas full of debatable science and opinionated nit picking.

1. are foamglas blocks (perinsul) certifed? i dont think so. will the supplier stand over the strength of the block and certify their structural integrity for your particular situation? if so, great, but i think you might run into problems here.

2. Although quinnlite blocks are certified for use below dpc level, basic science and testing shows they fill with water through capillary action and thus are rendered useless from an insulation point of view. Also theres a question as to the certification of their strength as well.

The best detail is to have a continuous closed insulation envelope all around the dwelling. This would mean a change from established traditional construction practises though.
You would need your details designed by a good arch tech IMHO.