Current roof build up - unsure of u value

[ADringer]

Hi all,

 

Currently having an extension done and the building reg drawings state:

 

 

Not the target U value of 0.18.

 

I've noticed they've gone for 100mm Xtratherm which has a U value of slightly above 0.2 - is this correct? 

 

If so, when looking at U values do I also need to take into account other layers such as the EPDM or is it just the insulation?

 

And if not, should I be getting them to top it up? The roof is finished so would have to be done from below.

 

 

Thanks 

 

[Redbeard]

Most PIR insulants are credited with a lambda of 0.022W/mK. 0.12(m)/0.022 = R = 5.45. 1/R = U = 0.183W/m2K. You can also take into account any othere elements which don't have cold ventilation air flowing through them, so that's the ceiling, deck board, EPDM, internal and external surface resistances. All that should just get you 0.18. Assume you already have Bldg Regs approval (prior to the June changes (if you are in England).

[Redbeard]

Ah, sorry, read that too quickly. You are saying they have ignored the spec of 120 and fitted 100? If so ask them to fit as per the spec which will get you 0.18. 100mm won't, unless there's something else not listed.

[ADringer]

19 minutes ago, Redbeard said:

Most PIR insulants are credited with a lambda of 0.022W/mK. 0.12(m)/0.022 = R = 5.45. 1/R = U = 0.183W/m2K. You can also take into account any othere elements which don't have cold ventilation air flowing through them, so that's the ceiling, deck board, EPDM, internal and external surface resistances. All that should just get you 0.18. Assume you already have Bldg Regs approval (prior to the June changes (if you are in England).

Hi @Redbeard,

 

thanks for the response. I noticed you used a thickness of 1.2m 8n your calculations whereas I have 1.0m - does you statement still stand saying that all other elements will get me to 0.18, or was that only for 1.2m?

 

Just saw your follow up message! I'm going tomorrow to double check everything but pretty sure there's nothing else there

Edited August 29, 2022 by ADringer