Hello all,

I’m trying to achieve a cavity wall construction with external render finish, internal plaster on blocks with an overall thickness of 300mm to achieve a U-value no greater than 0.18 W/m2K. Is this even possible? 😉

  • Rachel Haynes

    Hi…..and to answer your question, very tricky and not possible with the conventional insulations we use. You are looking at around 0.24ish for full fill 100mm cavity insulations (glass and rock wool type) and pretty similar with partial cavity fill rigid insulations such as Kingspan (assuming you are maintaining a 50mm clear cavity that NHBC will be looking for).

    Is it new build or renovation? If you are for a render finish and new build you could consider a single wall of blockwork with an external rendered insulation on the outside and plaster on the inside or consider frame type constructions such as timber of lightweight steel (Metsec for example). There are a lot of suppliers of external rendered insulations out there now (Knauf-Marmorit, Alumasc etc) who can give good advice.

    For more accurate u-values speak direct to the insulation manufacturers and discuss different construction solutions with them – I assume you are trying to keep the wall as thin as possible?

    There are new thin insulations available on the market now but make sure that they have approval from your Building Control Officer and warranty supplier (if you have one) before using them – they are a bit new and untested at the moment.

    Hope that’s of use!

  • Vince Holden - Construction Project Manager Holden

    Hope you are still checking this.
    Extratherm full fill 100mm with Celcon standard blocks will get you down to the value you are looking for. Just done it recently

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