Go to Forum Home Building Refurbishment and Retrofit Wall cavity insulation – to fill or not to fill?

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    • #30743

      One of our current projects is a renovation/residential conversion of a 1950's Hop-picker's Barracks. This is a long narrow building, with solid gable ends and long sides of repeated doors and windows. It has a single brick thickness shell wall, of common brick. We want to bring this up to AECB Silver Standard.

      We plan to line it with 100mm medium density block work. NHBC specifies an air gap between outer shell and insulation, but would it be satisfactory to completely fill the cavity with 100mm of polyisocyanurate (Celotex GA3000z), as this will be much easier for the Self-Builder to install? Exterior cladding on the long walls will be vertical cedar, and brick slips below windows. The gable ends could be clad with better quality brick, with or without an air gap between these and existing brickwork. Attention will be paid to cold bridging.

      So, can we have a fully filled 100mm cavity, and still meet Silver Standard?

      Advice will be welcomed!

    • #34327
      Mark Siddall

      Have a look at the following threads for guidance and directions to the appropraite documentation.

      Hope this helps.


      P.S. 100mm achieves a U-value of about 0.36. Silver standard looks for a U-value of 0.24. At 200mm insulation your getting closer to the mark. (Used BuildDesk software. May I suggest that you down load a copy. It's free and very intuative. That said the BRE tool is a little better but costs £50.)

    • #34328

      Thank you for your help. We have resolved to try another approach, resulting in 200 mm insulation, which should give us better u-values. We do appreciate your guidance in this!

      Best wishes, Julia HM.

    • #34329
      Eco Design


      NHBC generally don't like fully filled cavities, soley because historically they have prooved to create more defects and claims than partially filled cavities. However I am a great believer in fully filled cavities. If the workmanship is controlled and site team understand the physics of the materials and how the detail is supposed to work then there is no problem. Mass built homes have been the culprits due to sloppy workmanship in the past.

      There are UK maps in the NHBC documents that give guidance on wherther fully filled cavities are acceptable for that particular area or not. However specific site conditions vary locally and if the full fill is built wrongly it will cause problems. So the devil (as always) is in the detail.

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