Skip to main content

Rendering system on existing insulated cavity wall G#8288

By 16 December 2015August 16th, 2016Code, Encyclopaedia, Q&A

GBE > Encyclopaedia > Code > Q&A > G#8288

Rendering system on existing insulated cavity wall


Q “What rendering system is appropriate for an already insulated cavity wall on a small residential building?”

The easy answer:

  • The one that does not create condensation risk using BS 5250 calculation
  • (Static thermal & moisture vapour analysis: Lines not crossing)

The slightly less easy answer:

  • The one that does not create condensation risk using WUFI calculation
  • (Dynamic Hygro-thermal moisture analysis over time)

The slightly more expensive answer:

  • The one that does not create condensation risk using Delphin calculation
  • (Dynamic Hygro-thermal moisture analysis over time with full colour section display)

The next easy answer:

  • The one the BBA Agrément Certificate says its okay to apply over an existing insulated cavity
  • I doubt that such a statement exists in a BBAC

The non-answer:

  • On a small residential building
    • I cannot give advice on costs
    • Ask a GBE Supplier
    • But careful some do special handshakes

The Response Questions:

  • By Rendering system you might mean:
    • M20 Weather protection render
    • M20 Insulating weather protection render
    • M21 Rendered insulation system
  • Already insulated cavity wall
    • Masonry Cavity?
    • Timber frame and brick cavity
    • Is cavity insulation foamed plastic? Plastic beads? Stone fibre? Other?

Why is the question important?

  • Because one layer of insulation in the cavity will have lowered the temperature of the wall but may not have lowered the moisture level in the wall.
  • Adding an additional layer of insulation outside will push the temperature of the wall and insulation up
  • Depending upon the material of the insulation the moisture level may also go up or go down
  • There is a risk that the moisture level may be up and the temperature down at some part of the wall
  • This may permit precipitation of the moisture causing intersticial condensation in the wall
  • Embedded timber may be subject to the moisture and may rot leading to structural failure

There is no answer:

  • The answer to the question is reliant upon knowledge of the materials in the wall and the existing cavity,
  • without it there is no answer

The Compromise answer:

  • The compromise could be: ‘the most moisture vapour permeable but weather proof rendering system available on the outside’.

© GBE NGS ASWS BrianSpecMan aka Brian Murphy
16th December 2015  – 30th December 2015
modified here after the Word file

Leave a Reply