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
