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This video explains some of the risks when adding internal insulation.

 

Below, the various materials and approaches to solid wall insulation are shown along with some of the key considerations. 

 

External Wall Insulation Internal Vapour Closed Insulation Internal Vapour Open Insulation
Should keep the thermal mass warm Isolates thermal mass Limits access to thermal mass
Exposed to the elements so will require protection Is protected from the elements Is protected from the elements
Requires an intact vapour membrane Requires an intact vapour membrane Allows for moisture exchange and may help to regulate humidity and temperature
Will alter the external appearance Will not alter the external appearance of the building Will not alter the external appearance of the building
Will encroach on surrounding area Will encroach on internal floor area Will encroach on internal floor area
Usually requires scaffolding (when working at height) Should not usually require scaffolding Should not usually require scaffolding
Usually requires the disconnection and re-connection of gas and electric meters, phone, and broadband. Requires the removal of any: kitchen/bathroom units; internal plasters; skirting boards; etc. Requires the removal of any: kitchen/bathroom units; internal plasters; skirting boards; etc.
Should not be installed in wet conditions Can be installed all year round Can be installed all year round
Complications when the roof overhang is insufficient Complications with thermal bridges at joist ends, internal walls etc. Complications with thermal bridges at joist ends, internal walls etc.
Complications with adjoining buildings Reduces the temperature of the  external walls which may effect the moisture content and increase the risk of weather damage Reduces the temperature of the  external walls which may effect the moisture content and increase the risk of weather damage
Prevents any evaporation from covered walls Does not allow for internal moisture movement  Allows for internal moisture movement (provided not sealed with inappropriate paints/surface finishes)
Slow to heat up utilising thermal mass which should help to regulate temperature, usually slower to cool down Quick to heat up the air temperature but no access to thermal mass which may increase the risk of overheating. light/medium weight thermal mass available which may help to regulate temperature
Ventilation strategy required Ventilation strategy required Ventilation strategy required

 

This table shows various insulation types and their thermal conductivity. Vapour closed insulation such as XPS does offer better performance regarding heat-loss but will increase the likelihood of moisture problems in traditional vapour open walls.

Material Thermal Conductivity (W/m.K) Reference
  Vapour Open  
Woodfibre-board 0.038 (Steico, no date)
Hemp-lime 0.060 (Cérézo, 2005)
Oilseed rape/miscanthus-lime 0.060-0.075  
Expanded cork 0.04 (Ty-mawr Lime, no date)
     
  Vapour Closed  
Extruded polystyrene (XPS) 0.033 (Knauf, 2019)
Expanded polystyrene (EPS) 0.032 (Jablite, no date)
Polyurethane (PIR) 0.022 (Celotex, no date)

 

 

Solid wall insulation