How can I add external insulation using natural/renewable materials?

Click here for more general advice on external insulation.

Here are a few examples from the displays in CAT's visitor centre:

External insulation with rigid wood fibre boards

external insulation with wood fibre boardThe boards for external use are rigid & impregnated to resist moisture. They are fixed to the wall using specialist screws supplied by the panel manufacturer, and finished with a mesh reinforced two coat render system. This lime-based breathable render allows moisture trapped in the wall to be gradually released.  A plinth detail of cork insulation with tile facings is shown; this would be either glued or fixed to the brickwork with plugs & screws. Typical U-value for the improved wall with the thicknesses of insulation shown: 0.52 W/m2K.

  • Rigid tongue & groove Wood-fibre board insulation available in thicknesses up to 160mm plugged & screwed to brickwork.
  • Two-coat render with reinforcing mesh bedded into base coat to specification approved by wood-fibre board supplier.
  • Flashing over plinth.
  • Plinth detail: 50mm thick cork faced with slate or tile glued or plugged & screwed to brickwork.


External insulation with wood-fibre boards batts in a timber frame with rendered cladding

 external insulation with batts and claddingA grid of timber battens is fixed to the wall - the first, horizontal battens plugged & screwed to the brickwork & the second, vertical, battens screwed to the first. The two layers of insulation between the battens act like a quilt to reduce cold-spots. A layer of breathable but waterproof polyester fabric is then stapled to the outer battens to keep the insulation in place & a final layer of vertical battens added over the top to create an air gap to provide ventilation behind the cladding. This could be horizontal timber boarding or, as here, a rendered calcium silicate board. Fly mesh should be incorporated at the top & bottom of all the ventilation gaps.

  • 50x50mm horizontal softwood battens at 600mm centres plugged & screwed to brickwork. A layer of natural insulation made from wood fibre, hemp, flax or wool is added between the battens
  • 50x50mm vertical softwood battens at 600mm centres screwed to horizontal battens. A second layer of insulation is added reducing the cold spots to a minimum.
  • Breather membrane of non-woven polyester fabric stapled to vertical battens
  • 50x25mm vertical battens screwed to vertical battens to create ventilation gap
  • 10mm calcium silicate board as sub-strate to render screwed to battens to manufacturer's specification. Include flymesh to top & bottom of ventilation gaps
  • Two coat render with re-inforcing mesh bedded into base coat to specification approved by calcium silicate board supplier.
  • Flashing over plinth
  • 150mm deep plinth detail of 50mm thick cork faced with slate or tile glued or plugged & screwed to brickwork.


External slate hanging with cellulose fibre insulation.

 external insulation slate wallSlate or tile hanging is a traditional way of improving weatherproofing for an external wall. Here this is combined with an additional layer of insulation supported on a timber frame fixed to the original wall. This timber frame is sheathed with a breathable bitumen-impregnated fibre-board & filled with cellulose fibre (recycled newspaper) insulation.

  • 50x50mm softwood battens securely plugged & screwed to slate - avoid splitting or spalling slates
  • 100x50 vertical softwood studs screwed to battens at 600mm centres
  • 15mm bitumen-impregnated fibre-board screwed to studs
  • 50x25mm vertical softwood battens to create ventilation gap
  • 38x25mm slating battens nailed or screwed to vertical battens
  • Slates or tiles nailed to battens with copper or other rust proof fixings
  • Flashing over plinth
  • Plinth detail: 100mm cork, faced with slates glued or plugged & screwed to slate.


If you want more detailed advice about these techniques, and some hands-on experience, then we run a short course here at CAT on eco-refurbishment.

Please contact us if you need to find suppliers of some of these materials.


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