by Tim Renouf August 15, 2005
Ignorance exists about what is the correct technique to thermally insulate clad walls, either timber or fibre-cement, and also permit effective breathing. Sarking is a material in continuous roll form commonly used for waterproofing behind clad walls to prevent moisture penetrating into timber wall framing. Sarking must be impermeable to liquid moisture but still allow free flow of water vapour from the inner surface of the cladding to vapourise and dissipate. Fire retardant breather building papers or white Tyvek are possibly the best examples of breathable and waterproof sarking materials.
"Breather foil" is another option and is conventional anti-glare house wrap foil with one pure visible aluminium surface facing inwards, and has machine pin-pricked holes. In clad walls this product works best with adjacent airspaces on both sides of the foil wrap, airspaces which greatly assist moisture vapour to freely migrate inwards and outwards of clad wall structures unimpeded. If moisture is trapped behind timber claddings, subsequent moisture vapour build-up can cause warping of the timbers. Fibre cement claddings, such as Hardie planks, also need breathing airspaces, and breather foil is specifically listed as an option in Hardie texts.
Enhanced airspaces can be formed by dishing or depressing breather foil between studs, ie a loose foil wrap, or by battened airspaces over the foil with claddings fixed onto the battens. James Hardie wall cladding manuals as well as texts from fibre batt manufacturers and energy advisor centres show dished breather foil in clad walls in conjunction with fibre batts. This is effectively impossible and misleading to consumers.
When R1.5, 2 or 2.5 batts are part of clad wall insulation systems, they are inserted into the stud cavity, and make contact with the inner foil surface, unless the batt is stapled or strung in a manner which would create a formed breathing airspace against the foil surface. 20mm is the minimum width for optimising thermal performance. Also the fibre batts will indent the foil into the cladding profile thereby eliminating the two breathing air-pockets, as well as cancelling 100% of the foil’s thermal performance R-value, both for the visible inward aluminium airspace as well as the outward anti-glare airspace.
Effectively, fibrous batts in clad walls will restrict free movement of air and it is more likely for a moisture build up to occur between the foil and the cladding. Even with greater permeable breather materials, there is the ever present possibility of moisture forming in the matrix of the fibre batts, particularly in humid climates, which increases the risk of slumping and degradation in thermal performance. The new Insulation Standard AS/NZS4859.1:2002, clause 2.3.1, states that “the moisture content of materials will affect their thermal performance.”
The best choice for insulating clad walls with maximum breathing is to use breather foil combined with “certified” Concertina FOIL BATTS, and no use whatsoever of fibrous wall batts
= Total R-value R2.5. This R-value can be directly used in FIRST RATE 5 Star House Energy rating, in the “Total R”column. Further technical notes, refer www.concertinafoilbatts.com