All constructions suffer from Interstitial Condensation - whether or not it is an acceptable risk depends on the conditions under which condensation occurs. The wise designer will seek to eliminate condensation through the judicious use of vapour control layers, insulation and ventilated cavities.
Moisture arriving in a ventilated cavity through condensation will be dispersed harmlessly to the outside. In calculating the overall effect of the ventilated cavity the external layer or layers of the construction by-passed by the ventilation duct should be omitted from the calculations or significantly reduced in performance value. In Germany the ISO standard totally ignores the effect of the layers external to the ventilated cavity. In the UK, BS5250 does not make an allowance for this effect and only allows for a slight reduction in the thermal resistance of the cavity. In practice the actuality will be somewhere in between.
Breathing Walls do not include any significant vapour control layers and utilise materials of very low vapour resistance which allow the flow of moisture through the construction. Insulation is commonly mineral wool (glass or rock) , or recycled newspaper impregnated with borax to prevent insects and reduce flammability. (see Warmcel).
Breather Membranes
Breather membranes which allow the flow of moisture vapour whilst preventing actual water passing through are used behind external (timber) cladding. The most successful of these are made from woven polypropylene (Tyvek Supro & Klober Permo are the ones most well known, many others are on the market but check with your Building Control Officer before specifying).
For external cladding of timber framed buildings a well-used product for external sheathing is Hunton Bitvent
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