It appears that in their latest S.175 guidance MBIE have taken on Auckland Council’s unique and untested interpretation of what constitutes a cladding system to include " all substantive components within the complete wall assembly. This includes sheet cladding materials, framing, rigid air barrier, any insulation or sheet materials or blanket and the internal lining" .i.e. the complete wall assembly from inside to outside, not just the weather resistant layer.
They have stated that " _Timber framing (or combustible insulation products within a framed wall assembly) may be used if a robust protective lining material (being of limited combustibility) is fixed to the exterior side of the framing and can be demonstrated to remain in place and protect the framing during the period of external fire exposure. ‘Protect framing’ can be assumed to be achieved if the protective lining material as part of a light timber frame wall exposed to the test conditions of AS 1530.4 can be shown to prevent charring of the timber frame for a period of 30 minutes. One way to determine this is to limit the temperature on the cavity side of the fire-exposed protective lining material during the test period to be no greater than 300 degrees Celsius".
This requirement provides a solution to a problem that does not exist.
It appears that they have been assisted in preparing this Guidance by those with little or no cladding test experience, faculties or expertise and will enforce requirements for protection of timber framing that is unsupported by test evidence, real fire behavior or risk assessments.
Taking the case of non-combustible cladding for example (brick, sheet metal, etc) with timber framing behind, even if the timber framing did catch fire (highly unlikely) it would not lead to fire spread " 3.5 m vertically from the fire source over the external cladding of multi-level buildings . " as per C3.5. How could fire spread over the external non-combustible cladding? If there is a room fire it is accepted that a non fire rated wall will burn out, regardless of the cladding. Why is this all of a sudden a problem?
This Guidance will mean that unless an entire wall assembly has full scale test results fiber cement and calcium silicate boards will be prohibited with timber framing. There are currently no approved plasterboard system for external use. The solution provided in MBIE determination 2017/88, with fyreline behind the RAB board, is not tested for durability, and the GIB weatherline product is not yet on the market.
The Guidance applies to C3.5 and C3.7 and therefore will present some interesting issues when it comes to two storey timber frame buildings such as apartments, offices, shops etc constructed of light timber framing and within 1m of the relevant boundary.
Yet another piece of MBIE guidance rushed out before the holidays with no industry consultation and unintended consequences. Not to mention the increase in building costs and consenting delays.
Best wishes for Christmas and the New Year,
Vulcan Fire Engineering