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Roof membrane and ventilation

  • 06-08-2023 8:44am
    Registered Users Posts: 56 ✭✭

    A mid terrace victorian property, getting a re-slate....New battons and membrane.

    Tyvek suppo membrane says it offers "airtightness Vapour-permeable" protection. Product description also says..."Tyvek® Supro does not need any soffit, tile or ridge ventilation, which can save time and money in the construction process"

    My top floor is a Dormer floor with 4 rooms. The sloped portion of the ceilings are insulated between and below the rafters. A 50mm gap has been maintained between outside of rafter insulation and inside of membrane. There are no sofits, or ventilation gaps between rafters on wall plate. These inter-rafter gaps are torched shut with lime render/crete .

    I had expected to put a row of ublink ub11 inline slate vents at Low level to ventilate eaves spaces front and back. These slate vents penetrate the membrane to allow air into eaves. From eaves fresh air can travel up the slope (50mm gap), under the membrane and into attic space.

    Separately a row of high level, inline ubbink slate vents (front and back) that also penetrate the membrane allow warm air to escape.

    After reading the prod description for roof membrane and the claim that slate vents are not needed suggests that fresh air can pass from outside INTO and beneath membrane and vice a rate sufficient to ventilate the eaves, dormer and attic space.

    However my roof has no sofit vents so no ingress of fresh air to pass thru membrane.

    So my Q......despite the properties of the membrane, do i still need rows of slate vents at low and high level on front and back of house?

    Also, should slate vents penetrate the membrane...or if I 'let air in' under slates using slate vents, is the air permeability of membrane sufficient to let enough air thru underneath the membrane, to flush upwards in the 50mm gap and pass out thru the membrane again at high level to a set of slate vents (that do not penetrate the membrane) and exhaust outside?

    Wondering if I can a) avoid the cost of circa 16 slate vents. And b) If I do need the vents, do I need to penetrate the membrane in 16 locations......😵.

    Many thx