A method of re-lining a ventilation duct (20) for a building comprising the steps of: -providing (12) a flexible composite tube (30) of fire-proof polymer material having at least a flexible tubular fibre-core (32) being impregnated with at ambient temperature self-curing polymer material (34) through the duct (20) having inner walls (22) and outer walls (24); -blowing (14) compressed air through the flexible composite tube so that the flexible composite tube (30) is formed against the ventilation duct's (20) inner wall(s) (22) having a particular cross-section (T), and allowing the self-curing polymer (34) to self-cure at ambient temperature.
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