Abstract: A thermal insulating material is formed from layers of reflective sheet material (7) and has expanders (10,11) operable to convert it, on the application of transverse tension, from a closed state to a state in which the layers are held spaced apart. The material is provided with boundary portions for securing it e.g. to the underside of ceiling joists (4) and with deflector means (12) operable to space the material from a planar covering when such covering is applied. Advantageously the deflector means is provided flat and has a triangular cross-section when erected.