The invention provides cavity closer apparatus in strip form and suitable for use with ICF walls, the apparatus comprising a body member including a central portion for bridging the cavity between inner and outer wall leaves and respective front and rear projections for engaging the exposed ends of the leaves, in use, in which the body member incorporates means for lockable engagement of fixing cleats for a window or door frame. The central portion of the body member may include a housing defining one or more voids containing insulation material and the fixing cleats may comprise clips formed from a flat strip of springy or resilient material adapted at the forward end portion for attachment to a door or window frame element and at the rearward end portion for lockable engagement with the body member, behind the frame element. The apparatus may also include a spacer member in strip form, the body and spacer members including means for mutual engagement at a desired position of the spacer member relative to the depth of the body member, the spacer member including means for lockable engagement of fixing cleats for the window or door frame.