If there is slope or fall, then as the prior poster - user - recommends, furring that flattens out the floor, fastened to the slab is better. In either case, fully tape a 5-6' pc. of visqueen to the floor for a week. If moisture appears between the slab and plastic, you have a moisture problem to think about. Oct 19, 2013 · 20 Oct 2013. #6. The new roof trusses in the extension should be able to accommodate 300mm of insulation which meets the current standards. You could go further, and fix timber at 90 degrees to these, and add more insulation between then, then floor over the top.Insulating the rafters is pointless, your loft will have a breeze flowing through. Insulating Under A Concrete Slab . Insulating areas under a concrete slab like the basement of your home or building is important as heat is also lost through these areas. Placing an insulation barrier and a vapor barrier between the concrete slab and the ground underneath is great in maintaining the warm air and protecting the space from.