I'm looking to lay Luxury Vinyl Tiles over a concrete floor. I'm worried that 4.5mm or so of LVT will not provide sufficient thermal insulation. Underlay recommended for LVT is only 1mm albeit high tech. Could I and should I lay Screwfix Selotex PIR Insulation Board below the LVT underlay? LVT shop advice is that this can't be done but they've no answer to the U-Value of the LVT and it's underlay. Thanks in advance for any advice.