Don't use water tank lagging jackets (red or black) for internal tanks stored in attics in the olden days before on demand boilers) because when they are wet they fall to pieces.
Use reflective bubble wrap, then a layer of wood or celotex or similar on top, the thicker the better for retaining heat. Does keep the heated water fairly warm/hot if the tank is insulated well as I insulated my tank in my campervan