High density firm foam is a near perfect option for a Pop-Top roof mattress. The firmness and the density of this foam provides near optimal support for a thin mattresses found in most Pop-Tops. There a 3 elements to support Firmness, Density & Volume. Given most Pop-Top are less than 5cm thick one of the three elements (volume) is greatly reduced hence why a firm, high density foam is a good choice. Don’t be put off with the notion of firmness. It support that matters. Firm V38 Great choice for pop top mattress