Simon L H

Im looking at retrofitting u/f heating to existing cottage with no insulation under ground bearing slab. I think its going to need some insulation, even if minimal 25mm. spoke to Nu Heat who suggested it wasnt required with their Lo Pro. until I mentioned Building Regs and they suggested .. oh yes, it might need insulation in that case. looking to demonstrate ground floor is insulated enough to limit heat losses no more than 10W/m2. Any heat loss calculation guides recommended?