A bare dn16 pipe carrying 80 °C water through a 10 °C roof space loses roughly 45 W per metre. Over 20 metres that is 0.9 kW of collector output that never reaches the tank — on a system that may only collect 3 kW at peak.
Pre-insulated twin pipe with 19 mm closed-cell insulation brings that down to around 12 W/m at the same ΔT, and it does the return leg in the same sheath, so you dig one trench and clip one run.
Watch the ends. The weakest point of any pre-insulated run is where the insulation stops at the fitting. We supply end caps with every coil; if they are left in the box, the first metre does the condensing for the whole line.
