The brief
The Golf's aircon had lost its bite, the airflow felt weak even on the highest setting, and the owner had topped up the refrigerant twice in a few months and watched it disappear again each time. There was a musty smell when the AC ran, and the front passenger carpet had a damp patch. He brought it in. Refrigerant that keeps escaping plus weak cooling plus a wet footwell all point at the cooling coil, the evaporator buried behind the dash. Refrigerant runs through it, cabin air blows over it and comes out cold, and the moisture that condenses on it is supposed to drain away outside. When the coil leaks it can't hold a charge, which is the warm air, biofilm on a tired coil gives the musty smell, and a degraded drain seal lets the condensate run into the footwell instead of out the bottom. All three trace back to that one part.
The diagnosis
A pressure check showed the system undercharged with no leak visible under the bonnet, and a leak test traced it to the cooling coil behind the dash, the same coil whose condensate was being routed past a degraded drain seal into the footwell. The lines, the compressor and the condenser all checked clean. That's a coil replacement, not another recharge. A leaking evaporator only leaks more, and topping it up just buys a few weeks, so the coil had to come out, which on this car means going behind the dash.
The work
The remaining refrigerant was recovered, the dash dropped back far enough to open the HVAC box, and the failed cooling coil lifted out. The housing was cleaned of the biofilm causing the smell, and a new genuine VW-spec coil went in with fresh seals, including a new drain seal so the condensate routes out properly. Then the dash was rebuilt, the system pulled down to a long, deep vacuum, and recharged with the correct weight of refrigerant. A check at the vents confirmed cold air and a dry footwell.
The outcome
Cold air at the vents at idle, strong airflow, no musty smell, and the footwell dry. The Golf went home with the aircon working the way it should. A leaking evaporator is a big job because of where it sits, but it's a once-and-done one, and doing the housing clean and the drain seal while it was open means the cold, the smell and the wet footwell are all sorted in the same visit, not just the temperature.