When you know conditions will be cold and damp you probably want a sleeping bag with synthetic insulation. Synthetics are inherently hydrophobic, so they won't absorb moisture or lose their loft.
Besides maintaining loft when the humidity is high, synthetic insulators like OmniTherm® are less expensive and easier to maintain.
To increase the insulation of our synthetic sleeping bags we use shingled construction.
When properly oriented the shingles will loft up, or increase in overall thickness, when you pull the top up towards your neck.
How thick the insulation is determines the temperature range. For most conditions you want to select a bag with a comfort range around the temperatures you expect. With good ground insulation most people will still be comfortable at the Tlimit rating wearing a light baselayer. Our extreme rating (Textreme) would only be useful if you wore extra layers inside the bag. YMMV.