I am not sure that wallpaper offers any increase in insulation. Maybe the wall may initially feel warmer when touched, but the thickness of barrier is insignificant when it comes to heat loss.
Frankly, it is the fashion these days simply to paint the whole room a neutral shade with white ceilings. This goes for all properties - rental or not. Really what is going to be the decider between paint and wallpaper is the quality of the plasterwork. This is why so many rental properties use woodchip (yuk) paper. It's easy for a quick overpaint between lets. Spalsh and dash as it's known.
if your flat is that cold that you think wallpaper is going to make a difference then you want to be looking elsewhere for a solution to your winter shivers! loft and cavity wall insulation (tricky in a flat admittedly) double glazing, draught proofing, a more energy efficient heating system and woolly jumpers will keep you warm, not wallpaper!
the thermal insulation properties of a sheet of paper less than 1mm thick are so negligible as to be imperceptible. as to whether or not you paint or paper, that's a question of taste and style. don't base your decision on the warmth factor. you'll gain very little in that respect through papering.
as plumbbob says it might make the walls warmer to the touch but that's about it. and as you have acknowledged yourself, it's still cold with your existing wallpaper.
lining the walls with thin sheets of polystyrene prior to papering used to be popular and was said to aid insulation but it's not so common these days, largely down to the fact that it's effectiveness is hotly disputed.
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