A thick layer of wool
We got insulation put in the roof last week. What a revelation! We still need heating on cold days, but the HEAT STAYS IN. And there's not that slightly chill draughty feeling at the far end of the woodburner's reach. And the windows are dry in the morning. We could do plenty more, and may as budget allows, but for a quick, long-term, not terribly expensive improvement to your house, it would be hard to go past insulation.
When I say 'not terribly expensive', I mean around $1200, so not peanuts either by any means, but not in the realms of a new roof, piling, major redecoration, etc. And I cannot understand why landlords don't all do it. For sure, they don't get the direct benefit — not living there and not paying the power bill — but wouldn't some sort of social conscience prompt you to give your tenants a fair shot at being warm? And wouldn't it be worthwhile to make your house warmer, therefore drier, therefore likely to stay in good nick for longer?
We need it now, too. The colours have turned across the city, the days are getting shorter, it's the letting-go time. Some days I want to step out into it, others to stay home, light the fire, brew coffee, hunker.