Some might argue that they simply didn’t keep warm! Houses weren’t warm as they are now, as heating wasn’t very effective, and tended to smoke up the house. People tended to wear lots of clothing, as the cold inside could be the same level of cold as outside.
For the lower classes, the outer layer tended to be made of wool, with the under layers made of linen. These linen garments were washed occasionally, but it was unusual to wash the woollen layers. The smoke that was almost a constant feature from the fires seemed to permeate the outer layers and act as a sort of deodorant, to help stop everyone becoming too smelly.
The wealthy were able to line their clothing with fur to keep warm, a luxury the poor could never afford. The use of fur was covered by Sumptuary laws, which governed who could and could not wear particular fabrics, veils, and other things like that. Even who could eat what!
Farm families typically lived in a cottage which had one big single room, in the middle of which would be a hearth for the fire. Above the hearth was a hole in the rood for the smoke, and ‘hanging chimneys’ may have been used to help guide the smoke. The windows in the cottage were usually also unglazed, which led to houses and cottages being rather draughty and chilly.
Wealthy families were better off though, as they had less draughty building and more furnishings to trap in heat. The personal attendants of the lord and lady were sometimes able to stay in their sleeping quarters, wrapped up in a blanket on the floor where they could absorb some of the heat from the fireplace. The lord and lady, and their families, also had heavy blankets, feather mattresses, fur covers and wall tapestries to help block out breezes and the cold, and those with four poster beds were able to use the heavy curtains to trap in heat to keep even warmer.