Yes, they did. However, not like we think of insulating houses today. Rather, they built houses that were designed with insulation through how the houses were built.

Three main types of houses existed - pit houses, which were sunk into the ground, longhouses and weaved wicker and clay houses.

Pit houses were of an older design, and during the viking era it seems they were rarely the living dwellings of people - rather they were outhouses, storage houses, workshops and so on. Basically people dug a pit in a well-drained area and consctructed a house above it, saving building materials on the walls and gaining a well-insulated home since the frost will not penetrate especially deep into the ground. The walls could be constructed from timber (standing or horisontal) and plugged with moss which isolates quite well, while the roof would be from birch-bark and peat, with the peat being a good insulator. The walls could also be made from weaved wicker as an internal framework with mud mixed with straw and/or grass-feeding animal dung on both sides, which is also quite a good insulator.

Here's a modern image of a reconsctructed pit house at Foteviken's outdoor museum.

Longhouses were huge buildings, often constructed out of timber. Some claim there were knot-timbered, but the earliest buildings surviving are form the 1200s, and there's really no evidence knot-timbering was used before the 1100s. It is possible, but in no way certain. Long-houses often had standing timbers to carry the roof and could have either horisontal timbers or standing timbers for walls, or the earlier mentioned weaved wicker and clay (and/or dung) with straw walls. Roofs were out of birch-bark and peat, which as mentioned earlier, was a good insulator. Longhouses would also have an "earth bench", meaning dry earth or sand were placed along the lower part of the wall on the inside of the house, with wood planks or other materials covering it to create a surface to sit on or place things on. This was a common way to insulate houses in Sweden until the mid-1800s.

Here's a photo of an earth bench in a more modern setting, when a floor has been extended from wall to wall instead of just a bench over the stone and earth.

It is important to remember that viking era houses did not have windows, nor did they have chimneys. In general, they had a single, high-placed small opening for the smoke from heating and cooking fires to escape, called wind-eye. This means that draft and "leaking" of heat is limited.

So, the bottom line is that the building types, building materials and methods of consctructing buildings meant that they were insulated in their own, rather than any further insulation.

Still, even if the houses were pretty well insulated on their own, it did not mean they were warm. Experiemental archeology in Gene, reconstructing iron age longhouses (just before the viking age) showed that during winter, temperatures indoors were about 10-16 degrees celsius without burning unsustainable amounts of firewood.

Source: Långhus i Gena, PhD with experimental archeology by Lena Edblom

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