It is essential to have adequate insulation between your sleeping bag and the snow. A closed-cell foam sleeping mat is the lowest cost and most reliable option. Thin groundsheets or space blankets are not sufficient. Use your pack or a folded parka under your feet. Ordinary foam rubber is too heavy and absorbs moisture, while plastic air mattresses are unsatisfactory as they puncture easily, and the air inside does not warm up. Self-inflating, sealed-cell foam insulated sleeping mattresses (e.g. Thermarest), are excellent but expensive and can puncture. Another possibility is a closed-cell foam mat but with 1 cm egg-carton type bumps on one side. This doubles the insulation without doubling the weight, but is somewhat more bulky.
Closed-cell foam is also available in 2–3 mm thickness, which is excellent for providing insulation and reducing condensation by covering all the tent floor. Sleeping mats are then placed over the top.