Failing to properly ventilate your attic can allow moisture to build up and will eventually cause mould and damage to your sheathing and shingles. Over the years the required insulation for homes has changed. Newer homes will typically have R-50 of insulation installed.
Prevention is possible by ensuring all your insulation is covered by a vapour barrier, most codes call for a minimum of 6 mm poly, and all penetrations are overlapped and taped by Tuck tape, the red tape not the silver duct tape of Red Green fame. All electrical boxes, windows, switches, and vent penetrations should be tightly sealed.