When undertaking DIY insulation of your roof space it is important to get the right depth of insulation. However, from the roof structure's point of view it is also essential to make sure that the roof space is properly ventilated. The there should be air space in the eaves - where the roof trusses sit on top of the house walls so that moist air can be vented. The insulation should not prevent air circulation or initially it will become filled with moisture and lose effectiveness and of course the end result will be rotten timber. For the same reason the attic access hatch should be draught proofed, insulated and held firmly shut to prevent moist, warm air escaping into the roof space to condense.