No point putting insulation in attic and then compressing it! Will reduce the R factor considerably. We have a total of about ten to eleven inches; six inch batts with some further six inches of blown in glass wool, added some 15-20 years ago, which has settled a bit.
Also and very important is that the cold attic space above be ventilated properly to the outside!
Here the minimum attic venting under Canadian Building Codes is 0.3 percent of the floor area. In other words a 1000 sq foot attic requires "A minimum 3 sq feet of venting, arranged in such a way to permit 'cross' ventilation". This is to dissipate any moisture that condenses from warmer (and therefore more humid) house air that may leak up past the warm side ceiling vapour barrier of the rooms below.
Problems with mould, mildew and or moisture dripping off the colder roofs and/or forming as black stains and wood rot of roof trusses are often discussed where that rule has not been followed. Some sad stories about rotting roofs! And they are expensive!
To ensure adequate ventilation we have added extra soffit vents along our approximate 180 feet of front, back and ends, eave overhang*. Although any variety of vents can be used including ridge vents and/or gable ends or even powered fans that cut in if/when humidity levels are reached. Single storey bungalow roof is 60 by approx 40. Hence
60+60+40+40=3D 180. And periodically we check for air flow.We chose soffit vents because easier to add, facing almost horizontally downwards are less prone to snow blowing in and it avoids any work on or cutting of the roof surface. Which in our case is s a low slope pitch/gravel. Most homes however use same rules but are built with composite 'roof shingles'..