I've got some cellulose insulation in the loft, but it doesn't quite come up to the top of the joists (2-3" short). If I put fiberglass on top, should I put it perpendicular to the joists (which would leave extra air space and humps by each joist), or parallel (which would leave space between the fiberglass at each joist). Or do I have to fill the 2-3" first? Thanks for your help!