So, a spring project this year is putting/replacing the insulation in the attic at my parents place. And air sealing it. Maybe a radiant barrier. I want to do this before it gets too hot up there. However, I did a quick look on Saturday, measured the ceiling joists, they are 2 x 6, but the first three beams I measured are spaced uneven - not all are 16" on center. One had a gap of 14", the next 15", and then 13". Is this usual for the builders to space them uneven? maybe there is a reason?