I've been doing some research and getting some estimates on insulation (wow is it expensive!). And the debate between fiberglass and cellulose is driving me crazy. Do you guys have any opinions on what's best? Because from my end it seems to depend on who you ask. Does fiberglass lose r-value in frigid temps? Is sealing attic bypasses a doable DIY project or should I leave to a pro? Would the weight of cellulose be a concern for an attic with 2X4 joists and (I assume) standard dry wall ceilings? Is chemical treatment of cellulose a concern-does it wear off over time? Does the settling of blown fiberglass reduce the r-value? My head is spinning with information and the difference in the prices on the estimates is staggering. I'd love to hear any knowledgeable opinions from people who aren't trying to make money off me.
Thanks so much.

I think that most of you concerns are unfounded. What do you mean by sealing the attic bypasses? Fiberglass loose R-value, settling of blown insulation, loss of chemical treatment of cellulose not concerns. The settling of cellulose use to be but no more. The treatment of cellulose is fire resistance treatment. The product itself does not provide the insulation. The product creates air pockets and prevents or reduces air movement. The trapped air is what provides the insulation.

Just go on R-value.

Again what is attic bypasses.

Attic bypasses are the unsealed spaces leaking air into your attic---through overhead light fixtures, attic stairs, chimney, whole house fans, etc. Research shows these breaches can significantly reduce the performance of insulation as well as increase moisture in the attic from warm house air settling on cooler attic components and condensing. I've never heard of it before either but two insulators and several websites have all provided the same information.

All of these will naturally be sealed by blown in insulation except for attic stairs and whole house fans. Attic stairs can be insulated, generally by having some batt insulation that can be pulled over opening prior to closing stairs, quite a contraption though. If by whole house fan you mean a fan that pulls air out of the entire house, this should have a grill that can be closed when not in use. This would be different than an attic fan which circulates hot air out of the attic during the summer.