Hi all.. My small, superinsulated house is coming along. today I had the attic blown. I THOUGHT he was going to do cellulose, but it was Johns Manville fluffy white fiberglass. How bad is this? Is there any upside to fiberglass over cellulose? They did 20-24" worth. I'm worried about air infiltration or convection currents, especially in very cold weather. I'm also worried the glass doesn't attenuate sound as well...... What say you...?? Thanks mucho. -michael in WI

You'll get a number of opinions on cellulose vs. fiberglass. I'd bet most on this site would favor cellulose. It has beneficial properties on sound as well as stopping air movement. I've seen strong cases made for a topping layer of 3" of cellulose over your blown FG. That could prove beneficial in stopping air currents. I do know that cellulose has much higher performance in hot weather. It seems the IR energy penetrates several inches into FG, thus negating the benefits of the first few inches. Dana will probably show up later and quote you gospel chapter and verse.

The only trouble with fiberglass is that it can easily be over blown and installed well short of it's settled density though it meets height requirements.

ANother benefit of 3" overblown cellulose would be to help compact the FG where needed

Guess I am not the only one that has thought of cellulose over fiberglass to help cut down on air infiltration into the fiberglass. I would think wet blown cellulose over the fiberglass would work best. Moisture in the cellulose shouldn't hurt the ceiling assembly if it's performed during the hot summer months as it would dry quickly.

The amount of moisture in damp sprayed cellulose is small enough to install in an attic during any season. It's drying speed is only an issue in wall assemblies where closing it in with drywall too soon (especially if the drywall is immediately painted.) The thicker the wall assembly, the more drying time is prudent. In the summer in warmer more humid climates leaving it open INCREASES the moisture content of the cellulose beyond the amount used for activating the adhesive (!). But for overtopping fiberglass in a vented attic, forget about it- it's not a problem.

It only takes 3" of ~1.5lb density overtopping (about R11) of cellulose to fix any air-retardency related performance issues of the fiberglass. The air retardency of some of the newer fiberglass products is far less of a problem than 20 years ago. If the stuff appears to be clumps or flakes of higher density fiberglass overtopping it with cellulose isn't necessarily going to be worth it. But if it's a more uniform fluff of unknown density, a 3" overtopping of cellulose would fix it.

You asked if there is any benefit to fiberglass over cellulose. In new construction, not much, but in retrofits, its lighter weight can be an advantage. Old thin drywall ceilings might sag a bit under the weight of R60 cellulose. You could consider that as one more reason to put some cellulose on top of your fiberglass--with the light weight fiberglass underneath, you won't be putting much weight on your ceiling even if you put more than the recommended 3" of cellulose on top.