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We purchased a house with an insufficiently insulated attic. There's currently about 2-3 inches of loose-fill fiberglass insulation in between the 2x6 joists. I'd love to just roll unfaced R-30 fiberglass batts across the joists (simple, no equipment rental, very little mess), but that would leave a 2 to 3 inch air pocket between the bottom of the batts and top of the loose fill insulation. Is this a bad idea?

(We have no current problems with mold, high humidity, etc. I'd rather not create any. I don't understand the physics of this, whether it would cause problems.)

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Yes, you can do this, but...

Any gaps between the layers effectively negates the benefit of the upper layer. If unconditioned air is allowed to flow between the layers, the upper/outer layer does almost nothing. By putting the layers in contact with one another, the negative effect of small gaps is minimal.

I'd fill the cavities with R-11 and run R-19 crossways on top of that. Problem solved for minimal extra expense.

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in actual fact, that trapped air between the old insulation and the new will work as a dead air space which helps lower your passive thermal emissivity. the real crus of the matter would be can you roll the insulation out in such a way that you can ensure contiguous installation ie. there can be no gaps between the insulation from batt to batt or roll to roll. insulation is usually installed between the roof joists to ensure a constant insulation blanket (high r value glass fibre, low r roof joist, over and over and over). when you go over the joists longitudinally or transversely, you have to notch around all girders, webs, collar ties, etc. to make sure you have no air gaps in the insulation field.

mold and humidity issues usually come from non-intact vapour barrier, poor attic and eave ventilation or blockage of the transition space between attic and eaves. you wont create any new issues as long as you don't block the eaves off with the new insulation (just use eave chutes or venting profiles)

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