Can I add fiberglass insulation over rock wool in an attic?

We recently bought a 160-year-old farmhouse that has 2inches of rock wool between the joists in the attic. We would like to add fiberglass batts to increase the R-value. Can we simply lay the batts over the rock wool? There is not a vapor barrier, and we have been advised not to install one as our home does not have an air-exchanger (it is old and drafty--that IS our air exchange!). Or, do we need to remove the rock wool, put down faced batts first and unfaced over that? We really would prefer to leave the rock wool in place, but we do not want to have a moisture disaster in our house. Thanks for any advice!
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Yep, just Batt right over the rock wool, un-faced. The drafts or most of them aren't coming from the ceiling anyway. However, you do want to thoroughly inspect the attic and block-off any openings, holes or gaps to stop existing hot and cold columns of air within the lower walls. These aren't breathing leaks and only efficiency and comfort leaks.
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