Don't know for sure, but I've never heard of a max limit.
You do need airflow in your attic space though - cut that and you risk mold and rot. So that's a limiting factor of sorts right there.
But I think there's the rule of diminishing returns at play with insulation.
r120 would be twice as good as r60 but the difference would probably only save you about 15 cents a year in energy costs.
Keep in mind too that - depending on the materials used - if you stack insulation too high, it's going to start crushing the airspace out of the material at the bottom - lowering the rvalue.
In other words if you lay a strip of r 20 fiberglass insulation on top of another - it doesn't give you r40. It'll certainly be BETTER than r20 - but it may not be as good as you think because the lower layer is squeezed more than it should be.
And on top of that, as somebody else mentioned, the weight between joists directly on your ceilings is a serious concern. Put too much insulation and over time you could get some bowing and cracking in your ceilings.
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Today's Featured Article - Restoring a John Deere 2010 Diesel Tractor - by Jim Nielsen. Following seven years working in California's Silicon Valley, my wife, baby son and I moved back to Australia to retire. We bought a small 'farm' of about 50 acres near Bendigo, in the state of Victoria. I soon found that it would be very useful to have a tractor around the place for things such as grading our long drive and brush-hogging the fields. I was also embarking on planting 1000 eucalyptus trees, and hence I would need a ripper, small disk plow, sprayer etc. to get these things accompli
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