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 - Tuning-Up Your Tractor: Plugs & Compression Testing - by Curtis Von Fange. The engine seems to run rough. In the exhaust you can hear an occasion 'poofing' sound like somethings not firing on all cylinders. Under loaded conditions the tractor seems to lack power and it belches black smoke out of the exhaust. For some reason it just doesn't want to start up without cranking and cranking the starter. All these conditions can be signals that your unit is in need of a tune up. Ok, so what is involved in a tune up? You say, swap plugs and file the points....now tha
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