loft insulation

Hi my loft is insulated with what appears to be shredded glass fibre which seems to have shrunk over the 14 years it has been there. It was a new built house in Dec 1991. It has also been floored with tongue and groove chipboard i.e the same stuff the floors are made of. Would there be any point in lifting up the loft flooring and putting a layer of 60mm (or more or less) glass fibre insulation on top of it? The joists are about 84mm deep. And if so where is the cheapest place to buy that stuff?
I don't know if it me and my wife getting older but we seem to need the thermostat set slightly higher than when we first moved into the house that is why I was looking at the loft.
Thanks Ron.
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