Does the R value of blown insulation reduce if the insulation is compressed. The reason I ask is I have a section in my attic which is over a room and therefore has blown in insulation. If I place attic flooring (just a 4x8 sheet of wood) over it does it reduce the effectiveness. I know they measure the r value by inches so it would go from lets say 18" now to maybe 12".. Does it really even matter?

Yes it matters a great deal. Compressed insulation has a lower rating. You basically trap air inside insulation, the trapped air is what does the work. If you compress the blown in from 18 to 12" you will loose nearly 33% of the R valve but a small amount rreturns if you plywood flooring is tight and seals in some air loose.

Thanks for the feedback. I will not floor that section of the attic.