You add them together. But if the first layer is compressed, it reduces its R value. So let’s say you have r-13 fiberglass batt insulation, meant for 3.5” cavity. You shove that into a 2.5 or 3” cavity, and your R value is reduced, maybe to R-10.
So, if you really want to know, you have to measure the depth.
And if you want to know the total envelope, you have to factor in your joists, which pine runs about 1.5/inch. So if you don’t have insulation covering that, your total ceiling value is much much lower. Look up thermal bridging and how that effects your total.
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