How would I prove the following: If the normalization of $X$ can be covered by $G$-invariant quasiprojective open subsets, then the spectral sequence $E_{2}^{pq} = H^{p}(BG, IH^{q}(X)) \Rightarrow IH_{G}^{p+q}(X)$ converging to $IH^{*}(E_{n}G \times _X{G})$ degenerates at the $E_2$ term.
This is because of decomposition?