Bousfield-Kan completion of homotopy limits

We consider the commutation of R_∞, the Bousfield-Kan R-completion functor, with homotopy (inverse) limits over categories I with compact classifying spaces. We get a generalization of the usual fibre lemma regarding preservation of a fibration sequence by R_∞. The basic result is that for such I-diagrams N of nilpotent spaces the canonical commutation mapR_∞holim_I N → c holim_I R_∞N is always a covering projection. This has clear implications for Sullivan-Quillen localization and completion theory and for rational models. On the way we are lead to a sufficient condition for the homotopy limit over a finite diagram to be non-empty or in fact r-connected for a given r≥-1.