Separation Logic (SL) provides a simple but powerful technique for reasoning about imperative programs that use shared data structures. Unfortunately, SL supports only "strong updates", in which mutation to a heap location is safe only if a unique reference is owned. This limits the applicability of SL when reasoning about the interaction between many high-level languages (e.g., ML, Java, C#) and low-level ones since these high-level languages do not support strong updates. Instead, they adopt the discipline of "weak updates", in which there is a global "heap type" to enforce the invariant of type-preserving heap updates. We present SL w, a logic that extends SL with reference types and elegantly reasons about the interaction between strong and weak updates. We also describe a semantic framework for reference types; this framework is used to prove the soundness of SL w.