diff --git a/boards/sim/sim/sim/scripts/Make.defs b/boards/sim/sim/sim/scripts/Make.defs
index 22c4835d681..e64b3b5098e 100644
--- a/boards/sim/sim/sim/scripts/Make.defs
+++ b/boards/sim/sim/sim/scripts/Make.defs
@@ -106,6 +106,15 @@ CPPFLAGS := $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) \
 AFLAGS := $(CFLAGS) -D__ASSEMBLY__
 
 ifeq ($(CONFIG_LIBCXX),y)
+  # Why c++14?
+  #  * libcxx seems to require c++11.
+  #  * The compiler defaults varies:
+  #      clang/macOS (from xcode): 199711L
+  #      gcc/ubuntu:               201402L
+  #  * There is a precedent to use c++14.
+  #    (boards/arm/stm32l4/nucleo-l476rg/scripts/Make.defs)
+  CXXFLAGS += -std=c++14
+
   ifeq ($(CONFIG_HOST_MACOS),y)
     # macOS uses libc++abi
     CXXFLAGS += -DLIBCXX_BUILDING_LIBCXXABI