diff --git a/include/qbsp/qbsp.hh b/include/qbsp/qbsp.hh
index 09aaf02d..c4820576 100644
--- a/include/qbsp/qbsp.hh
+++ b/include/qbsp/qbsp.hh
@@ -115,6 +115,7 @@
 #define CFLAGS_STRUCTURAL_COVERED_BY_DETAIL (1U << 0)
 #define CFLAGS_WAS_ILLUSIONARY           (1U << 1) /* was illusionary, got changed to something else */
 #define CFLAGS_DETAIL_WALL  (1U << 2) /* don't clip world for func_detail_wall entities */
+#define CFLAGS_BMODEL_MIRROR_INSIDE		 (1U << 3) /* set "_mirrorinside" "1" on a bmodel to mirror faces for when the player is inside. */
 
 // Texture flags. Only TEX_SPECIAL is written to the .bsp.
 // Extended flags are written to a .texinfo file and read by the light tool
diff --git a/man/qbsp.1 b/man/qbsp.1
index db7cf4fc..8fbc8d2b 100644
--- a/man/qbsp.1
+++ b/man/qbsp.1
@@ -239,6 +239,9 @@ Similar to func_detail_wall except it's suitable for fence textures, never clips
 .IP "\fB""_lmscale"" ""n""\fP"
 Generates an LMSHIFT bspx lump for use by a light util. Note that both scaled and unscaled lighting will normally be used.
 
+.IP "\fB""_mirrorinside"" ""n""\fP"
+Set to 1 to save mirrored inside faces for bmodels, so when the player view is inside the bmodel, they will still see the faces. (e.g. for func_water, or func_illusionary)
+
 .SH MAP COMPATIBILITY
 In addition to standard Quake 1 .map files, TyrUtils QBSP is compatible with:
 
diff --git a/qbsp/brush.cc b/qbsp/brush.cc
index ff92375f..06fbf2d9 100644
--- a/qbsp/brush.cc
+++ b/qbsp/brush.cc
@@ -1118,6 +1118,11 @@ Brush_LoadEntity(mapentity_t *dst, const mapentity_t *src, const int hullnum)
         i /= 2;
     }
     
+    /* _mirrorinside key (for func_water etc.) */
+    if (atoi(ValueForKey(src, "_mirrorinside"))) {
+        cflags |= CFLAGS_BMODEL_MIRROR_INSIDE;
+    }
+    
     for (i = 0; i < src->nummapbrushes; i++, mapbrush++) {
         mapbrush = &src->mapbrush(i);
         contents = Brush_GetContents(mapbrush);
diff --git a/qbsp/csg4.cc b/qbsp/csg4.cc
index 913ce7a8..85a65946 100644
--- a/qbsp/csg4.cc
+++ b/qbsp/csg4.cc
@@ -356,7 +356,11 @@ SaveFacesToPlaneList(face_t *facelist, bool mirror, std::map<int, face_t *> &pla
                 || face->contents[1] == CONTENTS_DETAIL_FENCE
                 || (face->cflags[1] & CFLAGS_WAS_ILLUSIONARY)
                 || face->contents[1] == CONTENTS_SOLID) {
-                newface->texinfo = MakeSkipTexinfo();
+                
+                // if CFLAGS_BMODEL_MIRROR_INSIDE is set, never change to skip
+                if (!(face->cflags[1] & CFLAGS_BMODEL_MIRROR_INSIDE)) {
+                	newface->texinfo = MakeSkipTexinfo();
+                }
             }
             
             for (int i = 0; i < face->w.numpoints; i++)