diff --git a/lib/mixins/dir-mixin.js b/lib/mixins/dir-mixin.js
index 89522aa9..ea2db1ca 100644
--- a/lib/mixins/dir-mixin.js
+++ b/lib/mixins/dir-mixin.js
@@ -17,6 +17,10 @@ const HOST_DIR_REPLACMENT = ':host([dir="$1"])';
 const EL_DIR = /([\s\w-#\.\[\]\*]*):dir\((ltr|rtl)\)/g;
 const EL_DIR_REPLACMENT = ':host([dir="$2"]) $1';
 
+const DIR_CHECK = /:dir\((?:ltr|rtl)\)/;
+
+const SHIM_SHADOW = Boolean(window['ShadyDOM'] && window['ShadyDOM']['inUse']);
+
 /**
  * @type {!Array<!Polymer_DirMixin>}
  */
@@ -107,7 +111,9 @@ export const DirMixin = dedupingMixin((base) => {
      */
     static _processStyleText(cssText, baseURI) {
       cssText = super._processStyleText(cssText, baseURI);
-      cssText = this._replaceDirInCssText(cssText);
+      if (!SHIM_SHADOW && DIR_CHECK.test(cssText)) {
+        cssText = this._replaceDirInCssText(cssText);
+      }
       return cssText;
     }