diff --git a/src/plugins/clangcodemodel/clangutils.cpp b/src/plugins/clangcodemodel/clangutils.cpp
index 63f97122462..dea79f66250 100644
--- a/src/plugins/clangcodemodel/clangutils.cpp
+++ b/src/plugins/clangcodemodel/clangutils.cpp
@@ -120,16 +120,20 @@ private:
 
         if (m_projectPart.qtVersion != CppTools::ProjectPart::NoQt) {
             const QString wrappedQtCoreHeaderPath = wrappedQtHeadersPath + "/QtCore";
-            add(includeDirOption() + QDir::toNativeSeparators(wrappedQtHeadersPath));
-            add(includeDirOption() + QDir::toNativeSeparators(wrappedQtCoreHeaderPath));
+            add(includeDirOption());
+            add(QDir::toNativeSeparators(wrappedQtHeadersPath));
+            add(includeDirOption());
+            add(QDir::toNativeSeparators(wrappedQtCoreHeaderPath));
         }
     }
 
     void addDummyUiHeaderOnDiskIncludePath()
     {
         const QString path = ModelManagerSupportClang::instance()->dummyUiHeaderOnDiskDirPath();
-        if (!path.isEmpty())
-            add(includeDirOption() + QDir::toNativeSeparators(path));
+        if (!path.isEmpty()) {
+            add(includeDirOption());
+            add(QDir::toNativeSeparators(path));
+        }
     }
 };
 
diff --git a/src/plugins/cpptools/compileroptionsbuilder.cpp b/src/plugins/cpptools/compileroptionsbuilder.cpp
index 8de175fa169..9f11c02bb95 100644
--- a/src/plugins/cpptools/compileroptionsbuilder.cpp
+++ b/src/plugins/cpptools/compileroptionsbuilder.cpp
@@ -136,7 +136,8 @@ void CompilerOptionsBuilder::addHeaderPathOptions()
             break;
         }
 
-        result.append(prefix + QDir::toNativeSeparators(headerPath.path));
+        result.append(prefix);
+        result.append(QDir::toNativeSeparators(headerPath.path));
     }
 
     m_options.append(result);
@@ -399,7 +400,7 @@ void CompilerOptionsBuilder::addDefineFunctionMacrosMsvc()
 
 QString CompilerOptionsBuilder::includeDirOption() const
 {
-    return QLatin1String("-I");
+    return QLatin1String("-isystem");
 }
 
 QByteArray CompilerOptionsBuilder::macroOption(const ProjectExplorer::Macro &macro) const
@@ -501,7 +502,8 @@ void CompilerOptionsBuilder::addPredefinedHeaderPathsOptions()
 void CompilerOptionsBuilder::addClangIncludeFolder()
 {
     QTC_CHECK(!m_clangVersion.isEmpty());
-    add(includeDirOption() + clangIncludeDirectory());
+    add(includeDirOption());
+    add(clangIncludeDirectory());
 }
 
 void CompilerOptionsBuilder::addProjectConfigFileInclude()