diff --git a/lib/rubygems/installer.rb b/lib/rubygems/installer.rb
index 0c3c28e9af..b4375c3f1c 100644
--- a/lib/rubygems/installer.rb
+++ b/lib/rubygems/installer.rb
@@ -675,16 +675,20 @@ class Gem::Installer
 
     @build_args = options[:build_args]
 
-    @gem_home = @install_dir || Gem.dir
+    @gem_home = @install_dir
 
-    # `--build-root` overrides `--user-install` and auto-user-install
-    if @build_root.nil? && @install_dir.nil?
-      if options[:user_install]
-        @gem_home = Gem.user_dir
-      elsif !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
-        say "Defaulting to user installation because default installation directory (#{Gem.dir}) is not writable."
-        @gem_home = Gem.user_dir
+    unless @gem_home
+      # `--build-root` overrides `--user-install` and auto-user-install
+      if @build_root.nil?
+        if options[:user_install]
+          @gem_home = Gem.user_dir
+        elsif !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
+          say "Defaulting to user installation because default installation directory (#{Gem.dir}) is not writable."
+          @gem_home = Gem.user_dir
+        end
       end
+
+      @gem_home ||= Gem.dir
     end
 
     # If the user has asked for the gem to be installed in a directory that is