diff --git a/Library/Homebrew/brew.sh b/Library/Homebrew/brew.sh
index 1f27b455ef..be9fd12085 100644
--- a/Library/Homebrew/brew.sh
+++ b/Library/Homebrew/brew.sh
@@ -104,6 +104,10 @@ HOMEBREW_CASKROOM="${HOMEBREW_PREFIX}/Caskroom"
 HOMEBREW_CACHE="${HOMEBREW_CACHE:-${HOMEBREW_DEFAULT_CACHE}}"
 HOMEBREW_LOGS="${HOMEBREW_LOGS:-${HOMEBREW_DEFAULT_LOGS}}"
 HOMEBREW_TEMP="${HOMEBREW_TEMP:-${HOMEBREW_DEFAULT_TEMP}}"
+if [[ ! -w "${HOMEBREW_TEMP}" ]]
+then
+  HOMEBREW_TEMP="${HOMEBREW_DEFAULT_TEMP}"
+fi
 
 # commands that take a single or no arguments.
 # HOMEBREW_LIBRARY set by bin/brew
diff --git a/Library/Homebrew/bundle/commands/exec.rb b/Library/Homebrew/bundle/commands/exec.rb
index b0c00a7a47..e84764383c 100644
--- a/Library/Homebrew/bundle/commands/exec.rb
+++ b/Library/Homebrew/bundle/commands/exec.rb
@@ -111,6 +111,14 @@ module Homebrew
           # For commands which aren't either absolute or relative
           raise "command was not found in your PATH: #{command}" if command.exclude?("/") && which(command).nil?
 
+          %w[HOMEBREW_TEMP TMPDIR HOMEBREW_TMPDIR].each do |var|
+            value = ENV.fetch(var, nil)
+            next if value.blank?
+            next if File.writable?(value)
+
+            ENV.delete(var)
+          end
+
           if subcommand == "env"
             ENV.sort.each do |key, value|
               # No need to export empty values.