diff --git a/lua/pl/app.lua b/lua/pl/app.lua
index 736e166e..bfe69793 100644
--- a/lua/pl/app.lua
+++ b/lua/pl/app.lua
@@ -10,6 +10,18 @@ local path = require 'pl.path'
 
 local app = {}
 
+
+--- Sets/clears an environment variable default, to use with `utils.getenv`.
+-- This links to `utils.setenv_default`.
+-- @function setenv_default
+-- @usage -- override the global with this implementation to control
+-- -- environment variables with defaults on application level.
+-- os.getenv = require("pl.utils").getenv
+--
+-- utils.setenv_default("MY_APP_CONFIG", "~/.my_app")
+app.setenv_default = utils.setenv_default
+
+
 --- return the name of the current script running.
 -- The name will be the name as passed on the command line
 -- @return string filename
@@ -69,8 +81,8 @@ function app.require_here (base, nofollow)
 end
 
 --- return a suitable path for files private to this application.
--- These will look like '~/.SNAME/file', with '~' as with expanduser and
--- SNAME is the name of the script without .lua extension.
+-- These will look like `'~/.SNAME/file'`, with '~' expanded through `path.expanduser` and
+-- `SNAME` is the filename of the script without `'.lua'` extension (see `script_name`).
 -- If the directory does not exist, it will be created.
 -- @string file a filename (w/out path)
 -- @return a full pathname, or nil
@@ -78,8 +90,8 @@ end
 -- @usage
 -- -- when run from a script called 'testapp' (on Windows):
 -- local app = require 'pl.app'
--- print(app.appfile 'test.txt')
--- -- C:\Documents and Settings\steve\.testapp\test.txt
+-- print(app.appfile 'some\test.txt')
+-- -- C:\Documents and Settings\steve\.testapp\some\test.txt
 function app.appfile(file)
     local sfullname, err = app.script_name()
     if not sfullname then return utils.raise(err) end
@@ -94,7 +106,9 @@ function app.appfile(file)
 end
 
 --- return string indicating operating system.
--- @return 'Windows','OSX' or whatever uname returns (e.g. 'Linux')
+-- @treturn[1] string 'Windows' on Windows platforms
+-- @treturn[2] string 'OSX' on Apple platforms
+-- @treturn[3] string whatever `uname` returns on other platforms (e.g. 'Linux')
 function app.platform()
     if path.is_windows then
         return 'Windows'
diff --git a/lua/pl/path.lua b/lua/pl/path.lua
index 600bf057..a890f9b6 100644
--- a/lua/pl/path.lua
+++ b/lua/pl/path.lua
@@ -14,11 +14,11 @@
 -- imports and locals
 local _G = _G
 local sub = string.sub
-local getenv = os.getenv
 local tmpnam = os.tmpname
 local package = package
 local append, concat, remove = table.insert, table.concat, table.remove
 local utils = require 'pl.utils'
+local getenv = utils.getenv
 local assert_string,raise = utils.assert_string,utils.raise
 
 local res,lfs = _G.pcall(_G.require,'lfs')
diff --git a/lua/pl/utils.lua b/lua/pl/utils.lua
index 7c641035..8636cfba 100644
--- a/lua/pl/utils.lua
+++ b/lua/pl/utils.lua
@@ -585,6 +585,49 @@ end
 --- OS functions
 -- @section OS-functions
 
+do
+  local env_defaults = {} -- table to track defaults for env variables
+  local original_getenv = os.getenv -- in case the global function gets patched with this implementation
+
+
+  --- Gets an environment variable, whilst falling back to defaults. The defaults can
+  -- be set using `utils.setenv`. All Penlight modules will use this method to retrieve
+  -- environment variables.
+  -- The name is case-sensitive, except on Windows.
+  -- @tparam string name the environment variable to lookup.
+  -- @treturn[1] string the value retrieved
+  -- @treturn[2] nil if the variables wasn't found, and didn't have a default set
+  -- @see utils.setenv_default
+  -- @see app.setenv_default
+  function utils.getenv(name)
+    utils.assert_string(1, name)
+    name = is_windows and name:lower() or name
+    return original_getenv(name) or env_defaults[name]
+  end
+
+
+
+  --- Sets/clears an environment variable default, to use with `utils.getenv`.
+  -- The name is case-sensitive, except on Windows.
+  -- @tparam string name the environment variable name to set a default for.
+  -- @tparam[opt] string value the value to assign as a default, if `nil` the default will be cleared.
+  -- @return nothing
+  -- @see utils.getenv
+  -- @see app.setenv_default
+  function utils.setenv_default(name, value)
+    utils.assert_string(1, name)
+    name = is_windows and name:lower() or name
+    if value == nil then
+      env_defaults[name] = nil
+    else
+      utils.assert_string(1, value)
+      env_defaults[name] = value
+    end
+  end
+end
+
+
+
 --- execute a shell command and return the output.
 -- This function redirects the output to tempfiles and returns the content of those files.
 -- @param cmd a shell command
@@ -598,8 +641,8 @@ function utils.executeex(cmd, bin)
     local errfile = os.tmpname()
 
     if is_windows and not outfile:find(':') then
-        outfile = os.getenv('TEMP')..outfile
-        errfile = os.getenv('TEMP')..errfile
+        outfile = utils.getenv('TEMP')..outfile
+        errfile = utils.getenv('TEMP')..errfile
     end
     cmd = cmd .. " > " .. utils.quote_arg(outfile) .. " 2> " .. utils.quote_arg(errfile)