diff --git a/README.md b/README.md
index 65222b2..39be971 100644
--- a/README.md
+++ b/README.md
@@ -112,8 +112,9 @@ gpu-turnstile.exe --remove-service
 ```
 
 Layout: `C:\Program Files\gpu-turnstile\` holds the exe and
-`gpu-turnstile.env`, logs go to `C:\ProgramData\gpu-turnstile\` (set
-`LOG_FILE` in the env file — there is no console). The service always runs
+`gpu-turnstile.env`, logs go to `C:\ProgramData\gpu-turnstile\` — the
+installer sets `LOG_FILE` in the env file by default (there is no console;
+your own `LOG_FILE` setting is kept). The service always runs
 as the virtual account `NT SERVICE\gpu-turnstile` (low-privilege,
 per-service, no password); the installer automatically grants it write
 access to the install and data directories — nothing else to do.
diff --git a/SPEC.md b/SPEC.md
index c57f8e4..442cda2 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -186,8 +186,13 @@ By default install creates the canonical layout and copies the binary into
 it (Windows: `%ProgramFiles%\gpu-turnstile\`, plus
 `%ProgramData%\gpu-turnstile\` for logs; Linux: `/var/lib/gpu-turnstile/`
 with the config at `/etc/gpu-turnstile.env`). An existing config in the
-target location is never overwritten. `--no-copy` registers the current
-executable location as-is instead.
+target location is never overwritten. On Windows the install also makes
+sure the env file sets `LOG_FILE` to
+`%ProgramData%\gpu-turnstile\gpu-turnstile.log` (appended only when no
+`LOG_FILE=` line exists) since a service has no console; on Linux logs go
+to the journal via stderr, so no default is set there. `--no-copy`
+registers the current executable location as-is and leaves the config
+untouched.
 
 ### Windows
 
@@ -195,8 +200,9 @@ executable location as-is instead.
   `%ProgramData%\gpu-turnstile\`, copies the exe and (if none exists there
   yet) the `gpu-turnstile.env` into the Program Files directory, and
   registers that copy as a Windows service; recovery actions restart it
-  5 s after any failure. Logs go to the ProgramData directory via
-  `LOG_FILE` since there is no console.
+  5 s after any failure. The install ensures the env file sets `LOG_FILE`
+  to `%ProgramData%\gpu-turnstile\gpu-turnstile.log` since there is no
+  console — an existing `LOG_FILE` setting is kept.
 - **Account**: the service always runs as the virtual account
   `NT SERVICE\gpu-turnstile` — a per-service low-privilege identity the
   SCM manages (no password, automatic logon-as-a-service right, no admin
diff --git a/internal/service/service_windows.go b/internal/service/service_windows.go
index 6da86b9..c8dea52 100644
--- a/internal/service/service_windows.go
+++ b/internal/service/service_windows.go
@@ -152,25 +152,33 @@ func Install(configPath string, copyBin bool) error {
 		}
 	}
 
-	installDir, _ := installDirs()
-	if copyBin && !strings.EqualFold(filepath.Dir(exe), installDir) {
-		if err := os.MkdirAll(installDir, 0o755); err != nil {
-			return fmt.Errorf("create %s: %w", installDir, err)
-		}
-		installedExe := filepath.Join(installDir, "gpu-turnstile.exe")
-		if same, _ := sameFileContent(exe, installedExe); !same {
-			if err := copyFile(exe, installedExe); err != nil {
-				return fmt.Errorf("copy binary to %s: %w", installedExe, err)
-			}
-		}
-		exe = installedExe
+	installDir, dataDir := installDirs()
+	if copyBin {
 		targetCfg := filepath.Join(installDir, "gpu-turnstile.env")
-		if configPath != "" && !strings.EqualFold(configPath, targetCfg) {
-			if _, statErr := os.Stat(targetCfg); os.IsNotExist(statErr) {
-				copyFile(configPath, targetCfg) //nolint:errcheck // best effort
+		if !strings.EqualFold(filepath.Dir(exe), installDir) {
+			if err := os.MkdirAll(installDir, 0o755); err != nil {
+				return fmt.Errorf("create %s: %w", installDir, err)
+			}
+			installedExe := filepath.Join(installDir, "gpu-turnstile.exe")
+			if same, _ := sameFileContent(exe, installedExe); !same {
+				if err := copyFile(exe, installedExe); err != nil {
+					return fmt.Errorf("copy binary to %s: %w", installedExe, err)
+				}
+			}
+			exe = installedExe
+			if configPath != "" && !strings.EqualFold(configPath, targetCfg) {
+				if _, statErr := os.Stat(targetCfg); os.IsNotExist(statErr) {
+					copyFile(configPath, targetCfg) //nolint:errcheck // best effort
+				}
 			}
-			configPath = targetCfg
 		}
+		// A service has no console: without LOG_FILE the output vanishes, so
+		// the installed env file defaults to logging into ProgramData. An
+		// existing LOG_FILE setting is left alone.
+		if err := ensureLogFile(targetCfg, filepath.Join(dataDir, "gpu-turnstile.log")); err != nil {
+			return err
+		}
+		configPath = targetCfg
 	}
 	binPath := fmt.Sprintf(`"%s" -config "%s"`, exe, configPath)
 
@@ -304,6 +312,30 @@ func sameFileContent(a, b string) (bool, error) {
 	return bytes.Equal(ba, bb), nil
 }
 
+// ensureLogFile makes sure the env file at path sets LOG_FILE, creating
+// the file or appending the line as needed. An existing LOG_FILE= line
+// (even an empty one) is respected and left untouched.
+func ensureLogFile(path, logPath string) error {
+	data, err := os.ReadFile(path)
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("read %s: %w", path, err)
+	}
+	for _, line := range strings.Split(string(data), "\n") {
+		if strings.HasPrefix(strings.TrimSpace(line), "LOG_FILE=") {
+			return nil
+		}
+	}
+	s := string(data)
+	if s != "" && !strings.HasSuffix(s, "\n") {
+		s += "\n"
+	}
+	s += "LOG_FILE=" + logPath + "\n"
+	if err := os.WriteFile(path, []byte(s), 0o644); err != nil {
+		return fmt.Errorf("write %s: %w", path, err)
+	}
+	return nil
+}
+
 // copyFile copies src to dst (0755 on the new file).
 func copyFile(src, dst string) error {
 	in, err := os.Open(src)