diff --git a/nixos/modules/installer/cd-dvd/iso-image.nix b/nixos/modules/installer/cd-dvd/iso-image.nix
index c2836b5a9a1..ef618d102a7 100644
--- a/nixos/modules/installer/cd-dvd/iso-image.nix
+++ b/nixos/modules/installer/cd-dvd/iso-image.nix
@@ -644,7 +644,11 @@ in
       ];
 
     fileSystems."/" =
-      { fsType = "tmpfs";
+      # This module is often over-layed onto an existing host config
+      # that defines `/`. We use mkOverride 60 to override standard
+      # values, but at the same time leave room for mkForce values
+      # targeted at the image build.
+      { fsType = mkOverride 60 "tmpfs";
         options = [ "mode=0755" ];
       };
 
diff --git a/nixos/modules/installer/netboot/netboot.nix b/nixos/modules/installer/netboot/netboot.nix
index fa074fdfcc6..d3a61436093 100644
--- a/nixos/modules/installer/netboot/netboot.nix
+++ b/nixos/modules/installer/netboot/netboot.nix
@@ -30,7 +30,11 @@ with lib;
           else [ pkgs.grub2 pkgs.syslinux ]);
 
     fileSystems."/" =
-      { fsType = "tmpfs";
+      # This module is often over-layed onto an existing host config
+      # that defines `/`. We use mkOverride 60 to override standard
+      # values, but at the same time leave room for mkForce values
+      # targeted at the image build.
+      { fsType = mkOverride 60 "tmpfs";
         options = [ "mode=0755" ];
       };