Contextual alternate

I have an /i.ss01 which needs to have only half a /space after.

Would this be best done with features or kerning?

At the moment I have calt
sub [i.ss01 space] by [i.ss01 uniE000];

where uniE000 is set as a half space, but all the subsequent spaces are now half spaces too.

Any help would be greatly appreciated.

Your code just replaces all spaces by half spaces. You need to do something like this:

sub i.ss01 space' by uniE000;

One advice on the space thingy: There are many different kind of spaces officially encoded (u+2000—200F) and I think it’s better to use one of them which is thinspace (u+2009), instead of PUA glyph (u+E000).

Thanks @jeremie . This has fixed it.

Updated the glyph code, thanks @Tosche