Is there something between singlespace and onehalfspace?

Yes. Using the setspace package with a 10pt document font, a factor between 1.0 and 1.25 would lie "between" \singlespacing and \onehalfspacing. An idea of the appropriate spacing that is dependent on the default font size is available from Why is the linespread factor as it is?.

Here's an example:

\documentclass[10pt]{article}
\usepackage{setspace,lipsum}% http://ctan.org/pkg/{setspace,lipsum}
\begin{document}
\singlespacing\lipsum[2]
\setstretch{1.125}\lipsum[2]% Line spread between \singlespacing and \onehalfspacing
\onehalfspacing\lipsum[2]
\doublespacing\lipsum[2]
\end{document}

Note that lipsum inserts a paragraph break at the end of every \lipsum by default, which is required to have different \linespread across paragraphs in the example.

With the setspace package you can use the spacing environment with an optional argument containing the desired factor like this:

\begin{spacing}{1.25}
...
\end{spacing}