We usually blanket our horses between -30 and -50 degrees and feed them lots, but a good way to judge is wether they are shivering... If they are give them lots of extra hay, if they are still cold blanket them, you can even double blanket them if you have to... It really depends on where you live. I live in alaska so our conditions might be a little bit different than yours.
~He knows when you're happy
He knows when you're comfortable
He knows when you're confident
And he always knows when you have carrots.