I agree with Rbarata on the heat mat. My phals that are in the window set up are all on a heat mat, and it really makes a difference when the plants are in an area of less than ideal temps. Warmth at the roots is important with these plants.

15C is probably the minimum you can get by with, and only at night. The next day should be warmer if nights are this cold. Phals are warm-growing tropical plants.

Especially when they're grown cold, it is better to underwater than overwater them. At temperatures like this, they hardly grow, so they don't need much water. It is best to let the pot completely dry between waterings. Elaborating on what jcec1 wrote, a pot of that size, at those temperatures, might take a month to dry out.

The quickest fix for you might be getting an electric reptile terrarium heat mat, or a gardener's heat mat, and putting that under your plant. Take care to check the temperature before putting your plant on the mat so you don't cook it.