There is a minimum in that the bhikkhu must be properly and modestly dressed, especially in public.

During the cold season in South Asia, the Buddha allowed a double-layered outer robe (sa nghaati) to be used and so — using the Great Standards as a guide — in even colder climates extra layers may be allowable.

In countries where hypothermia may be a danger, the use of extra layers seems sensible — especially if this cuts down on heating and medical expenses.

That a bhikkhu lives as frugally as possible is a major aspect of the Vinaya.

However, it is generally felt very important that the traditional robes remain the basic dress and “extra layers” should not obscure this.

Learn more about the Theravadin Buddhist Monk’s Rules here:

The Bhikkhus’ Rules – Guide for Laypeople