Time of year and Region of Nepal you are volunteering matters much. The climatic season of Nepal is divided into major four groups. Nepal’s weather in general is pleasant. The months March to May fall under spring, June to August as summer, September to November as autumn and December to February as winter. The monsoon starts at June 10 and onsets around September 23. Spring and autumn are pleasant months to travel Nepal but tourists come Nepal all round the year. In the winter there will be fall of temperature to freezing point with a lot of snowfall.
The maximum and minimum temperature in Terai region of Nepal in winter will be 7degree Celsius to 23 degree Celsius. The central valley gets temperature of freezing point to twelve degree Celsius maximum. In Kathmandu valley whose altitude is 1360m has 19-27 degree Celsius in summer and 2-20 degree Celsius in winter. The average summer temperature of hill region is 28 degree Celsius, terai 40 degree Celsius.
In Nepali houses in general there will be no heating system. The warm clothes are only way to fight against cold so do bring warm jackets, globes, socks if you are traveling Nepal during winter months. On a clear sunny day it is warm during winter too. You will be provided warm blanket but if your preference is sleeping bag do bring along with you.
In Nepal here are shop which sell all the goods as jacket, gloves, socks, sleeping bags etc so if you want to buy them here it is appreciable because they are comparatively cheaper here than in the part of your world.