I didn't Philippines weather is getting that cold. I don't know your condition, but many thing can be use 1) Electricity available: heat from light bulb, oil heater, other produce heat source can be use 2) No electricity: heat rock/brick, boiling water and put big metal pot/barrel can be use or just dig a large hole few meters away (add rock/brick to retain heat) down wind and use metal pip to route heat to the coop. To avoid smoke, the pipe must be lowest point as possible and put small hole screen to block any type object pass through beside heat. The heat source scrap (hay, straw, small branch, leaves, etc...) can be use. The other no heat source can use is the big pile of wet grass, mulch. wood chips. Unfortunately, this method require few weeks of decompose to produce heat which may not be valid. I am using deep litter method to keep the coop warm during cold season.