August is deepest winter in Namibia it can get below 0°C at night, after sunset it will get cold.
Even if you stay in lodges, most of them don't have central heating or any heating at all in the rooms, you get a bed bottle to help you, inside where you have dinner there often is a fire burning, grilling the part of your body which is turned to it, not too much help heating the whole room.
Lots of people still do camp in Namibia during August with the right cloth and a very good sleeping bag, the camp fire will keep you warm, I would plan to have dinner shortly before sunset so you can cuddle up to the fire after that, check with the camp site when they heat up the donkey so you can have a hot shower.
In Windhoek you're around 1675 m above sea level, Swakopmund was pretty cold in May but then the sea might even that out during winter, so bring hats, scarfes, fleece or wool cloth, ski underwear and you should be fine, during the day it will be warm enough.