Northern fowl mites look similar to red mite except the colour and definitely make you itchy. Whatever creepy-crawly they are louse powder is a good start and will need repeating. I've tried using household flea spray in the chicken house with some effect - you need to keep the chickens away from it as its not good for birds, so when I've done it its been on a nice day first thing in the morning and the chickens are shut out of the house all day - also needs well ventilating. If you can paint the inside of the house with a gloss paint this will help especially if it plugs any crevices and then the little bugs have fewer places to hide and if you use white paint they will be easier to see. It will probably be a battle whatever you do, until the weather cools down sufficiently to reduce their numbers.