Make your barracks out of thread so it will be pure. If you build it out of clay or bricks or wood, it will inevitably have some dirt in it. Build it out of thread to keep it pure of... well, impurities.
This pure thread barracks can only be used in June (じゅん), unfortunately. Any other time of year there will be snow and rain and mud and that will get tracked into the pure barracks and ruin them. But June is the cleanest, most pure month, so you can use it then.