Not all mesh faces shows properly

Hi i recently began to learn blender and modelling and i did my first house model and wanted to try it out in unity but when i export it well … not all faces are showing and i have no idea why any help ?

here is the blender file:

In edit mode recalculate your normal by pressing cntrl\N , some faces will remain off , just select them and from the shadow’s menu just hit flip direction .