Drafty Nest Box Design?

My father-in-law makes things out of wood to sell for charity. He's been making nest boxes recently but I'm thinking his design will not be good for the birds. Along the side, he's left about a 8mm gap between the side wall and the roof. Although the rain won't get in, I'm thinking there'll still be a fair draft! Before he starts making an industry out of it, I'd just like some advice on whether he needs to make sure that there is no gap there. 

  

Thanks very much

David