I birth somewhere between 150 and 250 rats a month. Now that small potatoes compared to others I realize. However, I can tell ya the key to rat breeding is being clean. Changing the soiled bedding and keeping fresh water and fresh food, ventilation and air circulation is a major part. Not only does it help keep them cool but it helps them get fresh air and in turn you don't get that stale pee smell you get sometimes with rats.

If they are kept in a rack for any length of time hair, dust, dander, little food bit's will start to just collect everywhere, Shop vac's are a huge help to keeping the tracks of the rack clean and free for debris. This also lends to healthy rats as they aren't living in a dust caked environment.

I house my rats in home made racks, with 90 breeders per rack. When I hear people talk about the "SMELL"...a couple of things enter my mind that can help with that
1: your ventilation and air circulation isn't right (keep the air moving over the rack)
2: your not using enough bedding to absorb 7 days of rat pee. (they tromp it down, nest in it ect. In a 7" tub I give at least 4-5" of pine shavings)
3: you have to many in a tub (you can house 8 small rats or 4 jumbos but not both)

Now granted on cleaning day there is a slight smell, is it over powering? No.