Trade tip: Working with Oval galvanized duct work. Trying to get a 5" round duct through a 2x4 may seem impossible without having to compress it and jam it in. It's also a pain in the ass for the drywaller if it bulges out of the stud cavity. The 5" oval duct is 3"x6-1/2" and it fits perfectly between a 2x4 cavity without compromising the air flow volume. Look at the way a 90 degree connection can be used & makes for a seamless connector from the oval to the round duct.