Bring pot of water to boil and cook rigatoni according to package directions.
Cut bacon into one inch pieces and place in skillet to brown.
Dice chicken.
When bacon is done, remove from skillet with slotted spoon or spatula and drain on paper towel.
Add diced chicken to bacon fat. Cover and cook until done.
When rigatoni is done cooking, drain. Melt butter in bottom of pan, adding minced garlic.
Add flour to melted butter and stir to make roux. Add milk and ranch dressing to roux and whisk over medium heat until slightly thickened and smooth, about 3 minutes.
Add 1 cup of cheese to white sauce and mix thoroughly. Fold in rigatoni, bacon and diced chicken.
Pour into greased 8×8 pan or equivalent sized casserole. Distribute remaining cup of cheese over top of casserole.
Bake at 350 degrees for 15 minutes, or until cheese is melted and casserole is hot through.