Why You Should Never Cover Your Oven With Foil
Aluminum foil is a very useful, flat and pliable material, ideal for making improvised grills, pie crust protection and simple food wrappers (for non-acidic foods). Lining the pans with foil can help speed up the cleaning process – just toss the foil in the trash – but there is one thing in your kitchen that you should never foil on, no matter how dirty you think it might be: your oven floor closet.
Even Reynolds (also known as Big Aluminum) sees this as a bad idea and discourages such activity on their website:
To avoid possible heat damage to the oven, we do not recommend using aluminum foil to wash the bottom of the oven. Instead, we recommend that you place a sheet of sturdy aluminum foil on the oven rack under the pie or casserole you are baking. The foil should be just a few inches larger than the baking dish to ensure proper heat circulation. The foil will catch all drops before they reach the bottom of the oven.
It would be unwise to ignore this advice. Placing foil on the bottom of the oven not only blocks proper heat circulation, but it can reflect heat back to the heating elements, potentially damaging them. If you have a gas oven, the foil can block or otherwise interfere with the flame.
If drips and the like bother you, you can place a sheet of sturdy foil directly on the wire rack under a simmering casserole or stew. You can also use a baking sheet (although you will have to empty the baking sheet). Whichever drip tray you choose, just make sure it doesn’t take up the entire grate – you need hot air to flow around your food in order to cook it, which, after all, is the whole point of the oven. (If drops get on the foil, you can always clean the oven .)