I've heard of both double and triple wrapping when it comes to shielding a food item in an oven with regards to Kashrus.
What exactly does double or triple wrapping accomplish that one wrapping can't? Why is there a need for more?
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In Isur Veheter there is a concept: "Ain Taam Yotzei Mchaticha Lchaticha blo Rotev". Taste can't transfer without liquid. So if a slice of bread absorbed meat taste (so there is no actual meat in the bread, just taste), then one puts that (hot) bread on (hot) cheese, the taste cannot go from the bread into the cheese.
Another example of this idea is "Shtei Kdeiros", if two pots, one cooking meat and one cooking milk touch each other on the outside, if there is no liquid there, the pots do not become traif because the taste doesn't transfer.
When one double wraps food, he is creating that situation. Let's say he is cooking food in a traif dirty oven (and there is steam, which halachically is considered like the food itself). The outer layer of foil (or whatever other material one uses) becomes traif. If it was only one layer, that traif taste in the foil would transfer directly to the food, possibly causing the food to become traif. Now, if there are two layers, the taste must go from the outside layer into the inside layer first, before getting transferred into the food. However, because there is no liquid between the two layers, there is no taste transfer and, therefore, the food stays kosher.
The third layer is probably a backup in case the second layer has a hole, gets slightly unwrapped, etc.
Check out this article from the OU, which discusses the different Kosher issues to be aware of when using an oven and why you have to double-wrap food.
Read the article for many nuances not mentioned here, but here's a short rundown.
We'll assume you're baking milk products in a meat oven.
In short. There are three possible areas of concern:
Zeiah (steam): When previously baking an uncovered wet meat dish, the steam may have gotten absorbed in the walls of the oven. Now when you're baking a milk dish, the steam goes from the food to the walls, connecting the two and transferring the taste taste from the walls of the oven into the food. You now have a mixture of milk and meat. Problem. This assumes a clean oven, if the oven is dirty (i.e. has particles of meat in it) this is a stronger problem, since now the dairy steam connects directly to meat particles in the oven.
Reicha (aroma): L'chatchila, we are worried that the aroma from one food may transfer to another food and make a forbidden mixture. If the oven is dirty, the meat food that is sticking to the oven may transfer the aroma into the food, which you are not allowed to do.
Oven racks: The oven racks may be dirty, and when you put the milk pan on the meat residue, you will transfer the taste of the meat into the milk and cause problems. There are those who are lenient in this regard, see the article.
Covering the food when cooking takes care of these issues. With Regards to Steam, and certainly Aroma, a single covering should suffice, but to ensure no leakage, a double covering is recommended.
Even with regards to the oven rack, some authorities maintain that any spilled residue on the rack is halachically inconsequential, since it gets burned up, but since other authorities disagree, it is best to cover the rack (or have a different rack). In lieu of that, it appears covering the food with a double covering should be sufficient.