Am I allowed to warm up my food in a non-kosher microwave and how?
1. One option is to use a double wrapping around the food.
The double wrap ensures that no hevel (steam) comes into contact with the non-kosher walls of the microwave and then with the food. The contact of steam with the walls and with the food ‘contaminates’ the food with the taste of non-kosher food. The double-wrap, which ensures that there is no contact, is the solution to this problem.
Note that the wrapping must be leak-proof, such as two Ziploc bags or Saran Wrap.
2. Another option is kashering the microwave oven.
Many microwave ovens without browning elements or convection can be easily kashered: If the walls do not exceed 120 degrees Fahrenheit during cooking, one only needs to clean it well, and no further action is required. To check whether your microwave gets hot, cook a potato in it for about five minutes and then open the door before it has finished its cycle. Then immediately check the temperature of the ceiling and walls. If you can keep your hands there for 15 seconds, the oven walls never reached yad soledes bo and it is sufficient to clean the inside thoroughly with a cleanser so that the surfaces are clean.
If the microwave walls get hot, place a cup or bowl of water inside and boil for at least twenty minutes.
A new glass tray or cardboard must be placed on the bottom (the area that comes into contact with food).