Moisture in the solar oven is something that we all deal with.
Many foods put off moisture when cooked. This moisture evaporates in the dry heat of a regular oven. Or boils away on the stove top. But moisture cannot evaporate or escape from a solar oven. Just like a hot steamy shower mists up the bathroom mirror, the condensation from the steam in a solar oven mists up the glass lid. You can’t see in the mirror. And sunlight cannot enter the solar oven.
Some solar cookers are more prone to this than others.
No Sun means No Heat
Obviously, if sunlight does not enter the solar cooker, it does not heat up. As with any other type of cooker – no fuel means no heat. And in fact, a hot solar oven will begin to lose heat and cool down if deprived of the fuel.
When the moisture in the solar oven causes condensation on the inside of the glass so bad that you can no longer see inside, it is time to wipe it away. However, every time the lid is opened, the box loses heat, sometimes as much as 25°. It is a vicious cycle.
There are several ways to combat the problem of steam.
Ways to Control Moisture in the Solar Oven
Wipe away the moisture
If moisture is beginning to build but the glass is still mostly clean, open the oven door very quickly, for only 2-3 seconds, and let the steam vent out. If moisture covers the entire glass so that you cannot see in and cannot read the thermometer, open the glass very quickly and wipe it away with a cloth or paper towel.
Tight Fitting Lid
The first and foremost way to prevent steam in the oven is to use a pot with a well-fitted lid. Although some foods, such as cakes, cookies, bread, granola, etc., should be cooked uncovered, most food should be cooked with a lid. If needed, metal office binder clips can often be used to hold the lid on tighter. As an added bonus, the lid holds the heat and steam inside the pot so that the food cooks faster.
Exercise caution with tight lids. Do not heat sealed jars, cans, or pots. If the lid is too tight, steam buildup can create a dangerously pressurized cooker. For that reason, do not seal lids and make sure to punch at least two holes in the top of cans and jar lids.
Reduce the Liquid
The next way to lessen – if not prevent – steam is to reduce the amount of water. Decrease the amount of water or liquid by ¼ to ½ cup. Obviously, if you are making soup, this might not help. But if you are steaming vegetables, boiling beans, cooking rice, or making a pot roast, decreasing the liquid will minimize the amount of steam.
Use a Cooking Bag
A third way to control steam is to drape a clean tea towel or dark cloth over the covered pot to absorb some of the steam. Likewise, you could put the pot into a cooking bag before putting it into the solar cooker. Be careful to let the steam vent when opening the bag.
Vent the Oven
The final way to reduce the steam buildup on the glass is to vent the lid. This can be done in several ways. First, one or more of the latches can be left loose so that the oven glass or cover is not tightly sealed. If that isn’t enough, a small stick can be placed between the glass and the top edge of the oven. This creates a gap to allow steam to escape. And yet, be aware, that the oven is also losing heat and might not reach a high temperature.
In view of these facts, there is a clear trade off between continually opening the lid and losing a lot of heat or venting the lid and losing a little heat. In either case, only the solar chef can determine what is best in his or her cooking situation.
Finally, even these measures will sometimes not be enough to control moisture in the solar oven, to keep the lid clear and the steam down. So be sure to wipe away the moisture and dry the cooker before storing it.
A Final Word of Caution About Steam
A lifetime of cooking has taught you how to properly open boiling pots and hot ovens to avoid those nasty steam burns. Do not forget these lessons. Steam produced in the solar oven is no less dangerous and no less painful.
Remember to be careful opening up a hot solar oven filled with steam. Stand away from the solar oven and open up the lid slowly on the opposite side to allow the steam to vent.
Remember to remove the lid from a boiling pot with the lid turned away from your face and hands to direct steam away.
Remember to use pot holders. 
Tell us your best way to control moisture in the solar oven in the comments below.
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