A coldframe, especially in New Mexico, must be ventilated on sunny days. Even when the temperature is below freezing outside, the temperature inside a cold frame can climb quite high once the sun hits it. Place a thermometer inside the coldframe and open the lid when the temperature climbs to 70 degrees or so. If it is freezing outside, be careful not to open the lid too much. If it is cloudy, windy, or a cold front is expected, be careful to monitor the temperature after ventilating the coldframe.
Some gardeners place plastic jugs filled with water in the coldframe between the plants.
These water-filled jugs will absorb heat and slow the rate of temperature increase in the cold frame. At night or when a cold front passes, they will release the heat, protecting the plants. Painting the jugs black will increase their ability to absorb the heat in sunlight. Some of these operate by expanding a gas in a cylinder as they warm and require no electricity. As the cool, the gas contracts and closes the vent.