To prevent freezing in an unheated greenhouse, focus on maximizing heat retention and minimizing heat loss. Start by insulating the structure with materials like bubble wrap or double glazing for the windows. This helps trap heat inside and reduces the likelihood of freezing temperatures. You can also incorporate thermal mass, such as water barrels or large stones, which absorb heat during the day and release it at night.
Positioning the greenhouse with the longest side facing south allows for better sunlight exposure, warming the interior during the day. During cold nights, using row covers or blankets over plants can provide an extra layer of protection. For extra warmth, place cloches or cold frames around more vulnerable plants. Keep the greenhouse well-sealed, ensuring doors and vents are tightly closed during freezing conditions to prevent drafts.
