The best way to insulate an urban planter box for winter vegetable growing is to use a layered approach rather than a single wrap. For most city gardeners, the most reliable setup is to insulate the sides with 1–2 inch rigid foam board, raise the planter off cold pavement, mulch the soil surface heavily, and cover the crops with a low hoop tunnel or cold frame. This protects roots from freezing and freeze-thaw cycles while still allowing drainage and ventilation.
Start with drainage. Winter wetness kills more container vegetables than cold alone. Check that drainage holes are open and that the box sits on feet, bricks, or a pallet so water can escape and the base is not in direct contact with frozen concrete or stone. If the planter is on a balcony, move it against a wall or into a sheltered corner, but do not block drainage.
For side insulation, cut rigid foam board to fit the outside faces of the planter. XPS foam is a good choice. Hold it in place with straps, battens, or removable wooden frames, and tape the seams. Insulating the outside keeps soil volume intact. If you must insulate inside, line only the sides, not the bottom, and leave the drainage holes clear. Expect to lose some growing space and water more carefully. Bubble wrap alone is not enough in hard frost, though it can be added as a windbreak.
Next, insulate the top. Lay a thick layer of straw, shredded leaves, or wood chips over the soil, but keep mulch a little away from plant stems. Then set hoops over the box and cover them with row cover fabric or clear greenhouse plastic. A cold frame lid can work, too. The cover should be vented on mild or sunny days to prevent condensation, disease, and overheating.
Add thermal mass if possible. Sealed water containers placed among the plants absorb heat during the day and release it at night. Choose cold-hardy crops such as kale, spinach, carrots, leeks, mâche, and claytonia. In very cold or windy cities, add a windbreak around the planter and use two layers of row cover on the coldest nights. The goal is not to keep the box warm like a house; it is to keep soil temperatures steadier, protect plants from wind, and prevent repeated freezing and thawing.