How to Build a Cold Frame from an Old Window, Step by Step

Measure the window before you buy a single board. The glass is the one part you can't resize, so the box gets built around it, and a frame cut to fit a salvaged sash will hold heat and shed rain better than one built from a generic plan. The example below uses a window 36 inches wide and 30 inches tall, a size you'll often find in old storm windows and sash. Swap in your own depth using the table in the sizing section.

What Tools and Materials Do You Need?

You need one old window, a sheet of exterior plywood or an equivalent amount of cedar boards, two hinges, and basic cutting and drilling tools.

Here is the materials list for the 36 by 30 inch example:

• One old window sash or storm window with intact glass and a sound frame
• One 4x8 sheet of 3/4-inch exterior plywood (you'll use well under half of it), or an equivalent amount of 1x12 cedar boards
• 2x2 stock for corner cleats, about 6 feet total
• 1 1/4 to 2 inch exterior screws
• Two 2- to 3-inch butt hinges with screws
• Closed-cell foam weatherstripping, 3/8 inch thick, about 12 feet
• A 1x2 prop stick, 12 to 18 inches long
• A min-max thermometer

For tools, you'll want a circular saw, a drill/driver, a tape measure, a speed square, a level, two clamps, safety glasses, work gloves, and a lead test kit from a hardware store.

We'd pick exterior plywood for a first frame. It's cheap, one sheet covers every panel, and it lasts several seasons if you prime and paint it. Choose cedar boards if you want a frame that holds up longer, but expect to spend more and do more cutting. Materials typically run $50 to $150 when the window is free, and lumber prices drive most of the spread.

Where Should You Put a Cold Frame?

Put it against a south-facing wall, fence, or hedge in the sunniest spot you have, ideally one that gets about six hours of direct sun in late fall and winter.

The lid should face south so the glass catches the low sun, and a wall or fence behind it blocks wind and gives back a little stored warmth at night. Pick ground that drains, because soggy soil under glass invites rot. Keep the frame within hose reach, since you'll be opening, closing, and watering it often.

If you plan to sink the frame a few inches into the ground, call 811 before you dig. The service is free, and utilities usually need a few days to mark their lines. Your USDA hardiness zone tells you typical winter lows, and your state's Cooperative Extension office can give you local frost dates and crop suggestions.

How Do You Size a Cold Frame for a Storm Window?

Make the box's outside dimensions about a half inch smaller than the window on each side, so the glass overhangs slightly and drips clear of the joints.

For a 36 by 30 inch window, that means a box 35 inches wide and 29 inches deep. Set the back about 6 inches taller than the front. That rise over 29 inches puts the lid at about 11 to 12 degrees, enough that rain runs off the front instead of pooling while low sun still hits the glass at a useful angle. A steeper lid catches more winter light but makes a taller box that catches more wind, so a slope of 10 to 15 degrees is a workable range for a salvaged window.

An 18-inch back clears lettuce, spinach, and most young kale. For taller crops, raise both heights by 4 to 6 inches and keep the same rise. Plan on at least 8 to 12 inches of loose soil under the frame so roots have room, and loosen hard ground before you set the box down.

How Do You Cut and Assemble the Box?

Cut two trapezoid side panels, a front board, and a back board, then screw them together over 2x2 corner cleats.

1. Mark the sides. On the plywood, draw a trapezoid 29 inches long, 12 inches tall at one end and 18 inches tall at the other. Use a speed square to keep the 29-inch edge straight, then draw the top edge as a straight line between the two heights.
2. Cut the sides. Wear safety glasses, clamp the sheet, and cut with a circular saw. Use the first panel as a template for the second so they match exactly.
3. Cut the front and back. The front is 12 inches tall and the back is 18 inches tall, both 33.5 inches wide. That width allows for the two 3/4-inch side panels.
4. Add the cleats. Cut four 2x2 cleats to match the front and back heights. Screw one to the inside of each side panel at both ends, set back 3/4 inch from the edge, driving exterior screws from the outside.
5. Assemble and check. Stand the sides up, screw the front and back to the cleats, and set a level across the top of the back board. Measure both diagonals. If they match within 1/4 inch, the box is square and the window will sit flat.

Prime and paint the plywood before you assemble, especially the cut ends. Raw plywood edges soak up water and fail first.

How Do You Attach and Hinge the Window Lid?

Fasten two butt hinges between the top edge of the back board and the window's upper rail, so the lid lifts from the front.

Set the window on the box with the half-inch overhang even at the sides and front. Mark the hinge positions about 4 inches in from each end. Screw the hinge leaves to the back board first, then to the window frame, using screws short enough that they won't poke through the sash. Pre-drill, because old window wood splits easily.

For venting, a hinged lid propped with a notched 1x2 stick gives you adjustable gaps. Cut three notches for openings of about 2, 4, and 6 inches, then screw the stick to the inside of a side panel so it stays with the frame.

How Do You Seal Gaps and Control Condensation?

Stick foam weatherstripping along the top edges of the box where the lid lands, and bank soil or mulch against the outside base to stop drafts.

Even small gaps leak heat on a cold night, so close the lid after pressing the foam in place and look for daylight. Fill cracks where the panels meet with exterior caulk. Check the window's own glazing too, and replace crumbling putty only if the paint tests lead-free.

Some condensation is normal. Heavy dripping is the problem, because wet leaves in cool air invite fungal disease. The sloped lid helps by sending droplets down the glass and off the front edge instead of onto your plants. Vent for 15 to 30 minutes each morning to clear the moisture, and wipe the glass if it stays foggy. Don't chase warmth by sealing the frame airtight. A little air exchange keeps humidity in check.

How Do You Vent a Cold Frame?

Prop the lid open 2 inches once the air inside passes about 50°F, and open it wider as the temperature climbs.

Frames overheat faster than most people expect. On a clear fall afternoon the air inside can climb well above the outside temperature within an hour, and cool-season crops like lettuce and spinach do best when daytime air stays in the low 70s°F or cooler. Seedlings that look fine at noon can be scorched by 2 p.m.

Check the min-max thermometer each morning and evening. Close the lid in mid-afternoon, about two hours before sunset, so the soil holds the day's warmth. On nights forecast to drop near or below 25°F, lay an old blanket over the lid and stack straw bales around the sides. Local conditions shift all of this, so treat these numbers as a starting point and adjust once you see how your frame behaves.

Water in the morning, and only when the top inch of soil is dry. Soil dries out under glass, but roots take up little in cold weather.

When Should You Call a Pro?

Call a pro for lead paint abatement, electrical heating or fans, structures big enough to need a permit, and any drainage or tree work around the site.

A small cold frame often needs no permit, but rules vary, so a quick call to your local building department settles it. If your window tests positive for lead and you want to keep it, a certified lead professional should handle the paint. Otherwise, skip that window.

Soil heating cables, fans, and thermostats need a GFCI-protected outdoor circuit, and a licensed electrician should run it. If you're thinking of three or more windows, a walk-in space, or something attached to the house, you're building a greenhouse. The difference is scale. A frame can add several weeks at each end of the growing season, while a greenhouse may need a foundation, wind-load planning, and a permit. Standing water, a steep grade, or a limb overhead are jobs for a landscaper or a certified arborist.

Your next step is simple: measure your window, pick the matching row in the sizing table, and cut the sides this weekend, while there's still time to sow cold-hardy greens before hard frost.