How To Build a Mini Greenhouse Using Recycled Windows

Test for lead

Wear safety glasses, gloves, and a respiratory mask throughout this process. When using old windows, it’s important to test for lead paint.  Test kits are available at most hardware stores and typically involve putting a sample in a vial and sending it in.

If your recycled windoss do test positive for lead, you can still use them. Just be sure to only work in a well-ventilated outdoor area and never sand or create dust while restoring the recycled windows.

Prep the windows

Lay down a plastic liner to catch debris. Remove all hardware (rope, handles, hinges) using pliers to pull stubborn staples or nails. Work gently with brass hardware to avoid stripping screws.

Using a putty knife, gently remove only loose, flaking paint—don’t force stuck chips. Never sand or scrape lead paint surfaces to keep particles from going airborne.

Fill cracks and split end grain with Bondo. Mix small batches for workability. Work the filler flush to the surface to minimize sanding. Once dry, carefully sand Bondo flush without disturbing the surrounding paint. Use a shop vac with a HEPA filter to contain the dust, then seal the vacuum bag and filter together and dispose of them.

Reglaze

If the window glaze has dried, cracked, or fallen out, reglaze the windows. Start by cleaning out and removing any loose pieces. Knead the glazing compound until soft and pliable, then roll it into 1/2-in. thick ropes, three to six inches long. Press the glaze along the glass edge at an angle using a putty knife. Smooth with your finger, then finish with the putty knife for a clean seal.

Prime and Paint

Apply two to three coats of triple-thick primer to both sides. This primer seals cracked, weathered and peeling surfaces, and allows for the natural expansion and contraction of window frames that will live in the elements. Consider tinting the primer toward your topcoat color (we tinted our primer gray for a black topcoat). Prime over the window glaze to help it harden and adhere.

Once the primer is dry, apply two to three coats of exterior-grade paint to all the sides of the window frames.

Build the Raised Bed

Assemble the side panels

To make the garden bed of the mini greenhouse, build four separate side panels (two long and two short). Each side panel is constructed from three side pieces (A or B) stacked on top of each other and held together with battens (C) on both ends and a central batten (D) in the middle.

Use wood glue and brad nails to pin the battens (C) to the back of the side pieces (A or B), then add a few deck screws after double-checking that everything is square.

Connect the sides

Using three-inch deck screws, attach the short side panels to the long side panels by drilling through the short side pieces (B) and the battens (C) on each side into the edge of the long side panel’s end batten (C). Don’t worry about the screw holes on the outside of the raised box, as they won’t be visible once the corner trim (K) is applied. Reinforce the connection by adding three 3-in. x 3-in. angle brackets to the inside of each joint. Attach two of them two inches from the top and bottom, and one centered on each corner.

Build the top cap

Before installing the top cap pieces (F + G), install the mitered gussets (E) at each corner of the raised bed to hold the sides square. Attach these with wood glue and brad nails, toenailing them from inside the box

Secure the black-painted mitered top cap pieces (F + G) to the top side pieces (A + B) using countersunk three-inch screws. Be sure to leave a 2-5/8-in. overhang around the outside perimeter of the raised bed box. If your miters are opening up or not aligning top to bottom, don’t hesitate to stick a screw in the end to hold them in place. There’ll be plenty of wood filling and touch-up later.

After that, screw a layer of varnished cedar stepback pieces (H + J) to the top cap (F + G), flush to the inside edge. This layer is purely aesthetic and breaks up the painted black top cap (F + G) and window framing we’re moving on to next.

Make the corner trim

Using 1-1/2-in brad nails and construction adhesive, nail together the eight corner trim pieces (K) to create four “L” brackets to cover the visible screw holes on the sides (A + B) of the raised bed.

Attach the corner trim “L” brackets to the raised bed sides (A + B) using the same construction adhesive and brad nails as before.
These are purely aesthetic and can be installed at any downtime during the build.

With this step, you have completed the raised bed portion of the mini greenhouse.  From here, you will strategically build up the top greenhouse section, which will support the windows.

Construct the Greenhouse Top

Build the greenhouse’s peak

As stated in the introduction, the size of the recycled windows determines many greenhouse specifications, including the height of the uprights (L) and the width of the center beam (M).

Install the hinges on the top edge of the recycled windows and play around with upright (L) heights until you settle on one where the pivot point of the window hinge makes the bottom edge of the window frame land centered on a 2×4 placed on top of the long stepback (H).

When your upright (L) height has been determined, toe screw the uprights flush to the inside edge and centered on top of the short stepback pieces (J). Then, attach the center beam (M) to the top of the installed uprights (L).

Note: The toe screws between the uprights (L) and stepbacks (J) are temporary, as we are building our greenhouse indoors and moving it outdoors in two pieces.

Build the bottom frame

Because our greenhouse will be moved outside in two pieces we need to create a bottom frame for the greenhouse made out of the painted sill pieces (N + P). Notch the two short sill pieces (P) around the uprights (L) using a jigsaw. Secure the sill pieces (N + P) to one another and the uprights (L) using screws and construction adhesive. Be sure to remove the toe screws between the uprights (L) and the stepback layer (J) before joining these pieces together.

Install a few screws between the stepback (H + J) and sill (N + P) layers to hold the greenhouse frame in place, but don’t attach them permanently until the backyard installation.

Install the windows

Having known the width of my windows from the onset, I designed the greenhouse to have 2×4 mullions between each window frame with 1/4-in. gaps between each to give the windows room to easily open and close. Not to sound like a broken record, but another way that the size of the windows determines nearly every dimension of this project.

Start installing the windows by surface mounting the second leaf of the window hinges to the top of the center beam (M) using the included hinge hardware. Start in the middle, centering the first window on the beam (M), and work your way to the outside windows, ensuring your hinges are aligned so your windows open and close smoothly and square.

Install the mullions

The mullions (Q) that framed out our greenhouse windows took the most machining (and finishing, as there are four painted and four clear-coated) of any other part on the build (see fig. C). The mullion’s purpose is to follow the same slope as the windows and add support to the structure, and give use something to attach the cam lifts to later.

Determine the top and bottom angles for your mullions (Q), ours were 41.5° at the bottom and 48.5° at the top, then cut off the point at the top so that the mullion doesn’t protrude past the top of the center beam (M).

Secure the mullions to the center beam (M) and sills (N + P) using construction adhesive and screws. Leave around a 1/4-in. gap between the edges of the mullions and the sides of the window frames.

Add window stops

Add 2×2 stops (R) between the mullions (Q) along the bottom of the window openings. Place them so they hold the bottom edge of the window frame 1/4- off the top of the sill (N). Secure the stops (R) to the long sills (N) using construction adhesive and 2-1/2-in. screws.

Install the polycarbonate ends

Use cardboard to create a template for the polycarbonate ends. You can make a template for one single piece on each end, but I suggest two triangles with a seam in the middle to allow for expansion and contraction. Trace the cardboard template onto an 1/4-in. polycarbonate sheet and cut out the triangles using a circular saw outfitted with a plastics blade.

Glue and screw the painted 2×2 plex supports (S + T) to the inside of the outermost mullions (Q) and sill (P) on the inside of each end triangle.

Install the polycarbonate ends by pre-drilling oversized holes along the bottom and sloped edges of the polycarbonate, and screwing them loosely to the supports to allow for expansion and contraction.

Then cut and install the center plex supports (U), toenailing them to the painted 2×2 plex supports (S + T) and sandwiching the unsupported polycarbonate seams between the center plex supports (U) and uprights behind them.

Finishing touches

To wrap up the project:

• Use wood filler to fill any holes from nails or screw heads in the painted pieces. Then sand or wipe back the filler to leave a smooth surface and touch up the paint at these locations.
• Apply rubber or felt bumpers to the edges where the window frames contact the stops (R). During the hot summer months, this keeps the painted surfaces from sticking together.
• Attach lift supports that stretch between the window frames and mullions (Q) in order to keep the windows from dropping while working in the greenhouse.
• Add cabinet handles centered at the bottom of the window frame for easy access.
• Add small end caps (V) to flush out the center beam (M) with the outside of the mullions (Q) and plex support pieces (S, T, U). This can be avoided by using a longer 99-inch 2×4 as the center beam (M).

Outdoor installation

This mini greenhouse was designed to be put together in two pieces.  The top section for the windows and the bottom raised bed.

When determining the final resting place for your mini greenhouse, look for a level area where water drains freely. Attach a plastic liner as a barrier to the inside of the raised bed before filling the greenhouse with soil.

Then line up and secure the bottom sills (N + P) on the top to the stepbacks (J + H) on the bed using construction adhesive and 2-1/2-inch screws, and plant!

FAQ

How long does it take for window glaze to dry?

Traditional window glaze is oil-based and requires a much longer drying time –14 days to set up and up to 30 – 60 days to fully cure and harden.  Latex window glaze will dry and cure in about three days.

What are the best plant choices for a mini greenhouse?

Consider two things when selecting plants to grow in a greenhouse: use plants that are appropriate for your weather zone or region, and choose plants that will not outgrow the height of your greenhouse. Read the label on a plant at your local nursery to help guide you with all the above.

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