Your garage door is probably a single skin of steel. In winter the garage is barely warmer than the street. If that garage shares a wall with your living room, or you heat it as a workshop, part of your heating bill is going out through that door.
Here's the short answer. Fit 20–25 mm rigid foam board or a reflective foil kit into each door panel. Then seal the bottom with a rubber strip and the sides and top with brush or rubber seals. A standard single door takes 3–5 hours and roughly 80–200 in materials, depending on the kit. An unheated attached garage often ends up 2–5 °C warmer in cold weather. The exact gain depends on the walls, roof and floor. The job pays back in money only if the garage is heated or attached to heated rooms. A detached, unheated garage gains comfort, not savings.
1. Decide whether it pays back
Run the numbers before you buy anything. Use this worked example and swap in your own figures.
- Door size: 2.4 m × 2.1 m, about 5 m².
- Bare steel door: heat loss rate (U-value) around 5–6 W/m²K.
- With 25 mm polystyrene: around 1–1.5 W/m²K.
- Difference: about 4 W/m²K × 5 m² = 20 W for every degree between inside and outside.
Say you heat a workshop to 15 °C while it's 5 °C outside. That's a 10 °C gap, so the door now loses about 200 W less. Heat it 8 hours a day for 150 days: 200 W × 8 h × 150 = 240 kWh saved. At an example tariff of 0.30 per kWh with an electric heater, that's about 72 a year. A 100 kit pays back in under two years.
An attached but unheated garage is different. The door doesn't touch your heated rooms. It only warms the buffer space next to them. Savings are real but small, often a few tens of kWh a year, and they vary a lot. Check your unit rate on your bill. UK readers can compare tariffs through the energy regulator's guidance on bills and tariffs.
2. Pick the material
| Option | Thickness | Insulation value | Single-door cost (approx.) | Best for |
|---|---|---|---|---|
| Expanded polystyrene (EPS) kit | 20–25 mm | Good, R about 0.6–0.7 m²K/W | 60–150 | Most steel panel doors |
| Foil-faced PIR board | 20–25 mm | Better, R about 0.9–1.1 m²K/W | 80–180 | Heated garages |
| Reflective bubble foil | 4–8 mm | Low unless there's an air gap in front of the foil | 40–100 | Light doors, tight budgets |
Costs vary by region and supplier. Add 30–70 for seals. The Energy Saver guidance on insulation and air sealing explains R-values if you want to compare other boards.
3. Check the door before you add weight
Insulation adds 2–6 kg to a typical single door. Sectional and up-and-over doors are balanced by springs. Extra weight can upset that balance.
- Lift the door halfway by hand and let go. It should stay put or move slowly.
- If it slams down or shoots up, the balance is already off.
- Pick the lightest material that fits the panel depth.
Never adjust garage door springs yourself. They hold a lot of stored energy. If the door won't hold position after the job, call a qualified garage door technician.
4. Measure each panel
Don't assume the panels are identical. Top and bottom sections are often a few millimetres different.
- Measure width and height of every recessed panel, inside the frame.
- Measure the depth of the recess. Your board must not stick out past the frame or it will hit the tracks.
- Write each size on a sketch and number the panels.
- Add 3–5 mm to width for a friction fit on foam board. Don't add anything for foil.
5. Cut the boards
Work on a flat surface with a long straightedge.
- Score foam board with a sharp utility knife, 2–3 passes, then snap along the line.
- Change the blade every 5–6 boards. Blunt blades tear the foam.
- Cut foil with scissors, leaving a 20 mm overlap if the kit tapes joints.
- Mark each piece with its panel number.
6. Fit the panels in order
Follow this sequence for each panel:
- Close the door and disconnect any electric opener so it can't move.
- Clean the panel with a damp cloth and let it dry for 15–30 minutes. Adhesive won't hold on dust or grease.
- Fix the kit's retaining clips or adhesive pads, usually 2–4 per panel.
- Bow the foam board slightly and press it into the recess so it locks behind the frame lip.
- Press it onto the clips or pads for 30 seconds.
- Tape any joints with foil tape so no steel shows through.
- Move to the next panel, working from top to bottom.
Leave clearance around hinges, rollers and the lock. Cut neat notches rather than covering them.
7. Seal the bottom gap
The bottom gap often leaks more than the bare panels. A 10 mm gap across a 2.4 m door is 0.024 m² of open hole, roughly the size of a small open window.
- Close the door and look for daylight along the floor.
- Fit a rubber bottom seal to the door's base rail. Most slide into a channel or screw on.
- If the floor is uneven, add a threshold seal fixed to the floor. Uneven floors are common.
- Check the door closes fully without the opener straining.
8. Draught proof the sides and top
Air sealing is the cheap, high-value step. ENERGY STAR's air-sealing advice applies here as it does to house doors.
- Fit brush strip or rubber seal to the frame at both sides and along the top.
- Set the seal so it just touches the closed door. Too tight and it drags.
- Use a lit incense stick on a windy day to find leaks. The smoke bends where air moves.
- Close any remaining gaps at the frame with exterior-grade sealant.
9. Test the door and measure the result
Reconnect the opener. Run the door up and down 3–4 times and watch for rubbing or hesitation. Repeat the halfway balance test from step 3.
Then measure. Put a cheap min/max thermometer in the garage for a week before the job and a week after, at a similar outdoor temperature. You'll see your real gain, not a brochure figure. If it's under 2 °C, the walls, roof or an uninsulated garage ceiling are now the weak points. Deal with those next if the garage is heated.
Common mistakes
- Skipping the seals. People insulate the panels and leave a 10–15 mm gap under the door. The air leak cancels much of the benefit. Do seals first if your budget only covers one job.
- Boards too thick for the recess. Foam that sits proud of the frame catches the tracks and rollers. On sectional doors it can also crack when the panels hinge. Measure the depth and stay inside it.
- Ignoring door balance. Extra weight on a door with tired springs strains the opener and can make the door drop. Check balance before and after. Leave spring work to a professional.
Figures are worked examples: check your own tariff, the appliance labels and your local rules before you decide.