An overhead door is the largest single opening in any shop or garage. A standard 10×12 door has 120 square feet of surface area, and when you factor in the gaps around the edges, it might as well be a window with no glass. Even when the door is closed, the effective R-value of a single-layer steel overhead door is about R-4. An insulated sandwich-panel door is R-8 to R-16. Compare that to an insulated wall at R-19 or better, and you start to see why the overhead door is responsible for 25 to 40 percent of total heat loss in most heated shops.
The problem is actually two problems stacked on top of each other. First, the door itself conducts heat because it is a thin metal panel with minimal insulation. Second, the door leaks air around every edge, through every panel joint, and through the track slots. The conduction loss is constant whenever the door is closed and the heat is on. The infiltration loss varies with wind speed and door condition, but it is almost always larger than the conduction loss. This guide covers both problems, quantifies the cost, and lays out the fixes from cheapest to most effective.
Conduction Through the Door Panel
A non-insulated single-skin steel overhead door has an R-value of about R-4, counting the air films on both sides. At a 60°F temperature difference across 120 square feet, the conduction loss is 120 × 60 / 4 = 1,800 BTU/hr. That is not catastrophic by itself, but it is a constant drain whenever the heater is running.
Insulated overhead doors come in two types. Polystyrene-injected doors bond a layer of EPS foam between two steel skins, reaching R-8 to R-12 depending on thickness. Polyurethane-injected doors use denser foam with higher R-value per inch, hitting R-12 to R-18. The better insulated door cuts conduction loss from 1,800 BTU/hr to 400-600 BTU/hr. That saves 1,200 to 1,400 BTU/hr, which adds up to real fuel over a 5-month heating season.
If replacing the door is not in the budget, you can retrofit insulation. Rigid foam panels (1.5-inch XPS at R-7.5) cut to fit inside each door panel and held in place with adhesive or retainer clips bring a non-insulated door up to roughly R-11. The cost is $100 to $200 in materials for a 10×12 door. The downside is added weight. A non-insulated door was balanced by the spring for its original weight. Adding 30 to 50 pounds of foam panels can make the door harder to open manually and may require spring adjustment. On a door with an opener, the added weight is usually within the motor's capacity, but check the opener's rated lifting capacity before adding insulation.
The thermal break at the panel joints is another weak point. Where two panels hinge together, there is typically a gap or a metal-to-metal contact that conducts heat. Better doors have a vinyl or rubber thermal break at each joint. Older or cheaper doors do not. You cannot easily retrofit a thermal break, but you can reduce the impact by ensuring the joint weatherstripping is intact and the panels close tightly against each other.
Single-skin steel (no insulation): R-4
Polystyrene-injected: R-8 to R-12
Polyurethane-injected: R-12 to R-18
Retrofit foam panels (1.5" XPS): adds R-7.5
Even a DIY foam retrofit cuts door conduction loss by 60%.
Air Leakage Around the Door: The Bigger Problem
Air infiltration through and around an overhead door is typically 2 to 5 times larger than conduction through the door panel itself. The leakage paths include the bottom seal against the floor, the side seals along the tracks, the header seal at the top, the panel-to-panel joints, and the track slots where the rollers ride. Each path is relatively small, but added together they create an effective opening of 1 to 3 square feet, depending on door condition and wind exposure.
The bottom seal is the worst offender. Overhead doors ride on tracks that pull the bottom edge tight to the floor, but concrete floors are never perfectly flat. Dips, cracks, and settled spots create gaps under the door that cold air pours through. A new bottom astragal (the rubber seal strip on the bottom panel) costs $30 to $60 and takes 20 minutes to install. If the floor is uneven, a double-contact or bulb-type seal conforms better than a flat wiper seal.
Side seals run vertically along both sides of the door where the panels meet the jamb. Wind-driven rain and air enter through this gap, which widens as the door ages and the tracks shift. Brush seals or compression seals mounted to the jamb close this gap. They cost $50 to $100 per side installed. The header seal at the top of the door is often overlooked because it is above eye level, but it is the same leakage path as the sides. A piece of EPDM rubber or foam tape across the header costs $10 and five minutes of effort.
Track slot leakage is harder to fix. The slots in the vertical tracks that allow the rollers to move also allow air to pass. Some manufacturers offer track covers or brushes that block airflow through the slots without impeding roller movement. These are most effective on doors that stay closed for long periods. If the door opens and closes frequently, the track covers take more wear and need periodic replacement.
Overhead Door Infiltration Loss Calculator
Calculate heat loss through overhead doors in shops, garages, and warehouses. Compares open-door vs closed-door losses, seal condition impact, and annual cost of infiltration with payback on door seals and high-speed doors.
The Door-Opening Penalty: Five Minutes Costs an Hour
Every time you open the overhead door, you replace the heated air in the shop with cold outside air. A 30×40×14 shop holds 16,800 cubic feet of air. At 70°F inside and 10°F outside, the energy stored in that air above ambient is approximately 1.08 × (16,800/60) × 60 = 18,144 BTU. Opening the overhead door for 5 minutes on a cold day with even light wind replaces 50 to 100 percent of that air volume, depending on the door size, temperature difference, and wind.
That 18,000 BTU recovery costs about 15 to 25 minutes of run time on a 50,000 BTU heater. If you open the door 4 to 6 times a day (pulling vehicles in and out, receiving deliveries, moving equipment), you lose 70,000 to 100,000 BTU just from door openings. That is 1 to 1.5 gallons of propane per day, or $3 to $5 at current prices, in addition to the steady-state heating cost.
Strip curtains are the most cost-effective solution for shops where the door opens frequently. Heavy-duty PVC strips hung in the door opening allow vehicles to pass through while blocking 80 to 90 percent of air exchange. A strip curtain kit for a 10×12 door costs $200 to $400. For shops that open the door more than 3 to 4 times a day in winter, the payback is measured in weeks, not years.
The other strategy is a rapid-roll or high-speed door, which opens and closes in seconds instead of the 15 to 30 seconds a standard overhead door takes. Rapid-roll doors are more common in commercial and industrial settings but are available for agricultural and shop applications. They cost $3,000 to $8,000 installed, and the energy savings justify the cost when the door cycles more than 10 to 15 times per day.
Putting a Dollar Amount on the Door
To calculate the annual cost of your overhead door, add the three loss components. Conduction through the panel: use Q = A × ΔT / R with your door's R-value. Infiltration around the edges: use Q = 1.08 × CFM × ΔT, estimating CFM from the effective gap area and wind exposure. Door-opening losses: estimate the number of openings per day, the volume exchanged per opening, and the recovery energy.
For a typical 30×40 heated shop with a non-insulated 10×12 overhead door, opened 4 times per day, in a climate with 6,000 heating degree days: the annual heat loss attributable to the overhead door is roughly 15 to 25 million BTU. At a propane price of $2.50 per gallon (91,500 BTU per gallon at 80% heater efficiency), that is $500 to $850 per year just from the door. In a colder climate with 8,000 HDD, the number climbs to $700 to $1,100.
Compare that to the cost of fixes. Full weatherstripping replacement: $150 to $300, saves 30 to 50 percent of infiltration ($100 to $250/year). DIY foam insulation panels: $100 to $200, saves 50 to 60 percent of conduction ($50 to $100/year). Strip curtains: $200 to $400, saves 70 to 80 percent of door-opening losses ($200 to $400/year). Total investment of $450 to $900 saves $350 to $750 per year. Payback: 1 to 2 years.
Replacing the entire door with a high-R insulated model costs $1,500 to $3,000 installed. It addresses conduction and some infiltration, but not door-opening losses. Unless the existing door is structurally damaged, the weatherstripping, foam, and strip curtain combination gives a better return on investment than a new door.
= Conduction loss + Infiltration loss + Door-opening loss
All converted to BTU/year, then:
Annual fuel cost = Total BTU ÷ (fuel BTU content × heater efficiency)
Propane: 91,500 BTU/gal
Natural gas: 100,000 BTU/therm
Typical heater efficiency: 80%
Shop Heater BTU Sizing Calculator
Calculate the exact BTU output your shop or garage heater needs. Factors in wall R-values, ceiling insulation, slab edge loss, overhead door infiltration, and air changes per hour to size propane, natural gas, and electric heaters correctly.
Fix Priority: Spend Smart, Not Big
Rank your overhead door improvements by cost per BTU saved. The order is almost always the same regardless of door size or climate. First: replace the bottom astragal if it is cracked, missing, or not making contact with the floor. Cost: $30 to $60. This single fix eliminates the largest single infiltration path. Second: add or replace side and header seals. Cost: $50 to $100 per side, $10 for the header. Third: install strip curtains if you open the door more than twice a day. Cost: $200 to $400. Fourth: add foam insulation panels to the door. Cost: $100 to $200.
After these four steps, you have captured 70 to 85 percent of the possible savings from the overhead door for a total investment under $800. The remaining 15 to 30 percent would require replacing the door entirely or installing a high-speed door, which costs 5 to 10 times more for diminishing returns.
One often-overlooked fix is the threshold seal. If the concrete floor under the door has settled or cracked, even a new bottom astragal will not make full contact. A concrete threshold seal kit ($50 to $100) creates a raised, uniform surface for the door to seat against. It fills dips and gaps in the floor and gives the bottom seal something solid to compress against. On shops with old or uneven floors, the threshold seal is the difference between a weatherstrip that works and one that looks good but still leaks.
Finally, consider whether you need the door at all during certain months. A temporary insulated wall panel that bolts into the door opening can completely eliminate the door as a heat loss path during the months when you do not need vehicle access. Some shop owners build a simple 2×4 frame with rigid foam insulation that drops into the door opening in November and comes out in April. The R-value of a 4-inch foam panel (R-20) beats any overhead door on the market, and the cost is under $200 in materials.