You bought a 75,000 BTU heater for a 30×40 shop and it still never gets above 50°F in January. The heater runs nonstop, the propane bill is brutal, and the concrete floor feels like an ice rink. Everyone tells you to get a bigger heater. They are wrong. Your shop is not cold because the heater is too small. Your shop is cold because the building loses heat faster than any reasonably sized heater can replace it.
Heat leaves a shop through three paths: conduction through the walls, roof, and floor; infiltration through gaps, seams, and that overhead door that might as well be a window; and radiation off the concrete slab, which acts as a massive thermal bridge to the frozen ground underneath. Until you understand which path is eating your BTUs, adding a bigger heater just burns more fuel without solving the problem. This guide breaks down each heat loss path, explains how to calculate what your shop actually needs, and shows where the cheapest fixes are hiding.
Conduction: Your Walls and Ceiling Are Radiators in Reverse
Conduction is heat moving through solid materials from warm to cold. Every square foot of your shop's exterior surface is conducting heat outward whenever the inside is warmer than the outside. The rate depends on the R-value of the assembly and the temperature difference. The formula is straightforward: Q = A × ΔT / R, where Q is BTU per hour, A is area in square feet, ΔT is the temperature difference in °F, and R is the total R-value of the wall or ceiling assembly.
A typical uninsulated metal shop has an R-value of about R-3 for the walls (metal panel plus air film on both sides) and R-3 to R-5 for the ceiling. A 30×40 shop with 14-foot eave walls has about 1,960 square feet of wall area and 1,200 square feet of ceiling. At a 60°F temperature difference (70°F inside, 10°F outside), the conduction loss through uninsulated walls is 1,960 × 60 / 3 = 39,200 BTU/hr. Through the ceiling: 1,200 × 60 / 3 = 24,000 BTU/hr. That is 63,200 BTU/hr just through the shell before you even count the floor or the doors.
Adding R-19 fiberglass batts to the walls brings the R-value to about R-22 total. The wall loss drops to 1,960 × 60 / 22 = 5,345 BTU/hr. That is an 86% reduction. Adding R-30 to the ceiling cuts ceiling loss from 24,000 to 2,400 BTU/hr. Insulating the shell typically costs $3,000 to $6,000 for a 30×40 shop, and it cuts the conduction load by 75 to 85 percent. No heater upgrade gives you that return.
The lesson is simple: insulate before you upsize. A 45,000 BTU heater in a well-insulated shop will outperform a 150,000 BTU heater in an uninsulated one, and the fuel bill will be a fraction of the cost. Every dollar spent on insulation pays back for the entire life of the building.
Q = A × ΔT / R
A = surface area (sq ft)
ΔT = inside temp − outside temp (°F)
R = total R-value of assembly
Q = heat loss (BTU/hr)
Insulating from R-3 to R-22 cuts conduction loss by 86%.
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.
Infiltration: The Invisible Hole in Your Building
Infiltration is cold outside air sneaking in through every gap, crack, and unsealed joint in the building envelope. In most shops, infiltration accounts for 30 to 60 percent of total heat loss. The biggest culprit is the overhead door. A standard 10×12 overhead door with worn weatherstripping leaks air around all four edges and through the panel joints. Even when closed, the effective gap around a typical overhead door is equivalent to having a window open 6 to 12 inches.
The infiltration heat loss formula is: Q = 1.08 × CFM × ΔT, where CFM is the volume of outside air entering the building per minute. A shop with 0.5 air changes per hour (ACH) has a volume of 30 × 40 × 14 = 16,800 cubic feet. At 0.5 ACH, that is 8,400 cubic feet per hour, or 140 CFM. At a 60°F temperature difference: Q = 1.08 × 140 × 60 = 9,072 BTU/hr. That sounds manageable.
But 0.5 ACH is a tight shop with good weatherstripping and no door activity. A shop with a leaky overhead door, a man door that does not seal, and some gaps at the eave easily runs 2 to 4 ACH. At 3 ACH, the infiltration loss jumps to 1.08 × 840 × 60 = 54,432 BTU/hr. That is more than a 50,000 BTU heater can handle just from air leakage. Open the overhead door for five minutes to pull a truck in, and you have replaced the entire volume of heated air with outside air. The heater has to start from scratch.
The cheapest fix is weatherstripping. Replacing the bottom seal, side seals, and header seal on an overhead door costs $100 to $300 in materials. Adding brush seals or foam compression strips around the man door is another $30. Sealing the eave joints and penetrations with canned foam or caulk is a weekend project. These simple measures can cut infiltration by 40 to 60 percent and are the highest-return investment in any shop heating project.
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 Concrete Floor: A 1,200 Square Foot Heat Sink
Concrete has a thermal conductivity of about 5.0 BTU/(hr·ft·°F), which is roughly ten times that of wood. An uninsulated 4-inch concrete slab sitting on frozen ground acts as a massive thermal bridge, pulling heat out of the shop and dumping it into the earth. The heat loss through a slab is not evenly distributed. The edges, where the slab meets the foundation wall and is exposed to outdoor temperatures, lose heat much faster than the center.
The standard engineering approach for slab heat loss uses a perimeter loss factor rather than an area calculation. The formula is: Q = F × P × ΔT, where F is the edge heat loss coefficient (typically 0.55 to 0.90 BTU/hr per foot of perimeter per °F for uninsulated slabs), P is the perimeter in feet, and ΔT is the temperature difference. For a 30×40 shop with 140 feet of perimeter: Q = 0.75 × 140 × 60 = 6,300 BTU/hr. That is 10 to 15 percent of the total load in a typical shop.
Slab-edge insulation is the practical fix. Adding 2 inches of rigid XPS foam around the perimeter of the slab, extending 24 inches below grade, cuts the edge loss coefficient roughly in half. For existing buildings, you can trench along the outside of the foundation and adhere rigid foam to the foundation wall. For new construction, the foam goes in before the concrete is poured. The cost is $500 to $1,500 depending on accessibility, and the payback is typically 2 to 4 years in fuel savings.
The other slab problem is comfort, not just energy. Concrete at 55°F pulls heat from your feet through conduction. Even if the air temperature is 65°F, standing on a cold slab makes you feel 10 degrees colder. Anti-fatigue mats or rubber flooring in work areas add insulation under your feet and make a 60°F shop feel like a 65°F shop. This is not an engineering solution, but it is a $100 comfort fix that reduces the temptation to crank the thermostat higher.
Concrete Slab Heat Loss Calculator
Calculate heat loss through concrete slab-on-grade floors. Uses edge loss and full-slab methods with perimeter F-factors, insulation R-values, and heating degree days to estimate annual cost and insulation payback.
Right-Sizing the Heater: Why Bigger Is Not Better
Once you know your actual heat loss, sizing the heater is arithmetic. Add up conduction losses (walls, ceiling, floor) and infiltration losses at your design temperature. That total is the steady-state load. Add 10 to 20 percent for a recovery factor if you open the overhead door regularly. The result is the heater capacity you need.
For a well-insulated 30×40 shop with good weatherstripping, the total load at a 60°F delta-T is typically 25,000 to 40,000 BTU/hr. For a poorly insulated shop with a leaky door, it can be 80,000 to 120,000 BTU/hr. The difference is entirely in the building envelope, not the heater.
Oversizing the heater creates its own problems. A heater that is twice the required capacity will short-cycle, turning on and off frequently. Short-cycling wastes fuel during startup (combustion is least efficient in the first few minutes), creates uncomfortable temperature swings, and accelerates wear on ignition components and gas valves. A modulating or two-stage heater is a better choice for shops where the load varies significantly, such as when the overhead door opens occasionally.
Heater type matters too. Forced-air unit heaters are cheap and common, but they blow warm air at ceiling level and create temperature stratification. A 14-foot shop might be 80°F at the ceiling and 55°F at the floor. Radiant tube heaters warm objects and the floor directly, eliminating stratification and typically delivering the same comfort at a 5 to 10 degree lower thermostat setting. For a shop where you work on the floor (welding, mechanic work, woodworking), radiant heat is almost always the right answer.
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.
The Cheapest Fixes First: Where to Spend Your First $500
If your shop is cold and your budget is limited, spend your money in this order. First, seal the overhead door weatherstripping: $100 to $300, saves 20 to 40 percent of infiltration losses. Second, seal the man door and any penetrations: $30 to $50, addresses the remaining infiltration gaps. Third, insulate the ceiling if it is bare metal: $800 to $1,500 for blown-in or batt insulation, cuts ceiling conduction by 85 percent. Fourth, insulate the walls: $2,000 to $4,000, cuts wall conduction by 85 percent.
The total cost of all four measures for a 30×40 shop is roughly $3,000 to $6,000. The fuel savings are typically 50 to 70 percent of the heating bill. At $200/month in propane costs over a 5-month heating season ($1,000/year), the payback is 3 to 6 years. After that, the savings continue for the life of the building. Compare that to upgrading from a 75,000 BTU heater to a 150,000 BTU heater: $2,000 to $3,000 for the new unit, and your fuel bill doubles because you are pumping twice the BTUs into a building that is still leaking them all out.
If you can only afford one thing, fix the overhead door seals. If you can afford two things, add ceiling insulation. These two measures together address the two biggest heat loss paths in most shops and cost under $2,000. The heater you already have is probably big enough once you stop feeding the outdoors.