Is the building leaking the heat you already paid for?
Seal and insulate first, or buy more watts?
Why an insulated door and weatherstrip often beat a jump from 5 kW to 7.5 kW, when infiltration and slab mass fake an undersized heater, and when more watts are still required.
Updated 2026-09-04
The heater that “doesn’t work”
The usual complaint is a 5 kW ceiling unit on a 30 A / 240 V circuit that runs all morning and the two-car still feels like a carport. The element is doing its job. The building is exchanging the air you paid to heat. Buying a 7.5 kW cabinet, or a second 5 kW unit, is how people turn a sealing problem into a panel problem.
This page is the envelope conversation that belongs after you know the circuit and the heat type, and after you have a wattage bracket from wattage by garage size. The size brackets already split insulated versus drafty. Seal-first here is the still-cold / upgrade path: when the bay stays cold, or you are about to jump from 5 kW to 7.5 kW, fix the envelope before you buy more watts. Then pick the mount.
We are not going to invent a payback year, an R-value that “pays for itself in one winter,” or a coverage claim for a door kit. Door insulation and weatherstrip are cheap relative to a new 240 V circuit and a larger unit heater. That is the comparison. Street prices for kits and doors move; treat any retailer row as a class, not a live price.
Seal-first checklist
Do these in order before you upsize the heater. None of them replace a load calculation. All of them change whether a 5 kW forced-air unit has a chance.
| Step | What you are fixing | Good enough signal | Not a substitute for |
|---|---|---|---|
| 1. Weatherstrip the overhead door | Side and top seals, plus the worn rubber at the jambs | Daylight around the panel is gone; you cannot feel a knife of air at the sides | A new heater |
| 2. Fix the threshold / bottom seal | The sill is often the largest hole in a two-car | The bulb or flap meets the floor along the full width; water and leaves stay out | A second 5 kW unit |
| 3. Insulate or replace the door | A single-layer steel or wood door is a giant uninsulated wall that also moves | An insulated door or a listed retrofit kit is on the panel, not a moving blanket taped up | Manufacturer “up to X sq ft” copy |
| 4. Man-door, vents, and obvious holes | The pedestrian door, broken glass, and the gap at the house wall | Sweep and weatherstrip on the man-door; foam or gasket on the ugly penetrations | Sealing combustion-air vents you actually need |
| 5. Ceiling and walls you can reach | Open rafters and bare studs dump heat into the attic or the outdoors | You have a plan for the planes you can insulate without burying junction boxes or dampers | A finished house-grade remodel as a prerequisite for any heat |
| 6. Then re-read the wattage bracket | The same square footage is a different load after the door stops leaking | The insulated column on the size page is no longer a fantasy | Skipping the circuit and panel check |
Door insulation and weatherstrip versus a 5 kW to 7.5 kW jump
On a typical two-car, the overhead door is the largest single surface that is also a moving gap. An uninsulated door plus a crushed bottom seal will undo a correctly sized 5 kW unit. The size guide already says the quiet part: weatherstrip and an insulated door often beat a jump from 5 kW to 7.5 kW.
That jump is not a bigger plug. A 7.5 kW FUH-class unit is about 31 A at 240 V and wants 8 AWG copper in the same manual family that puts 5 kW on 10 AWG and a 30 A fuse. You are buying a different circuit, not a software setting. Two 5 kW cabinets are two 30 A two-pole breakers. See 120 V vs 240 V for the continuous-load arithmetic.
Weatherstrip and a door kit stay on the building. They cut infiltration every hour the heater runs, and every hour it does not. A larger element only helps while you pay for the extra watts, and only if the air stays in the room. If the door is open for a bay that is also a driveway, neither the kit nor the 7.5 kW unit will make the cubic feet behave. That is the forced-air vs infrared problem: heat the person, or close the door.
We will not quote a dollar-per-therm comparison we did not measure. Fix the seals you can see. Insulate the door if it is a single skin. Then decide whether the insulated column on the size page still asks for more than 5 kW. Many attached two-cars land on one 5 kW unit after that work. Many detached shops still do not.
Infiltration is the load
Forced-air electric heat warms air. Every cubic foot that leaves under the door, through a missing threshold, or up an open rafter bay is a cubic foot you heat again. That is why a shop with the overhead door cracked “just a foot” feels like the heater is broken. The heater is a very expensive fan pointed at a hole.
Infiltration also explains the “it was fine until January” reports. Stack effect and wind get worse as the delta-T grows. A door that is merely mediocre at 40 °F outside is a gale at 10 °F. Adding watts without touching the sill scales the electric bill faster than it scales comfort.
Infrared changes the failure mode, it does not repeal infiltration. You can stand in a radiant beam with the door up and feel warm. The far corner and the slab still dump heat. If the job is whole-bay air temperature, you need the envelope. If the job is one person at a bench, spend the watts on the person and stop pretending the volume will catch up. That split is written out on forced-air vs infrared.
Do not tape over dryer vents, water-heater air, or a carbon-monoxide path to “gain R-value.” Seal the junk gaps. Leave the intentional ones.
Slab and thermal mass
Concrete and two parked cars will drink the first hour of heat even when the wattage matches the air load. The slab is a sink. It is also why first-start on a Monday feels weak and Saturday afternoon, after the heater has been on, feels fine. That is not proof you need 7.5 kW. It is proof you asked a resistance coil to warm a few tons of mass in forty minutes.
Practical responses that are not a larger breaker:
- Start the heater before you occupy the space, on a thermostat or a timer you trust.
- Put a listed mat or a throw rug at the standing zone if you only need the feet.
- Use infrared at the bench so the first watts hit you instead of the slab. Same 1.5 kW portable limit if you are on 15 A.
- Park the second car outside if this is a shop day and you can. Two engines are two sinks.
Insulating under an existing slab is reconstruction, not a weekend kit. Edge insulation and a closed door help. They do not turn the slab into a wood floor.
If you only occupy a 6×8 ft patch, stop heating the slab you are not standing on. That is a heat-type decision, not an insulation failure.
When more watts are still required
Sealing before more watts is not a slogan that forbids a larger heater. After the door and the obvious leaks, you still need more watts when:
- The footprint is a three-car or a “2-car plus shop” in the 600–900+ ft² band. The insulated column on the size page is already 6–10 kW.
- Ceilings are high. Ten-foot or open-rafter volume is more air, and forced-air stratifies. You can insulate the plane and still be short.
- The climate and the setpoint are the house, not “take the edge off.” Shirt-sleeve shop temperature in a cold climate is a different load than keeping tools above freezing.
- The building is detached, uninsulated, and you have decided not to insulate the walls. Honest path: size to the drafty column, or abandon whole-bay air heat.
A 1,500 W milkhouse heater does not become a two-car heater because you weatherstripped. Circuit limits do not move when the door gets a kit. If you are still on a 15 A outlet, the next adult step is staying on portables or installing 240 V, not stacking two 1,500 W units on one run.
When electric resistance is the wrong path
This site compares electric resistance heaters. That is a scope choice, not a claim that resistance is always the adult answer. The hard stop is the panel.
A 5 kW heater is about 20.9 A at 240 V. A 7.5 kW unit is about 31 A. Two 5 kW units are two 30 A two-pole breakers. On a 100 A service that already feeds a range, dryer, HVAC, and an EV charger, that load may not fit without a calculation, a panel upgrade, or a service upgrade. We will not pretend every suburban garage has a free 40–60 A of 240 V waiting.
If the panel cannot take the circuit, the useful electric options shrink to: keep the smaller heater and finish the envelope, or spend the same 1.5 kW on infrared at the bench. The non-electric options (gas unit heater, mini-split, wood) exist. They have their own combustion, refrigerant, and install rules. We will not write a fake comparison matrix for equipment this site does not specify from a current nameplate. If you leave resistance heat, leave this decision tree; do not bolt a 5 kW cabinet onto a lighting circuit as a protest.
Undersized service plus an uninsulated door is the expensive way to skip the hub order. Name the circuit, pick heat type, then size the watts. If the bay is still cold or you are about to jump from 5 kW to 7.5 kW, seal the envelope before more watts. Then decide wall versus ceiling. See wall-mount vs ceiling-mount when you are actually hanging steel.
Forced-air once the door holds
Heat type is already decided by this point. Sealing does not reopen that fork. It is what makes a forced-air choice actually hold air. Once the door stops leaking, whole-bay fan-forced heat can make the space itself less miserable. That is the closed-door, some insulation case on forced-air vs infrared. A 5 kW Comfort Zone / Fahrenheat-class ceiling unit is the common hardware for an attached two-car in that condition. Details live on ceiling-mount units under $200.
If you sealed what you could and you still work with the door up, insulation does not convert a driveway into a room. Infrared at the occupied zone remains the honest spend of limited watts. Sealing still helps the hours the door is down. It does not make fan-forced air stay in an open bay.
Affiliate buy links for doors, kits, and heaters are not live. When they are, they will be labeled. Do not treat a listing that says a weatherstrip “adds 10 degrees” as a measurement we will repeat.
Next: if the envelope now matches the insulated column and you are hanging a hardwired unit, pick the mount on wall vs ceiling. If the insulated column still asks for more watts than the circuit can feed, that is a panel conversation, not a second milkhouse heater. If you only occupy a bench, stay on 15 A portables or directed radiant, and stop shopping for more fan.
Related guides
By size
Start with square footage and insulation. A 1,500 W plug-in unit is a workbench heater, not a two-car garage heater.
120V vs 240V
Voltage is not a feature. It is the limit of the circuit you already have — or the one you are willing to install.
Forced-air vs IR
Forced-air heats the air. Infrared heats people and objects. Drafts punish the first; open-door work favors the second.
Ceiling under $200
Most sub-$200 ceiling units are the same job: 5 kW, 240 V, hardwired, 30 A, 10 AWG copper. The differences are controls and jumpers, not magic BTUs.
15 A portable
On a 15 A circuit, treat ~12 A as the continuous ceiling. A 1,500 W milkhouse heater is 12.5 A — it wants to be the only load.
Wall vs ceiling
Ceiling units throw across the bay if the joists can take 25–30 lb. Wall mounts win on low ceilings and one work zone. The manual’s clearance is the rule, not a blog height.