Why is my converted garage too cold in winter?

A converted garage is usually cold because it was built as an unheated shed for cars, and the conversion left some of that behind: thin or missing insulation, an uninsulated slab, leaky seams, and a heating or cooling supply that was never sized for the room. Fixing it starts with finding which of these is the main cause, not with buying a bigger heater.

The usual causes

Not enough insulation, or insulation that was skipped. A garage ceiling and walls are normally bare. For a garage converted to living space, Spotsylvania County's carport enclosure details, based on the 2021 Virginia Residential Code, require insulation and windows that meet its table, which lists R-60 for the ceiling, R-15 for walls and R-10 for a slab-on-grade, with a window and door U-factor of 0.30.^[4] Those are one county's published numbers, and your locality sets its own, but a converted room that was finished with a few inches of batt in the walls and nothing in the ceiling will be cold in January no matter what is plugged in.

The slab. Heat leaks out through a concrete floor, and the floor is where you feel it. Spotsylvania's table adds that insulation must be provided to protect the slab-on-grade from heat loss.^[4] A bare slab under a rug is a common reason the room feels cold at foot level even when the air temperature reads fine.

The old garage door opening. The wall that replaced the big door is the most likely place for gaps, because it was framed in after the fact. Air leaks at the sill plate, around the new window and along the old header let cold air in and warm air out. Air sealing is often skipped. A blower-door test, which a contractor or an energy auditor can run, shows where the leaks are.

Heating that does not reach the room. Many conversions extend the house's existing ducts into the garage. ENERGY STAR says that in a typical house about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts, and it lists ducts located in an attic, crawlspace or garage among the signs of poorly performing ducts.^[3] A long, thin branch off a system that was sized for the original house can leave the new room with a weak trickle of warm air, and the furnace or heat pump may not have the capacity to carry the extra load at all.

A heating plan that was never calculated. Spotsylvania's details say living spaces must be capable of maintaining a room temperature of not less than 68°F at a point 3 feet above the floor and 2 feet from the exterior.^[4] If the room cannot do that on a design-cold day, the system or the shell is falling short. ENERGY STAR says your HVAC contractor should verify the proper system size using a calculation called Manual J.^[2]

What a fix usually looks like

The cause decides the repair, and a good contractor will check in a sensible order. Is the shell tight and insulated? Is the heat reaching the room? Is the equipment big enough? Then decide.

• Air sealing and added insulation where the shell falls short, most often the ceiling, the closed-in door wall and the slab edge.
• Duct repair or a new supply if the room is on an existing duct run that loses heat before it arrives.
• A separate heating source. ENERGY STAR lists additions or outbuildings, including a garage, where extending ductwork or heating and cooling capacity is difficult as a good use for ductless mini split heat pumps.^[2] It also lists spaces adjacent to unconditioned spaces where ductwork would be exposed to harsher temperatures.^[2] ENERGY STAR says certification of a cold climate air-source heat pump requires third-party verified performance at low temperatures, with testing down to 5°F.^[2] A ductless unit does not rely on the house system, and it handles heating and cooling in the same room.
• A floor treatment if the slab is the culprit.

Portable electric heaters can take the edge off for a while, but they are a stopgap, and running one as the only heat is a sign the room was not finished properly.

If the work is under permit

A fix that changes insulation, framing, electrical or mechanical systems may need a new permit or an inspection of its own, so ask your building department before work starts. See do I need a permit to convert a garage into a bedroom in Virginia. If the conversion was done without a permit, tell the contractor up front; it changes what they can legally touch.

For background on what the full job includes, read what converting a garage into living space involves, and how much a garage conversion costs in Virginia covers what drives the price of redoing a floor or ceiling.

When to call a licensed pro

For almost every fix. Insulation behind finished walls, framing and window changes, new ducts, a ductless unit and any new electrical circuit all fall to a licensed contractor. A job of this size needs a contractor with at least a Class C licence, which covers a single contract over $1,000 but less than $30,000, or at least Class B for a single contract of $30,000 or more but less than $150,000.^[5] Virginia's Board for Contractors says the contractor should apply for and obtain any necessary building permits,^[1] and that you should not allow work to start until the contractor gives you copies of the officially issued permits.^[1]

Ask for a written scope that names the cause the contractor found, the R-values and equipment being added, and the room temperature the system is meant to hold. Check the licence number and class in DPOR's licence lookup before you pay a deposit. Loclis lists DPOR-licensed remodelers; browse remodeling services or read more remodeling answers.

To compare quotes for your own garage, describe the job once and licensed local businesses will respond.