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Thermal Breaks at the Garage Door Wall

By epoxy flooring allen record

An overhead door is one of the leakiest openings a house ever has. It is a big, thin, hinged panel with gaps along the sides, a rubber seal on the ground and a track that needs clearance to move. When that door is removed and the opening is closed in with a wall, many homeowners assume the problem is solved. Often it is only partly solved, because the strip of slab and framing along the old door line keeps leaking heat, cold and moisture long after the conversion looks finished.

Why that line is different

The rest of the garage slab sits inside walls on three sides. The edge under the old overhead door sits on the outside face of the building. The concrete there is often a thickened apron that extends toward the driveway, so it is physically connected to the outdoor pavement. Concrete conducts temperature easily, so a slab that is warm at one end and cold at the other moves heat along its length. In winter the cold of the driveway creeps into the room along the floor. In summer, the heat of the apron radiates into the cooled space.

A thermal break is any material or detail that interrupts that path of conduction. Think of it as a speed bump for heat.

Where the leaks actually are

Walk the old door line with a flashlight at dusk, or on a windy day with a stick of incense, and the weak spots tend to show themselves:

• The groove or lip where the door seal used to press. Often a shallow channel in the concrete that gathers water and lets air through.
• The bottom plate of the new wall. If it sits directly on the slab without a gasket or sealant, air slips underneath.
• The corners. Where the framing meets the old jamb, gaps hide behind trim.
• The slab edge. A cold bridge that sweats in humid weather.
• The sheathing seams. Panel joints on the exterior face that were never designed to be airtight.

Stopping air before heat

Air movement carries far more heat than conduction through a small strip of material, so sealing comes first. A continuous bead of flexible sealant beneath the bottom plate, a sill gasket made of compressible foam, and sealant around every penetration stop the draft. Spray foam in the narrow cavity between the plate and the slab works well for irregular gaps, though it should be a low expansion type so it does not push the framing out of position. Before any sealant goes on, clean the concrete of dust and oil so it sticks.

Insulating the wall bottom and edge

Next, address conduction. Rigid foam board can be installed against the inside face of the slab edge or beneath the bottom plate as a break between concrete and wood. If the new wall is built slightly inboard of the old door line, the leftover strip of slab outside the wall becomes outside space and the colder part is cut off from the room. This is a design choice that has to be made at the framing stage, and it can reduce heat loss measurably. A thermal break strip, a thin sheet of closed cell foam, plastic or composite, set between the slab and the plate also helps keep the bottom of the wall warmer.

Dealing with water

Garage aprons slope away from the building, but the old door line is often a spot where water pools after rain, then seeps back under the new wall. A flashing or a sloped sealant bead at the exterior, plus a drip edge on the wall's bottom course, send water outward. Inside, a sealed floor edge prevents any moisture that does enter from wicking into the framing. Keep the bottom of any wood sheathing a couple of inches above the finished grade outside and use materials that tolerate occasional wetting, such as pressure treated lumber for the bottom plate.

Raising the interior floor

Many conversions add a raised floor, which can hide the problem rather than solve it. If the new floor covers the slab edge, make sure the insulation continues to the wall and that nothing leaves a cold air channel along the base. Where a raised floor stops short of the wall by an inch or two for expansion, that gap should be covered by trim and backed with sealant or a backer rod, so air cannot move in the cavity behind the baseboard.

Checking the result

A thermal camera shows cold lines clearly, but there are simpler checks. On a cold morning, lay a hand along the baseboard at the old door line. A noticeable chill compared to the other walls means more sealing or insulation is needed. In humid weather, look for dark damp patches or condensation along the floor edge. A smoke pencil or incense stick held near the joint reveals draft direction. A blower door test, run by an energy auditor, measures the whole house leakage and can show how much improvement the work produced.

Why the old slope matters for sealing

Garage slabs are poured with a slight fall toward the door, so the gap that results under a new wall plate is often uneven, tight at one corner and wide enough to slide a finger into at another. Pack the wide sections with a firm backer material first and seal over it, since sealant alone will sag in a deep gap and eventually pull away.

Shims under the plate should be a solid, rot resistant material such as composite or plastic, not scraps of wood, and every shim should sit on a continuous gasket so there is no path for air around it. Taking the extra half hour to do this properly is much easier than chasing the draft after drywall and paint are finished.

Materials to avoid

Standard interior caulk is not enough for a joint between concrete and framing, because it cracks when the materials move at different rates. Choose polyurethane or a high quality hybrid sealant rated for exterior use. Avoid fiberglass batts stuffed loosely into the bottom of the cavity; they absorb moisture and settle. Skip expanding foam that is rated for large gaps in narrow cracks, since it can bow the wall.

Putting it in order

The best time to build a thermal break is before the wall goes up, when the slab edge is bare and accessible. Clean and repair the concrete, install a sill gasket and break strip, frame the wall, flash the exterior, seal every joint, then insulate the cavity. Doing the work in that order produces a conversion that is comfortable in January and July, and avoids tearing apart a finished wall to fix a draft that should have been handled at the start.