Insulated garage doors and a sweating Tupelo garage

Why does my garage floor sweat in the spring?

Spring in north Mississippi runs a pattern that is hard on a closed garage, and March through May is when it shows. A cool night drops everything in the room to the overnight low. Then a south wind pulls warm Gulf air over Lee County by mid-morning, and that air works its way into the garage under the door and around the jamb, with a fresh load arriving every time the door goes up.

Warm wet air touching a cold surface gives up its water. That is all condensation is. The slab is the classic offender because concrete holds last night’s temperature well into the afternoon, so you get a floor that looks mopped when nobody mopped it. But the slab is not the only cold surface in the room, and it is usually not the one that does the most damage.

Why the water lands on the door before anything else

On a May morning the inside of the garage door has a dull film of water on it, heaviest on the lower sections, with a damp line on the slab just inside the seal where it has been running off all night. The drywall a foot away is dry.

A single-skin door is one layer of painted steel and nothing else. No liner, no core, no back. Whatever the temperature is outside before dawn, the inside face of that door is sitting right at it, and it stays there while the garage air warms up around it. So the wettest air of the day arrives against the coolest broad surface in the garage, hung vertically, which means whatever condenses runs down the panels and pools at the bottom section and along the seal.

An insulated door is built in layers: a steel skin outside, a core of polystyrene board or sprayed polyurethane, and a second skin or a vinyl back on the inside. The core keeps the outdoor temperature from reaching the inside face, so the inside face tracks the garage air rather than the overnight low. Water condenses on whatever is below the dew point of the air around it, and a panel sitting at whatever the room is sitting at rarely gets there. It is the same reason your insulated side wall stays dry on a morning the bare door is beaded up.

It also explains why a kit of foam panels pressed into an existing single-skin door can make the sweating worse. If the foam is not sealed at its edges, humid garage air slides behind it and condenses on steel you can no longer see or wipe, against the panel and the hinge hardware. A factory two-skin door has no gap to get behind.

What sweating actually costs you

A wet garage is a slow problem, which is why it gets tolerated for years.

• Cardboard is the first casualty. Boxes stacked on a slab that sweats wick water up through the bottom flaps, and mildew sets into the corrugations where you cannot see it until the box smells. Stored clothing and old paperwork are what people most often find ruined, usually the season they finally go looking for it.
• Hand tools and anything cast iron bloom orange. A table saw top or a socket set left open through a spring of sweating mornings picks up rust you then have to sand off.
• The door hardware takes it too. The bottom section and the cable brackets sit at the exact place the water runs to, and rust on cables and springs is the part that eventually becomes a repair rather than an annoyance. The storm-season maintenance guide covers how to read it.
• Drywall on the shared wall of an attached garage wicks along the bottom edge, and mold in that cavity is on the house side of the fire separation.

The attached garage is a different problem than a detached one

Most garages around Tupelo are attached, often with a bedroom or a bonus room over them. Two things follow from that.

The first is that a conditioned house leaks into an unconditioned garage. The shared wall and the ceiling between the garage and the room above are insulated, but the garage side of them still runs cooler than the outdoor air on a humid afternoon, and cool surfaces are where water lands. An attached garage with a bare door gets the worst of both. It is fed cool from the house side and humid from the outside, and the door is the surface in between with no resistance to either.

The second is that the door drives the temperature of that shared wall. In July the inside face of a single-skin door radiates heat across the garage into the wall your air handler is fighting, and in January it does the reverse. People notice the bonus room being the worst room in the house and start pricing ductwork, when the one uninsulated surface touching that space is a door.

Be clear about what the door does not do. An insulated door leaves the humidity in the room exactly where it was. What it changes is surface temperature. Take the cold face out of the room and the daily film on the panels generally stops. The swing across the shared wall narrows too, and some of what was landing on the slab never condenses at all.

Do you need an insulated garage door, or is a bare one fine?

Condensation only costs you if it lands on something you care about. Weigh the door against what is in the room.

If the garage is detached and what lives in it is yard equipment and a spare tire, condensation is not costing you anything you will notice. Metal spots, you wipe it, the slab dries by noon. Nothing out there is going to be worse in five years for having been damp through a spring. Leave the door plain and put the money into a bottom seal and a tune-up.

The picture changes when the room holds things that do not recover: a second refrigerator, boxes you plan to keep, tools with flat machined surfaces, a project car under a cover that traps what condenses on it. It changes again if the garage is attached with a room over it, or if anybody spends real time working in there. Those garages are where the insulated door pays for itself in something other than comfort.

Insulated garage door prices in Tupelo, next to the bare one

Around Tupelo a non-insulated steel two-car door installed runs $1,100 to $1,700. Put a foam core in it and the range moves to $1,500 to $2,600. Single-car doors are roughly 40 percent less either way. The cost page lists both of those next to every repair line, so you can see where a new door sits against fixing the old one.

The install itself is a half-day visit. The wait is in the order. Doors are built to your opening, so delivery is usually one to three weeks. If the sweating is what pushed you into this, order in fall or winter and have it hung before the spring pattern starts rather than in the middle of it.

One more thing before you commit to a whole door. If your current door is sound and only one section is damaged, section replacement runs $250 to $600, though a single insulated section on an otherwise single-skin door buys you nothing on condensation. The installation page walks through which of those two conversations you are in.

What helps before the door gets replaced

A dehumidifier in a garage with a gappy door runs constantly and never catches up, so start with the gaps. A bottom seal that has shrunk back from the corners lets in a surprising volume of humid air, and so does a missing or curled piece of jamb weatherstripping. Keep boxes and tools off the slab on shelves or pallets, because the air up on a shelf is drier than the air lying against the concrete. Resist the urge to hose the floor out in the spring. And if you are leaving on a humid morning, the less time the door spends open, the less of that air you invite in to condense.

Before you call, go look at the door on the next humid morning and note where the water actually is. Beads across the panels point at the door. A wet slab under a door whose panels are dry points somewhere else, usually the seal and the ventilation. That one observation decides whether a new door is the purchase that fixes your problem. Then call (662) 407-6162 and ask for both doors priced, insulated and not, on the same quote. That is a normal request and it takes one call.