Why Frost Forms on the Inside of a Garage Door in Marquette

The first hard cold of the season tends to show up in the garage. You open the house door on a dark morning, flip the light on, and the whole inside face of the garage door is furred white while the walls around it stay dry.

That frost is water, and nearly all of it came into the garage with you, most of it on the car. What the door does with it, and where on the door it lands, says a fair amount about the door itself.

Why is there frost on the inside of the garage door in the morning?

The air in an attached or heated garage holds moisture, and overnight it settles on the coldest thing in the garage. On most older doors that is the inside skin of the door, a thin sheet of steel sitting close to the outdoor temperature with little or nothing behind it. The water condenses there and freezes. That frost is one of the plainer reasons people start asking about garage door installation in Marquette.

The framed walls stay clear. They have insulation and drywall between the garage and the weather, and an old door is a single steel skin. It is the thinnest wall in the garage, so it frosts first, and on a hollow door it frosts edge to edge.

Where does all that water come from in a garage?

From outside, carried in by whoever and whatever comes through the door. A garage in Marquette County takes on a lot of water on a snowy day, and almost none of it is from a leak:

• snow packed on the roof of the car and in the wheel wells after a lake-effect afternoon, melting off all evening
• slush off boots, shovels and the dog
• the snowblower, parked back inside wet after the driveway
• warm, damp air from the house every time the door to the kitchen opens
• the heat itself, in a garage that is kept warm and so holds more moisture in the air than a cold one

Brushing the snow off the car before pulling in cuts down the water more than anything done to the door. The rest of the water is a house and garage matter that no door work will touch.

On a morning after a dry, clear day the frost is thin or missing. After a day when the car came home under a load of lake snow, it is thick enough to scrape.

Why does the frost collect along the seams and edges first?

Two different things put frost on a door, and they leave different marks. An even sheet across the flat faces of every panel means the steel itself is cold, with nothing behind it to keep the inside face near the garage’s own temperature. Heavier lines of frost along the joints between sections, up both sides where the door meets the jambs, and across the top under the header mean outside air is getting in.

Those lines follow the weatherstrip. The rubber along the jambs and header and the seals between sections dry out and crack over enough winters, some pulling away from the steel, and each gap lets a thin stream of cold outside air across the inside of the door. The steel on either side of that stream runs colder than the rest of the panel, so the frost stacks up there first and thickest.

Plenty of doors show both. Frost on the window glass before anything else is normal on any door with windows and says little about the rest of it.

Where does the water go when the frost melts?

Down. On a thaw day, or once the sun gets onto the outside of the door in the afternoon, the frost turns to water and runs down the inside face in streaks, collecting along the ribs and joints of each section on its way to the bottom. It ends up on the bottom section and in the bottom seal at the slab.

From there it causes two problems, both of them on the bottom of the door. The water in and under the bottom seal refreezes when the sun goes down, and the next morning the door is bonded to the concrete, the situation covered in the piece on a garage door frozen to the floor. The water that collects on the inside face of the bottom section lands right where the slush line leaves its band of rust, the one in the U.P. winter maintenance guide, and keeps that steel wet through the season.

On an older wood door the same meltwater soaks into the bottom rail instead, which is how those rails go soft.

What does a foam-core door change on the inside face?

It moves the cold surface somewhere else. A door with an injected polyurethane core has foam filling the space between the outer and inner steel skins, with a thermal break so the two skins are not joined by solid metal. The inside skin stays near the garage’s own warmth rather than the outdoor cold, so the moist air in the garage no longer finds its coldest surface on the door. The frost that used to cover the panels mostly stops forming there.

New perimeter weatherstrip and new seals between the sections close the lines of frost along the joints and jambs. A new door comes with both, and on a door that is staying, worn weatherstrip is a repair we do on its own.

What a new door does not change is the water. The car still brings in the same snow, and the moisture still settles on whatever is coldest. In a garage with a foam-core door that tends to be the window glass, or the bare steel of the tracks and hardware along the sides.

Is a frosting door a reason to replace it before spring?

Sometimes. On an attached or heated garage, a hollow or thin steel door that frosts over every cold morning and is already rusting along the bottom section is the weak wall of the garage. It lets the heat out and feeds the ice at the slab all winter, and that door is worth replacing. An insulated single-car door runs $1,200 to $2,500 installed here and a double $1,800 to $4,500, with tracks and springs included.

A door on a detached, unheated garage that frosts but runs well and shows no rust at the bottom is not a reason on its own.

Whether to do it now or once the snow is gone is its own question, and there is a separate piece on timing a new door around the first snow and the spring thaw.