Ice on the Inside of Your Windows: What It Means

Ice on the inside of your windows is one of the first things many Michigan homeowners notice when the cold sets in. A thin layer of frost forms along the bottom of the glass overnight, or a ridge of ice builds up on the frame. It can look alarming, but it is a message, not a mystery. Something is letting warm, moist indoor air meet a surface that is cold enough to freeze it. Figuring out which part of the window is the cold spot tells you what to fix.

Why ice forms on the inside of a window

Indoor air always holds some moisture from cooking, showers, laundry, houseplants and people breathing. When that air touches a surface below its dew point, the moisture comes out as water. When the surface is below freezing, the water turns to frost or ice. In a Metro Detroit January, the inside face of a window can get cold enough for that to happen, especially near the bottom edge and the frame.

So ice on the inside always comes down to two things working together: moisture in the air and a surface that is too cold. You can work on either one, and the best results usually come from working on both.

Three common causes

• High indoor humidity. If frost shows up on most windows in the house, and worst in kitchens, bathrooms and bedrooms, the air may simply be holding more moisture than your windows can handle in deep cold.
• Air leaks around the sash or frame. Cold outside air slipping past worn weather stripping chills one part of the window much more than the rest. Ice that forms in one corner or along one edge, often with a draft you can feel, points here.
• A failed seal on insulated glass. Double pane glass depends on a sealed space between the panes. When the seal fails, the glass loses much of its insulating value and the inside pane runs colder. Fog or moisture between the panes is the giveaway.

How to tell which one you have

Start by walking the house on a cold morning and looking at every window. Pay attention to where the ice is and how much there is.

1. Frost on many windows, spread fairly evenly along the bottom of the glass, usually means humidity is the main issue.
2. Ice in one spot on one window, such as a corner or the meeting rail of a double hung window, usually means air is getting in at that spot.
3. Fog, streaks or droplets between the two panes that you cannot wipe off mean the seal has failed. Our post on foggy window glass explains what that means and how it gets fixed.
4. Ice on the frame itself, especially on older aluminum frames, is often the frame conducting cold straight through to the inside.

Hold the back of your hand near the edges of the sash on a windy day. If you feel moving air, the seals around the window need attention whatever else is going on.

What you can do about humidity

Bringing indoor humidity down a little often clears up frost on windows that are otherwise in good shape. A few simple habits help:

• Run the bathroom fan during showers and for a while after.
• Use the range hood when cooking, especially when boiling water.
• Keep curtains and blinds open a few inches at night so warm room air can reach the glass.
• Check that a whole house humidifier on the furnace is not set too high for very cold days.
• Make sure dryer vents run outside and are not blocked.

Our earlier post on steps to control humidity in your home has more on this.

When the window is the problem

If humidity is under control and one window still ices up, the window is likely letting cold in. Worn or flattened weather stripping is the most common cause. New weather seals can make a window 20 to 25% more efficient, and replacing them is far less work than replacing a window. A sash that does not close tight, a lock that no longer pulls the sash together, or cracked caulk around the frame can all add to the problem.

A failed seal on insulated glass is a different repair. In most cases the glass unit itself can be replaced without replacing the whole window, which keeps the frame, trim and hardware you already have. Our glass replacement and weather stripping work covers both of these.

Is ice on the inside of windows harmful

A little frost on a very cold morning is not an emergency. The trouble is repetition. Every time the ice melts, the water runs down onto the sill and into the bottom of the sash. Over a few winters, that can lead to peeling paint, mold on the sill and, on wood windows, soft spots that turn into rot. If you see dark stains or soft wood where the melt water collects, read our post on wood rot on window sills.

Wipe up melt water when you see it, and do not chip at ice on the glass with anything sharp. Let the room warm up and it will come off on its own.

Getting it looked at

We work on windows in homes across Metro Detroit (Macomb, Oakland and Wayne Counties), from older wood windows in Detroit to newer vinyl in Shelby Township and Sterling Heights. If you have a window that ices up no matter what you do, we can take a look, tell you whether it is the seals, the glass or something else, and give you a free quote on fixing it. Our work comes with a 5-year workmanship warranty and a 10-year glass warranty.

Request a free quote and include a photo of the iced up window if you can.