If your window cracked in cold weather and nothing hit it, the cold alone is rarely the cause. What breaks glass on a winter morning is a temperature difference inside one pane: part of the glass warm, part of it cold, and the warm part trying to expand against cooler glass that holds it back. Glass makers call that thermal stress, and the shape of the crack usually tells you whether it is what happened.
This guide is for North Jersey homeowners looking at a fresh crack: how to read it, which winter conditions inside the house set it up, what to do tonight and what gets replaced, which is usually the glass and not the whole window. It also says when the cause is something else, such as a point of impact, a forced sash or a window installed only weeks ago.
Why a window cracks in cold weather: a temperature difference across the glass
Glass grows very little when it warms. Pilkington, a large flat-glass maker, notes that a 50 by 100 inch pane heated from 0 to 100 degrees Fahrenheit grows only to about 50.025 by 100.049 inches. The trouble starts when warm and cool areas of one pane try to grow by different amounts.
Pilkington's thermal stress bulletin works through the textbook case. The edge of the pane sits in the frame and stays cooler than the exposed center, the warmer center pushes outward, and the edge is stretched into tension of about 50 psi for every degree of difference. A 40 degree difference against a fully shielded edge works out to roughly 2,000 psi, and the bulletin says the probability of breakage becomes significant once edge stress passes about 2,000 to 3,000 psi, depending on how clean the cut edge is.
Those numbers are written for sunlit commercial glass, so treat them as an illustration of the mechanism, not a forecast for your house. The same bulletin lists heating vents blowing hot or cold air straight onto glass as a cause of excessive edge stress, which is the winter version most homes have. A window does not need to be very cold to crack this way. It needs one part of the pane to be much warmer or colder than the glass next to it.
Reading the crack: thermal, impact, pressure or bending
Each crack shape points to a different cause. Pella describes three; Cardinal, an insulating glass maker, adds a fourth.
- Thermal stress. A line that starts at the edge of the glass and runs inward, leaving the edge at roughly a right angle, with no center point. Cardinal says thermal break patterns generally occur at 90 degrees to the edge.
- Impact. A starburst of cracks radiating from one point where something struck. Cardinal adds that impact pieces can be small enough to fall out of the opening, which hides the pattern.
- Pressure. A curved crack Pella compares to an hourglass, in an insulating glass unit, often with no point of impact. Pella ties it to installation at too high or too low an elevation or to sharp swings in weather pressure; Cardinal calls it uncommon.
- Bending (racking). Cardinal's example is a casement forced open while one of its locks is still engaged, which bends the sash and the glass with it.
What sets up a thermal crack in a North Jersey winter
The same short list appears across the manufacturers' literature, and in January most of it is inside the house.
- A heating register or space heater aimed at the glass. Pilkington lists vents blowing hot or cold air directly against glass; Pella names HVAC vents and space heaters.
- Blinds, drapes, shades and stick-ons. Pilkington counts insulating blinds and heat-absorbing labels or decorations on the glass among the heat traps that can create larger temperature differences than ordinary annealed glass allows. Cardinal lists indoor shading, and Pella notes dark blinds or curtains add heat by absorbing sunlight.
- Part of the pane shaded, part in sun. For heat-absorbing glass, Pilkington reports that an overhang more than 3 inches deep casts a shadow that stays put and develops about 35 percent more stress than no shadow at all. It also describes snow collecting on a deep sill in front of sunlit heat-absorbing glass, chilling the lower part while the sun heats the rest.
- A flaw at the edge. Cardinal says scratches, chips and digs concentrate stress and that a thermal break can start at an edge flaw, which is why one window cracks while its twin does not.
- Ordinary annealed glass. Pilkington says heat-strengthened and fully tempered glass are generally strong enough to resist the thermal stresses typical of building windows; annealed glass is the type at risk.
Double-pane units: which pane cracked, and what happens to the seal
Check from both sides which pane cracked. A double-pane window is a sealed unit, and a crack through either pane opens that sealed airspace, so a unit that looked clear the day it cracked can fog weeks later. Foggy double-pane glass explains what fog between the panes means and why the sealed unit, not the whole window, is what gets replaced.
A crack is not something to watch for a season. Pella notes stress cracks may keep spreading as the glass expands and contracts, and Cardinal says any glass fractured in the field should be replaced. Photograph the crack with a ruler beside it to see whether it grows by morning.
Tonight: is it dangerous, and what helps
A cracked pane has lost strength. It may hold for weeks or release a piece, and you cannot tell which by looking, so treat it as glass that can come out.
- Turn the heat source away from the glass first: close or redirect the register, or move the space heater. The crack will not close, but you stop adding stress.
- Keep children, pets and furniture back from the window, and do not lean on it, slam the sash or force it open or shut.
- Do not count on tape; it does not give the pane its strength back. Cardinal warns that even film sold to hold broken glass in place cannot be relied on with every type of break.
- If pieces are loose or missing, do not push on them. Cover the opening with heavy plastic or cardboard fixed to the frame, not to the cracked glass.
- If there is an open hole, a pane moving in its frame or a ground-floor window that no longer secures the house, call for emergency glass repair instead of waiting for a scheduled visit.
Tempered, laminated and annealed glass break differently
Annealed glass is the ordinary kind, and it cracks in the single edge-to-center line described above. Cardinal describes tempered glass breaking into relatively harmless fragments with no jagged edges, usually from impacts such as lawn mower debris, stones or tools, so it leaves little pattern to read. Cardinal also notes tempered glass can, rarely, break on its own from a nickel sulfide inclusion. Laminated glass has a plastic interlayer that holds the broken pieces in place, so it can crack and stay in the frame.
The location decides what the replacement must be. Federal rule 16 CFR 1201 sets safety requirements for glazing in storm and combination doors, doors, bathtub and shower doors and enclosures, and patio-type sliding glass doors. New Jersey adopted the 2024 International Residential Code effective August 17, 2026, with a six-month grace period after adoption, and that edition moved the glazing section from R308 to R324. Its hazardous locations include glazing in walls or enclosures at tubs, showers, spas and pools whose bottom edge is less than 60 inches above the standing or walking surface (R324.4.5), and glazing whose bottom edge is less than 36 inches above stairs, ramps or their landings (R324.4.6).
If the cracked glass is in a door, a sliding patio door, a tub or shower enclosure or beside stairs, expect the replacement to be safety glass, and ask the glazier to confirm before anything is ordered. This is general information, not legal advice; your town's construction code office decides which rules apply to your job.
Fixing it: usually the glass, not the window
If the frame and sash are sound, the cracked glass is replaced on its own: a new sealed unit made to size in a double-pane window, or a new pane glazed into a single-pane sash. That is glass repair, not window replacement, and the Garfield glass shop page covers what to bring in or have measured.
The exception is a crack the frame caused. If a sash is racked, a lock binds or the frame has shifted and squeezes the glass, new glass in the same frame can crack the same way. That is window repair first, then glass.
Before you accept a quote, ask what the replacement glass is (annealed, heat-strengthened or tempered) and whether the location requires safety glass; Pella notes standard annealed glass may be more vulnerable than tempered or heat-strengthened. If the crack appeared soon after the window was installed, read the manufacturer's warranty before paying anyone, which is Pella's advice too.
Stopping the next crack
Prevention comes down to keeping the temperature even across each pane.
- Keep register and space-heater air from blowing straight onto the glass. A deflector that turns register air into the room is a small fix; moving ductwork is an HVAC job, not ours.
- Leave a gap between blinds or drapes and the glass so room air can reach it. A blind pulled tight to the pane is the heat trap Pilkington describes. If you added insulating shades for a draft, mount them so room air still reaches the glass.
- Be careful with retrofit film on double-pane glass, meaning film applied directly to the glass rather than the heat-shrink plastic sheet taped to the trim for drafts. Cardinal says reflective film on the indoor pane raises that pane's center temperature and significantly increases the potential for thermal breakage, and Cardinal will not warrant units that have retrofit film applied. If you are weighing window film, get the window maker's position in writing first.
- Have a pane with a visible chip or deep scratch near the edge replaced before a cold snap finds the flaw.
- Do not force a sash that is frozen or still locked; Cardinal's bending-crack example is a sash forced against an engaged lock. If a draft is why you want to cover the glass, fixing a drafty window covers what to try before replacing anything.
Sources (read October 20, 2026)
The documents behind the facts above. The Pilkington bulletin dates from 2013 and is written for commercial glass.
- Pilkington North America, Technical Bulletin ATS-123, Thermal Stress, January 14, 2013: https://www.pilkington.com/-/media/pilkington/site-content/usa/window-manufacturers/technical-bulletins/ats123thermalstress20130114.pdf
- Cardinal IG Company, Technical Service Bulletin IG24, Insulating Glass Breakage, December 2020: https://www.cardinalcorp.com/wp-content/uploads/2023/01/IG24_12-2020.pdf
- Cardinal IG Company, Technical Service Bulletin IG04, Use of Retrofit Films, August 2022: https://www.cardinalcorp.com/wp-content/uploads/2023/01/IG04_08-2022.pdf
- Pella, Why Do Windows Crack? Understanding the Three Types of Window Cracks: https://www.pella.com/ideas/windows/why-do-windows-crack/
- AIS Glass, Laminated Glass: https://www.aisglass.com/products-services/architectural-glass/laminated-glass
- 16 CFR 1201.1, Safety Standard for Architectural Glazing Materials, scope (Cornell LII): https://www.law.cornell.edu/cfr/text/16/1201.1
- Minnesota Department of Labor and Industry, 2024 IRC technical advisory group worksheet quoting R324.4.5 and R324.4.6: https://dli.mn.gov/sites/default/files/pdf/TAG-residential-090925-worksheet.pdf
- New Jersey Department of Community Affairs, adopted amendments to N.J.A.C. 5:23-3.14 through 3.22 (2024 model codes), 58 N.J.R. 8(2), effective August 17, 2026: https://www.nj.gov/dca/codes/codreg/pdf_rule_adoptions/model_codes_2024_adopt.pdf