Quick answer
To find where air is coming from, close and lock the window, then check whether you feel moving air at the sash joint, the frame-to-wall edge, or somewhere else in the room. A paper strip can help identify a sash that does not press firmly against its weatherstrip, but it does not diagnose a hidden installation gap. A cold room without moving air may be caused by cool glass or an HVAC or insulation issue. Identify the symptom before choosing a repair or considering replacement.
Why a draft can be hard to locate
When one seat, desk, or bed near a window feels uncomfortable, it is natural to blame the glass. Sometimes that is the source. Air may pass through a worn weatherstrip, a sash that does not close evenly, or a gap around the installed frame. But air can also move from a nearby wall, ceiling, floor, outlet, or duct and collect near the window. A cool pane can make nearby air sink and feel like a draft even when outside air is not entering.
The distinction matters because the fixes differ. Weatherstripping is intended for moving parts such as an operable sash. A gap between the window frame and the wall is a separate air-sealing detail. Fog between two panes may point to a failed insulated glass unit, while room-side condensation may be related to indoor humidity and surface temperature. Replacing a window without identifying the source can leave the original comfort problem unresolved.
ENERGY STAR notes that drafts are often noticed around windows and doors, while significant air leaks can also be hidden in the attic or basement. In other words, a cold spot near a window is useful evidence, but it is not a complete diagnosis. ENERGY STAR’s guide to locating air leaks describes why air movement elsewhere in a house can affect comfort around windows.
Start with the location of the air movement
Choose a time when the window is closed and the symptom is noticeable. Do not test during a storm or when the glass or frame is wet. Move curtains, blinds, or furniture enough to see the joints; window coverings can trap air and hide where a sensation begins. If you have a fan or HVAC system running, note that too, because pressure and airflow can change as equipment cycles.
Move your hand slowly around the perimeter without touching sharp edges. Check the horizontal meeting rail where the sash closes, the sides of an operable sash, the corners, and the interior trim where it meets the wall. Then compare the same locations on a nearby window. A leak isolated to one sash suggests a different issue from a cold band that continues along the exterior wall or floor.
Avoid trying to diagnose a leak with a candle, lighter, or open flame. Air movement can be observed without introducing a fire hazard. Do not pry off trim, cut sealant, or inject foam into a window opening as a first step; hidden drainage paths and moving components can be damaged by sealing the wrong location.
Use a paper strip to check the sash contact
With a double-hung or other operable window fully closed and locked, place a narrow strip of paper between the sash and frame. Close the sash on it and gently pull. Repeat at a few points along the same joint and compare the resistance. If the paper pulls out much more easily at one spot, the sash may not be pressing evenly against the weatherstrip there.
This is a simple comparison, not a calibrated leakage test. Paper that slides at one corner can reflect a sash alignment, lock, hardware, or weatherstrip issue. It does not show whether air is moving through the wall around the rough opening. On some window designs, the test may not be useful or may be awkward; do not force the sash to make it work. ENERGY STAR describes a paper-strip check as one way to assess whether a closed window fits tightly, not as a substitute for professional evaluation.
Also check the lock. A sash that is closed but not fully engaged can leave a gap. If the handle or lock does not operate normally, stop forcing it and record the window type, the location, and what happens when you operate it. A damaged lock or operator may need attention before any seal material is considered.
Distinguish the sash from the frame-to-wall edge
The sash is the moving panel that holds the glass. Weatherstripping seals the moving joint between sash and frame. The window frame is the stationary assembly installed in the wall. Around that frame is the rough opening, which is the framed opening in the house structure. Air at the sash joint and air around the outside perimeter of the frame are not the same condition.
If you feel air along the inside edge of the trim, look for visible separation, cracked finish, or a stain, but do not assume the surface caulk is the whole air barrier. The Building Science Education resource from the U.S. Department of Energy explains that gaps between a window unit and the framing rough opening can allow uncontrolled air leakage and that the seal requires careful detailing. Removing trim can reveal the opening, but it can also damage finishes or expose materials that should be handled by a qualified professional.
Do not caulk a window shut, cover weep holes, or block any visible drainage openings. Windows manage both air and water. A bead of sealant in the wrong location can prevent drainage, interfere with operation, or conceal evidence of water intrusion. If the suspected gap is behind trim, ask for the perimeter and water-management details to be assessed rather than applying a surface patch blindly.
Check whether the sensation is actually moving air
Hold a lightweight tissue near the glass without touching it, then move it toward the sash and frame joints. A tissue that moves consistently toward or away from a seam can help you compare locations. But movement near the glass can also result from natural convection: air cooled by the pane becomes denser and sinks along its surface. That downward flow may feel draft-like even when the window is closed tightly.
Fog or moisture provides another clue, but its location matters. Condensation that wipes away from the room-side surface is different from persistent haze trapped between panes. Moisture between panes can indicate a failure in the insulated-glass seal; surface condensation alone does not prove a seal failure. Take a photo when the condition appears, note whether it wipes away, and check if it occurs at the same time of day or under certain indoor humidity conditions.
Compare the window symptom with other parts of the room. Is the floor cold across the whole wall? Does the problem change when the heating or cooling system starts? Is one supply register blocked by a rug or furniture? Does the room receive less afternoon sun than neighboring rooms? These details help separate a window-specific issue from an airflow or insulation issue elsewhere in the home.
A quick symptom-to-location guide
| What you notice | First place to compare | What the observation can suggest |
|---|---|---|
| Paper pulls out easily at one sash corner | Sash-to-frame joint and lock | Uneven sash contact, hardware, or weatherstrip condition |
| Air seems to come from trim-to-wall edge | Frame perimeter and nearby finish | Possible gap around the installed frame; needs appropriate inspection |
| Tissue moves down the glass, but not at the seams | Glass surface and room airflow | Cool-glass convection may be contributing to the sensation |
| Haze stays between panes | Insulated glass unit | Possible glass-seal failure; verify product and warranty details |
| Cold feeling continues away from the window | Wall, floor, ceiling, HVAC register | Another building-envelope or air-distribution source may be involved |
Use the table to describe what you observed, not to select a repair by yourself. Several conditions can happen at once. A window can have worn weatherstrip and a cold glass surface; a room can also have air leakage at the frame plus a separate HVAC imbalance.
Keep a short symptom log before requesting an estimate
For a day or two, note the room, the window, the weather, the time, whether the HVAC was running, and the exact spot where you felt moving air or saw moisture. Take a wide photo of the whole opening and close-ups of the lock, sash joint, and any visible damage. If the glass has a product label, photograph it without removing the sash or trim.
This record is useful when comparing observations with an installer or window professional. It can also prevent a conversation from jumping directly from “the room is cold” to “all the windows need replacement.” Ask the person evaluating the opening to explain which part they believe is responsible, what evidence supports that conclusion, and what other sources were considered.
What Van Nuys homeowners should keep in mind
Van Nuys sits in the inland San Fernando Valley. Warm afternoons, cooler evenings, and periods of winter rain can make different conditions noticeable at different times. If the symptom appears after rain, mention any water stain, peeling paint, swollen trim, or dampness as well as the draft. Water intrusion around a window is not the same problem as air movement, even when both appear near the frame.
American Reliable Windows & Doors is based in Van Nuys and lists window sales, replacement, and installation. Its published service information does not confirm standalone weatherstripping or insulated-glass repair, so call to ask whether the specific repair is offered. If the inspection points toward replacement, the company can discuss replacement-window options and a window estimate. The core page on drafty windows and failed seals in Van Nuys explains how to compare common symptoms and possible next steps.
Make the observation repeatable
Check the same joint at a similar time on more than one day if the sensation comes and goes. Wind direction, sun exposure, and the heating or cooling cycle can change how air moves around a room. Record those conditions beside the location instead of relying on memory. If one sash corner repeatedly feels different from the others, that is a more useful observation than a single general report that the entire room is cold.
What to write down before you request help
A useful note is specific: “I feel air at the lower-left sash corner when the window is locked” is more helpful than “the bedroom window leaks.” Record the date, time, outdoor conditions, whether the heating or cooling system was running, and whether the tissue or paper check changed at different joints. Take a photo of visible gaps, stains, or fog, but do not remove trim to get a better view. If more than one window is affected, compare the same joint on each one. These observations can help an evaluator focus the conversation, though they do not replace an inspection or prove the source by themselves.
Frequently asked questions
How can I tell whether air is leaking through a closed window?
Check for moving air at the sash joint and frame perimeter while the window is closed and locked. A paper strip can compare sash contact at several points, but it cannot reveal a hidden rough-opening gap or diagnose the whole house.
Can a cold window feel drafty even if air is not leaking?
Yes. Air cooled by a cold glass surface can sink and move down the pane, creating a draft-like sensation. Compare the glass with the sash and frame joints, and consider HVAC airflow and nearby walls before assuming outside air is entering.
Should I add caulk around the window frame?
Not until you know where the gap is and how the window drains. Caulk can interfere with operation or block drainage openings if applied in the wrong place. A visible crack at the trim is not proof that the rough-opening air seal is accessible from the surface.
When should I ask about window replacement?
Consider an evaluation when drafts persist, the window will not close or latch, the frame is damaged, or fog remains trapped between panes. Ask the evaluator to identify the source and explain whether a repair is possible before comparing replacement options.
Next step: describe what you found
Write down the window location, where you felt air, whether the paper-strip check differed by location, and whether the glass or frame showed moisture or damage. American Reliable Windows & Doors offers window sales, replacement, and installation in Van Nuys; call first to confirm whether a repair is within its current service scope. You can request a window estimate or call (818) 787-1101.
Sources
- ENERGY STAR: Locating Air Leaks
- U.S. Department of Energy: Rough Opening Flashing and Air Sealing
- U.S. Department of Energy: Air-Sealing Essentials
Elliot Berookhim
With over 15 years of industry experience, Elliot Berookhim specializes in residential window and patio door replacement throughout Los Angeles, California. At American Reliable Windows & Doors, he emphasizes quality materials, trained in-house installers, transparent pricing, and installation practices designed for long-term performance. His firsthand experience helps homeowners avoid common problems such as poor installation, premature window failure, and costly water leaks.
