How to tell if your windows are costing you money — room by room, right now.
July 2026
You don’t need an energy auditor. You don’t need special equipment. You need five minutes, your own senses, and this guide.
Windows account for 25-30% of residential heating and cooling energy use, according to the U.S. Department of Energy. But most homeowners have no idea whether their specific windows are contributing to that problem — or how significantly.
This audit changes that. Walk through it once, mark your answers, and you’ll know exactly where you stand.
BEFORE YOU BEGIN
What you’ll need:
- Your hand (or a lit candle)
- A thermometer (your phone’s weather app works for context)
- Five minutes per room
- Natural daylight (do this between 10 AM and 3 PM)
Where to start: Begin with the room that feels least comfortable in summer. That’s almost always where the most significant window issue lives.
CHECK 1 — THE FOGGING TEST
Stand in front of each window. Look carefully between the panes.
What you’re looking for: Cloudiness, fogging, or visible moisture trapped between the layers of glass.
☐ Clear glass, no fogging between panes
☐ Slight haziness or discoloration between panes
☐ Visible moisture or condensation between panes
What it means:
A fogged double-pane window has a failed seal. The insulating gas that was helping that window perform has leaked out, and humid air has replaced it. The window is now performing closer to a single-pane unit — with significantly less thermal protection.
This isn’t a cleaning problem. It can’t be fixed by wiping the glass. The seal is gone.
Score this check: Clear = 0 points. Slight haze = 1 point. Visible moisture = 2 points.
CHECK 2 — THE DRAFT TEST
Close all windows and doors in the room. On a warm day, hold your hand slowly along the frame perimeter — the edges, corners, and the gap between frame and wall.
What you’re looking for: Any movement of air across your skin. Even subtle movement counts.
☐ No air movement detected anywhere along the frame
☐ Slight air movement at one or two points
☐ Noticeable draft at corners or frame edges
What it means:
Air leakage around window frames is one of the most direct ways your home loses conditioned air. The DOE estimates that air leaks through gaps around windows and doors contribute meaningfully to heating and cooling loss. Even a small, consistent draft represents conditioned air escaping — and hot or cold outdoor air replacing it.
A candle held near the frame works well if your hand isn’t picking up subtle movement. Watch for the flame to waver.
Score this check: No draft = 0 points. Slight movement = 1 point. Noticeable draft = 2 points.
CHECK 3 — THE HEAT ZONE TEST
Stand one foot from the window. Then walk to the center of the room. Notice any difference.
What you’re looking for: A distinct temperature change as you move away from the glass — warmth radiating toward you near the window in summer, or cold radiating toward you in winter.
☐ Room feels consistent — no noticeable difference near glass
☐ Slightly warmer or cooler near the window
☐ Distinct and uncomfortable temperature difference within a few feet of the glass
What it means:
This is the solar heat gain test. When windows allow significant solar radiation through, the surfaces near them heat up — and you feel it as radiant warmth. If standing near a closed window feels noticeably different from standing across the room, the glass is transmitting meaningful heat load.
In summer, this is the primary driver of rooms that “never cool down.” The AC removes air temperature, but the glass keeps radiating heat from the sun into the space.
Score this check: No difference = 0 points. Slightly warmer = 1 point. Distinct difference = 2 points.
CHECK 4 — THE GLARE TEST
At peak afternoon sun (2-4 PM), sit or stand where you normally would in this room. Look at any screen — TV, computer, phone — or try to read near a lamp.
What you’re looking for: Visible glare, eye strain from brightness near windows, or screens that are hard to see.
☐ Room is comfortably lit — no glare on screens or eye strain
☐ Some brightness or glare that’s noticeable but manageable
☐ Glare makes screens difficult to see or creates eye strain
What it means:
Glare isn’t just a comfort issue. It’s a signal that unfiltered solar radiation is entering the space at high intensity. The same light that washes out your TV screen is also generating heat and UV radiation.
If you’ve developed a habit of closing blinds during certain hours — even partially — that habit is acknowledging a solar control problem, even if you haven’t named it that way.
Score this check: No glare = 0 points. Noticeable but manageable = 1 point. Screen-obscuring glare = 2 points.
CHECK 5 — THE FADING INDICATOR
Look at the floor, furniture, and any fabric within four feet of the window. Then compare to the same items toward the center of the room.
What you’re looking for: Color differences, fading patterns, or visible discoloration on items that have been in consistent sun exposure.
☐ No visible fading difference between window and room areas
☐ Slight lightening or color shift near the window
☐ Obvious fading, discoloration, or “ghost” outline where a rug or furniture sat
What it means:
Fading is UV radiation made visible. Standard window glass allows 50-70% of UV-A radiation through — the wavelength responsible for breaking down pigments in floors, fabrics, and furniture over time. If you can see fading near your windows, UV damage has been accumulating — silently — for years.
The materials that fade fastest are natural fibers (wool, silk, cotton), hardwood floors, and leather. The damage is permanent without refinishing or replacement.
Score this check: No fading = 0 points. Slight shift = 1 point. Obvious fading = 2 points.
YOUR RESULTS
Add up your total points from all five checks for this room.
0 – 2 points Your windows are performing reasonably well. No immediate red flags. If you notice seasonal discomfort that doesn’t correspond to these checks, the issue may be HVAC-related rather than window-related.
3 – 5 points Your windows have measurable performance gaps. You’re likely feeling this in your energy bills and comfort levels, even if you haven’t connected it to the windows specifically. The most impactful improvement depends on which checks scored highest: seal failures call for window repair or replacement, while heat gain and UV issues are addressable with solar control film on existing glass.
6 – 10 points Your windows are a significant source of energy loss and discomfort. This room is working against you — your HVAC is compensating for ongoing heat gain, UV damage is accumulating on your interiors, and you’ve likely adapted your daily habits around the problem without realizing it. Addressing the windows here would have a meaningful impact on comfort, energy costs, and long-term interior protection.
WHAT TO DO WITH YOUR RESULTS
If you scored highest on Check 1 (fogging): Failed seals require either insulated glass unit replacement or full window replacement. Window film is not the solution here — the structural issue needs addressing first.
If you scored highest on Checks 2 (drafts): Weatherstripping and caulking are the first step. These are often inexpensive DIY repairs that meaningfully reduce air leakage.
If you scored highest on Checks 3, 4, or 5 (heat gain, glare, fading): These are all solar radiation problems — the same root cause, showing up in different ways. Solar control window film applied to existing glass addresses all three simultaneously: reducing heat gain, cutting glare, and blocking 99%+ of UV radiation without replacing the window or sacrificing natural light.
Repeat this audit in every room that concerns you. The patterns across rooms often reveal which windows need attention first — and what kind.
Get a Professional Assessment
Self-audits identify the symptoms. A professional assessment identifies the cause and the most effective solution for your specific windows, climate, and goals.
CoolVu offers free residential assessments that evaluate solar heat gain, UV exposure, and film options for your existing glass.
Find your local CoolVu installer: www.coolvu.com
Five minutes per room. That’s all it takes to know where your home stands.
