5 Signs of Drafty Windows (and How Curtains Can Fix Them)

Summer should feel bright, not blistering. If certain rooms turn into hot zones and your AC can’t keep up, you may be dealing with classic signs of drafty windows—problems that also show up as heat gain, glare, and longer cooling runtimes. This guide explains how to confirm the issue with simple at‑home checks and how to use heavy, well‑installed curtains to reduce heat and improve comfort, with renter‑friendly options along the way.

Quick diagnostic setup for summer

Before you start, gather a few basics and plan 15–30 minutes per window. Keep safety first and avoid open flames near drapes or finishes.

• Tools: incense or a smoke pencil, an infrared thermometer, a tape measure, painter’s tape or a marker for notes, and a flashlight for daylight‑bleed checks.

• Conditions: test during a hot, sunny period or when there’s a temperature difference between indoors and outdoors. For smoke testing, run the HVAC fan on for a couple of minutes to lightly pressurize/depressurize the home.

• Safety and method references: The U.S. Department of Energy’s contractor training materials outline safe envelope diagnostics, including IR scanning and air‑leak checks; see the 2024 guidance in the DOE Weatherization training overview. For the potential impact of window attachments in summer, Pacific Northwest National Laboratory summarizes DOE field results in the Window Attachments for Solar Control and Energy Efficiency guide.

Sign 1 — Smoke shows air movement at the frame and sash

If you hold a steady smoke source 1–2 inches from window seams and see it pull, push, or flicker consistently, you’ve spotted a leakage pathway. In summer, these pathways feed convective loops that move hot air around the window area and make nearby seating feel stuffy.

How to test: Close windows and interior doors. Turn your HVAC fan to On for two minutes. Move the incense or smoke pencil slowly along the weatherstripping, locks, meeting rails, and trim lines. When smoke deflects, mark that spot with painter’s tape so you can seal it later. Keep the flame away from fabrics; a smoke pencil is safest.

How to fix now: Add low‑residue foam weatherstripping at sash stops, tighten or replace worn gaskets, and use removable bottom draft stoppers on deep sills. For the covering, choose rods that return to the wall or wrap around at the sides so your curtain panels sit close to the wall and block edge leakage.

Why curtains help: Dense, lined drapes disrupt the convective loop behind the fabric and reduce air exchange at the edges. Their effect scales with coverage and sealing at the sides and bottom, so installation details matter.

Sign 2 — An IR thermometer shows hotter glass and frames than nearby walls

Point an infrared thermometer at the center of the interior glass, then at the edge, the frame, and an interior wall 6–12 inches from the frame. If the glass or frame reads about 5–10°F or more above the nearby wall during peak sun, that signals significant solar heat gain and weak thermal performance.

How to test: Stabilize indoor conditions and avoid direct supply air. For more reliable readings, place a small square of black electrical tape on the measurement spots to normalize emissivity, then measure each spot in quick succession and take an average. Repeat once or twice to confirm the pattern. The DOE’s training overview covers envelope scanning techniques, while PNNL offers surface‑comparison fundamentals in its infrared R‑value tool notes.

How to fix now: Close heavy, blackout‑lined drapes before peak sun hits the window. The goal is wall‑to‑wall coverage, near‑ceiling to floor, with tight side returns and a slight “kiss” at the floor to minimize bottom gaps. If allowed, layer a cellular or Roman shade behind the drapes for added solar control; PNNL’s attachment guidance explains how operation timing and layering reduce summer loads in practice in the window attachments guide.

Why curtains help: Velvet with blackout lining limits transmitted light and radiant heat, and when fitted with returns and bottom contact, it also slows the warm air loop that forms behind standard drapes.

Sign 3 — Daylight bleeds around the edges of your current coverings

Stand in the room at mid‑day with lights off. If you can see bright lines at the sides, top, or bottom of a blind or curtain, you’re witnessing uncontrolled solar entry, which also drives glare and localized overheating. As a rule of thumb, gaps around 1/8 inch or more are worth sealing.

How to test: Measure the visible light gaps at the sides, top, and bottom. Note the largest points and whether the window faces south or west, where afternoon sun can be especially intense.

How to fix now: Specify hardware and panels to close those gaps. Wraparound or return rods bring the fabric back to the wall at the sides; extend rods 6–15 inches beyond each side so panels can stack without uncovering glass; and add a center overlap of about 2–4 inches where the two panels meet. Mount higher to reduce the top gap and allow the hems to “kiss” the floor so the bottom gap disappears. For renter‑friendly sealing, use magnetic or hook‑and‑loop side tracks that can be removed cleanly. For measuring and hardware choices, see the Freshine guides on how to measure window treatments and the curtain hardware guide for wraparound rods and bracket spacing.

Why curtains help: Edge control is everything. A well‑sized blackout drape with true returns and a slight floor contact can transform a bright, hot perimeter into a calm, shaded surface with reduced convective spill.

Sign 4 — Persistent fogging between panes or edge condensation

Moisture trapped between panes typically means a failed insulating glass unit seal. While it shows up more in cool seasons, it’s a year‑round red flag for thermal underperformance and often correlates with summer overheating near that window.

What to do: Plan for window repair or replacement—curtains can’t fix failed seals. As an interim step, operate coverings tactically: close lined drapes and shades before the sun hits, then open at night if outdoor air is cooler. For context on how attachments improve comfort and energy use when operated well, see PNNL’s overview of window attachments and solar control.

Sign 5 — AC runs longer and bills climb in rooms with sun‑exposed windows

If your cooling cycles stretch and utility costs jump during heat waves—and the hot, uncomfortable spots are near the windows—you’re likely fighting solar gain and edge leakage. Don’t guess. Log it.

How to test: Note window orientation, time of day, outdoor temperature, and whether coverings were open or closed. Take quick IR readings at the glass and the adjacent wall, and repeat the daylight‑bleed check. Change one variable at a time—such as closing drapes two hours earlier—and observe whether AC runtime shortens. Energy Saver resources from the Department of Energy outline basic draft‑finding approaches for homeowners; see DOE’s “How to find drafts” overview for additional context.

How to fix now: Combine operations and hardware. Close heavy, lined drapes before peak sun, layer a shade where allowed, and seal the sides and bottom. You’ll feel the difference first as reduced glare and a calmer air column by the window, then as shorter AC cycles.

Get more from velvet blackout drapes by installing them like a pro

Here’s the deal: curtains only perform as well as they’re installed. To turn velvet blackout panels from decor into a summer comfort tool, focus on coverage, edge control, and support hardware.

• Returns to wall: Use wraparound or return rods so the fabric touches or nearly touches the wall at the sides. Typical return depth is 3–6 inches depending on casework and trim.

• Center overlap: Size a 2–4 inch overlap where two panels meet to minimize the bright seam.

• Floor contact and puddling: Aim for a gentle “kiss” at the floor. A slight puddle can help close micro‑gaps on uneven floors, though it adds cleaning needs. Keep hems even to avoid light leaks.

• Extend and mount high: Mount near the ceiling and extend rods 6–15 inches beyond each side so panels clear the glass when open but cover fully when closed.

• Hardware capacity: Velvet blackout panels are heavy. Choose sturdy rods or tracks with appropriate bracket spacing. The Freshine hardware guide explains when wraparound rods and extra brackets are needed. For crisp folds that hang straight and help the edges lay flat, consider heat‑setting; see Freshine’s thermal memory shaping overview.

• Renter adaptations: Use removable magnetic or hook‑and‑loop side channels and a low‑profile sill draft stopper. For large, heavy panels, avoid tension rods due to sag risk; reserve them for lighter treatments.

Practical example with disclosure: A custom pair of blackout‑lined velvet panels sized wall‑to‑wall with 4‑inch returns, a 3‑inch center overlap, and floor‑kiss hems can substantially reduce side light and slow the convective loop you feel as a hot draft near the window. See what this looks like with options from Freshine Curtains. Disclosure: Freshine Curtains is our product.

Quick reference

Below is a snapshot of common tasks, time, difficulty, and typical DIY cost bands. Use it to plan your weekend and set expectations.

For measurement fundamentals and where to place returns, review the Freshine how to measure guide.

What kind of improvement should you expect

Curtains aren’t a silver bullet, but they’re a powerful part of a window‑attachment strategy. Field studies summarized by PNNL report that well‑operated, high‑efficiency interior attachments can yield roughly 15–25% whole‑home summertime cooling savings, depending on the product and usage. See PNNL’s window attachments summary of DOE field results. Heavy, blackout‑lined curtains use the same principles—blocking light, slowing radiant transfer, and interrupting convective loops—but their impact depends heavily on coverage and edge sealing. Layering a cellular or Roman shade behind the drapes can push performance further in sun‑exposed rooms.

When to call a pro: If smoke testing reveals widespread leakage, if panes show persistent fogging, or if hardware spans are wide with heavy fabrics, bring in a window contractor or an experienced installer. They’ll help with sealing strategies, safe anchoring, and achieving the tight edge control that makes the biggest difference in summer.

Next steps

• Confirm the signs with smoke, IR, and a daylight‑bleed check.

• Decide where velvet blackout drapes plus edge sealing will make the most impact, then measure for returns, overlap, and a floor kiss.

• Operate smart: closed before peak sun, open when nights cool off.

Want an example of how custom sizing and hardware choices translate into better edge control and less heat spill? Explore velvet options and installation guidance with Freshine Curtains. Disclosure: Freshine Curtains is our product.