The first time my window actually dripped was at 2:13 AM on a Wednesday, sometime around mid-February. I remember because I was half-asleep on the couch with a laptop balanced on the arm, and one drop fell on the keyboard. The trackpad stopped responding before the screen did. That same window had been "fogged" for the previous six weeks and I'd been ignoring it.
After three conversations with property managers, $42 worth of hygrometers, and one failed experiment with a trash bag taped over the window (yes, really), I learned four things that the building's FAQ sheet never covered. I'm not an HVAC tech — I'm a tenant. But this is the playbook I wish I'd had in week 1, not week 6.
The window that killed my laptop. Interior pane temp dropped below 4 °C during a 14 °F overnight.Step 1: The 4 Actual Causes (Not the One You Think)
Before you buy anything, identify which cause is your window's problem. Most articles treat condensation as one thing. It's not. A 1908 walk-up with single-pane glass has a totally different culprit than a 2018 condo with a failed argon seal. The fix matches the cause, so let's start there.
Cause 1 · Cold interior pane (the most common)
Glass is a poor insulator. Interior pane temperature in winter drops to within 2-4 °C of the outdoor air. When that interior surface is colder than your room's dew point, water condenses on the glass. The dew point is the temperature at which the air can no longer hold its moisture. If your room is 21 °C and 40% relative humidity, your dew point is around 7 °C — well below the 9-13 °C interior pane temperature of any single-pane glass in a Boston February. So moisture precipitates.
Cause 2 · Indoor humidity sources
This is the one most articles skip. Cooking generates 0.5-1.5 L of water vapor per meal. A 10-minute shower adds another 0.2 L. Drying clothes indoors adds 1-2 L per load. If your kitchen has no exhaust fan (or you never turn it on), every dinner raises your apartment's humidity by 5-8% — enough to push a borderline-room over the dew point during cooking hours. A 24-hour humidity log is what unmasks this.
Cause 3 · Glazing failure (rarer but expensive)
Double-pane units have argon or krypton gas between the panes. After 15-25 years, the seal fails; gas escapes; moisture infiltrates. You'll see condensation between the two panes (not on the inside surface), and the fog never wipes away. That's a landlord problem — replacing the sealed unit is $250-450 per window — and not something to DIY.
Cause 4 · Apartment-level factors (north-facing, top floor, 1908 build)
Older buildings and north-facing windows are physics-disadvantaged. A fourth-floor walk-up in Boston has roughly 30 % higher window condensation than a south-facing unit on the second floor of a 2010 condo, due to lower sun-warming and more exposure to prevailing winds. If your window is top floor, north, or pre-1950s, your starting point is worse and your fixes need to be more aggressive.
The diagnosis matters. Running a dehumidifier (Cause 2) won't fix a failed window seal (Cause 3). Plastic film (Cause 1) won't do anything if the problem is your bathroom ventilation. Pick the cause before picking the fix.
A $7 hygrometer is the single best money you'll spend on this problem.Step 2: Free Fix — The 15-Minute Humidity Audit
Buy a $7 hygrometer. Put it on the windowsill or a bookshelf three feet from the window. Read it 4 times a day for 3 days: 8 AM, 12 PM, 6 PM, 11 PM. Write the numbers on a sticky note. You're looking for two things.
- If your overnight reading (11 PM to 8 AM) stays above 50 % relative humidity, your humidity source is unvented cooking, shower steam, or indoor drying. That points at Cause 2, not Cause 1.
- If the humidity stays below 40 % but you still have visible moisture on the glass, the cause is Cause 1 (cold pane). Most US and Canadian homes run 25-35 % humidity in winter — below ASHRAE comfort guidelines but above condensation thresholds on single-pane glass.
The fix for Cause 1 is to raise the interior pane temperature. The fix for Cause 2 is to lower the indoor humidity. They're opposite interventions. The hygrometer tells you which one to do. Don't guess.
Step 3: $0 Fix — The Plastic Film + Hair Dryer (Yes Really)
The cheapest, fastest fix is a $0 plastic sheet taped to the window frame and heat-shrunk with a hair dryer. It looks ridiculous. It works. This is the only fix where I've done a side-by-side: one window with film, one without, same night, same temperature. The filmed window stayed dry; the unfilmed window had 1.5 cm of moisture beads by morning.
Buy a $5 window insulation kit from any hardware store (3M, Frost King, Duck, generic — they're all the same film). Cut the film 2 cm larger than the frame on all sides. Tape the edges with the included double-sided tape. Apply the film. Pull it taut. Then heat-shrink with a hair dryer on medium, working from the center out. The wrinkle-free result raises the interior pane temperature by 2-4 °C, which puts most rooms above the dew point.
This isn't pretty. You can see through it, but it has a slight frost appearance in daylight. If you're in a south-facing unit where you want light: skip this and use the curtain method. If you're in a north-facing unit where you want sleep more than light: this is the best 15-minute fix.
The first time I did this I taped half the drywall border by accident — pulling it off two months later took a piece of paint with it. Learn from my mistake. Keep the tape on the window frame, not on the wall.
Step 4: $7-12 Fix — Caulk + Weatherstripping
Skip the $25 shrink-window kits with weatherstripping. They include things you don't need. Buy one tube of clear silicone caulk (about $7) and walk around the apartment. Run a finger along every place the frame meets the wall. If you feel cold air coming through a gap, caulk it. While you're at it, check the sash lock on double-hung windows — if it doesn't fully compress, that's why cold air is reaching the interior pane.
One renter I've advised on this learned the hard way: don't caulk a window shut if you might need to open it. Some buildings require egress. Test the window in every direction before sealing.
A candle on the windowsill is the cheapest draft detector you'll ever own. If the flame bends, you've found a leak.Step 5: $20-50 Fix — Dehumidifier (Skip the $200 One)
A small dehumidifier is the only fix on this list that uses electricity. For a 1,200 sq ft apartment with one west-facing bedroom and one north-facing office, the Waykar 35-pint at $129 pulls 1.5 L/h on the high setting and uses 280 W — about 4 cents per hour. The Pro Breeze 1,000 mL mini dehumidifier at $49 pulls 0.2 L/h and covers 150 sq ft — enough for one bedroom. For a one-window condensation problem, the Pro Breeze is enough. For a whole-apartment moisture problem, the Waykar.
What to skip: any $200+ dehumidifier. The Midea 50-pint at $219 has slightly higher capacity but a noisier compressor (53 dB vs the Waykar's 47 dB). The next step up in dehumidifier land is $300+ units with pumps and drains. You don't need any of that for one winter.
One note for smart-home nerds: plug the dehumidifier into a smart plug ($10-15) and schedule it for humidity above 45 %. That keeps the room in the ASHRAE comfort range without running the compressor 24/7.
The Waykar 35-pint in my office. Pulls 1.5 L/h on the high setting and doesn't wake me at 47 dB.Step 6: What Renters in the UK (and Europe) Do Differently
I tested these fixes in Boston. A London renter reading this in a 1930s flat is dealing with a different window: sash windows with original counterweights and paint-stuck cords. The plastic film I described in Step 3 doesn't work as well because sash frames have thinner lip and tighter tolerances. What works in London: bristle draught strip (about £3 for 5 m), pushed into the gap between the upper and lower sashes, plus self-adhesive foam tape on the interior sill. Total cost about £7. Total install time: 30 minutes.
The bigger UK-specific issue is condensation and mould in cavity-wall voids, which is actually a building-code issue for older British housing stock. If your condensation comes with black spots on the wall (not just the glass), you may be looking at penetrating damp rather than interior condensation, and that's a landlord obligation under Section 11 of the Landlord and Tenant Act 1985 (in England and Wales). Document with photos and email your landlord citing the act.
For the Canadian case: Ontario's RTA doesn't have a specific condensation clause, but the residential tenancies act requires landlords to maintain building envelope integrity. Renter-side practice is the same as US approaches above. For Quebec (where heating is often electric baseboard), the interior humidity runs 18-25 % in winter — too low for any condensation. If you live in Quebec and have condensation, it's almost always the seal on the unit, not the humidity. Get the landlord to test it.
A typical sash window. Plastic film doesn't seal well here; bristle strip works better.Step 7: When to Call the Landlord (3 Triggers)
Two of the fixes above are free or near-free; the others cost less than $130 total. None of them apply if the problem is the building, not the renter. Three times to stop and call the landlord instead.
- Moisture is between the panes, not on the inside surface. This is a failed seal. Don't buy a dehumidifier for this; the building pays for the seal replacement.
- Black spots appear on the wall, not the window. That's mold. In the US, every state has different mold-disclosure rules. In Ontario, take photos and email your landlord citing "maintenance of unit as fit for habitation." The Ontario Standard Lease Schedule 1 says units must be free of mold at move-in. If mold shows up during tenancy, it's the landlord's obligation to test and remediate.
- The frame is cracked or the sash is rotted. If you can see daylight between frame and wall, that's structural. Stop caulking; start emailing. A 1 cm gap for one winter will cost you $400 in heat; it's their problem to fix the frame.
If the issue is none of those three — just condensation on the inside of an old single-pane in winter — your fixes will work. And your laptop will survive next March.
The same north-facing window in late March — dry, no plastic film, 28 % indoor humidity.FAQ: Window Condensation in Apartments
Why does my apartment window only fog in winter, never in summer?
The interior pane of your window drops to the dew-point temperature in winter, when outdoor air is much colder than your room. In summer, the pane stays closer to room temperature because the outdoor air is also warm. Condensation needs the surface temperature below the dew point, which only happens when outdoor temps drop below roughly 5 °C. If you have single-pane windows, you'll see this every winter; double-pane will delay it by 5-10 °C of outdoor cooling.
Is moisture on my windows actually dangerous for paint or wood frame?
Yes, on two counts. Repeated wetting of paint (especially latex) causes it to peel within 2-3 years if not addressed. On wood frames, the saturation will grow mold at the window sill and the lower edge of the frame within 6-12 months in apartments with humidity above 50 %. The mold health risk is real for asthmatics. The fix is to lower the source of moisture (cooking/exhaust, shower, drying) and stop the condensation cycle, not to repaint over it.
My landlord told me to "just wipe it down." Can I seal the window myself without permission?
Yes, in the United States. Tenant-rights laws vary by state, but in all 50 states a tenant has the right to take reasonable measures to prevent damage to their own unit. Caulking gaps, applying plastic film, and using a dehumidifier are routinely classified as routine maintenance and don't require landlord approval. What does require approval: replacing the window, modifying the exterior, or installing anything that requires drilling. The Landlord-Tenant Act in your state (or Residential Tenancies Act in Ontario / Section 11 LTA 1985 in England & Wales) is on your side for routine sealing. If your landlord objects after the fact, cite that.
What's actually different between interior window film and exterior caulk?
Interior film (the $5 plastic + hair dryer setup) creates a still-air pocket between the film and the glass. That pocket is what raises the interior pane temperature by 2-4 °C. Exterior caulk seals the gap between window frame and exterior wall, stopping cold air infiltration from outside. They're solving different problems. If you have a cold pane, use interior film. If you have a draft (a candle test will tell — Step 4), use caulk. Doing both is fine and is what most 1908-build Boston apartments need.
I left a candle burning for 3 hours and the window cleared. Why?
The flame is a small, dry-heat source that locally raises the air temperature and slightly lowers relative humidity. With dry, warm air next to the glass, the dew-point drop condition goes away and condensation stops. This is a useful diagnostic — if your window clears when you have a candle, fireplace, or kitchen stove on, your problem is mostly humidity (Cause 2), not cold glass (Cause 1). The fix is then ventilation, not insulation. Most people mistake this for "the candle fixed it" and end up over-relying on heat rather than fixing the cause.
Will sealing my window make my apartment stuffier?
Yes, slightly. Modern ventilation in apartment buildings (HRV/ERV systems in newer builds) depends on a controlled amount of leakage around windows. If you seal every gap aggressively, you may raise CO2 indoors to 1,200-1,800 ppm — sleep-affecting levels. The fix is balanced: caulk big gaps and weatherstrip the sash, but leave intentional room-ventilation (open the bedroom window 5 minutes every morning). If your building has a mechanical ventilation system, leave it running and don't disable it. If your building doesn't, a window vent ($15) plus weatherstrip gives you both sealed + ventilated.
Does any of this apply to newer apartments with HRV or ERV systems?
Newer Canadian and US buildings (post-2018) often have heat-recovery ventilators (HRVs) that bring fresh air in while recovering heat. These reduce condensation because they lower indoor humidity below 35 % year-round. If your 2018+ condo has condensation, the HRV is probably off, the filter is dirty, or the bathroom-kitchen exhaust fans are venting into the HRV intake instead of outside. Check the HRV filter (clean or replace $20-30) before doing any of the fixes above. In Ontario, building code requires HRVs in units built after 2018 — your superintendent should be maintaining it; report it if it's broken.
I'm a UK renter in a 1930s flat. Does the same plastic-film trick work?
Plastic film works less well on sash windows because the frame profile is too thin to seat the tape reliably. Use bristle draught strip ($3 for 5 m) pushed into the sash gap and self-adhesive foam tape on the interior sill instead. If the condensation is heavy on the upper sash (a "sash slot" problem), rub candle wax on the meeting rails — old British practice, surprisingly effective, and you'll see the original technique used on Georgian-era homes that haven't been refit. If you see black mold on the walls around the window, that's penetrating damp and the landlord's responsibility — cite Section 11 of the Landlord and Tenant Act 1985.
The fix that worked for me was the $7 hygrometer first, then the $5 plastic film on the worst window, then the Waykar dehumidifier in the office. None of these are silver bullets. But none of them require calling a contractor, and a single March's worth of laptop damage cost me more than all three combined.
Sources: ASHRAE Standard 55-2023 (thermal comfort guidelines); Health Canada humidity targets; Energy.gov window insulation guidance; UK Landlord and Tenant Act 1985 (Section 11); Ontario Residential Tenancies Act (Standard Lease Schedule 1). Personal test data: my Boston apartment, 1,200 sq ft 1908 walk-up, four windows north-facing, January-March 2026. Photos: Unsplash royalty-free.
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