Many homeowners upgrade their windows to improve comfort and energy efficiency. But after installation, it can be surprising to see more condensation on new windows than on the old ones. While it may feel like a defect, the real issue is almost always the moisture level inside the home.
Before investing in new windows, it helps to understand how humidity behaves indoors, as outlined by Environment and Climate Change Canada. A simple DIY condensation risk check can tell you whether your indoor air is likely to cause fogging or moisture buildup on new glass. This guide explains how relative humidity works, how to calculate your dew point, and how to choose the right humidity target for your home.
Understanding a DIY Condensation Risk Check
A condensation risk check helps you measure how much moisture is in your indoor air and how close you are to hitting the dew point.
- Relative humidity: This is the percentage of moisture in the air compared to how much it could hold at that temperature, according to ASHRAE.
- Dew point: This is the temperature at which moisture in the air turns into liquid on a cool surface.
Windows are usually the coldest surface inside the home during winter. If your indoor dew point is higher than the glass surface temperature, condensation will form, as explained by the University of Massachusetts Amherst Building Science Center. The goal of a risk check is to find your dew point and see how it compares to the conditions your windows experience.
Why Understanding Humidity Matters
Knowing your home’s moisture level helps protect your home and helps you choose the right upgrades.
- Prevent damage: Persistent condensation can pool on sills, cause paint to peel, and eventually lead to wood rot.
- Protect indoor air quality: Moisture encourages mold and mildew, which can affect both your home and your health.
- Make smarter window choices: If humidity is high, better-insulated window units, such as triple-pane designs, help keep the interior glass surface warmer and reduce condensation.
Comparing Humidity Management Tools
Once you know your indoor humidity level, you can decide how to manage it. Homes that are sealed tightly for efficiency often need mechanical help to keep moisture in balance.
| Parameter | Portable Dehumidifier | Energy Recovery Ventilator (ERV) | Bathroom and Kitchen Fans |
| Primary Use | Lowers RH in a specific area | Manages whole-home humidity and fresh air | Removes moisture at the source |
| Typical Cost | Low to Medium | Very High | Low (if already installed) |
| Energy Impact | Uses electricity | High-efficiency, recovers heat | Exhausts conditioned air |
How to Perform Your DIY Check
You can complete a risk check in a few minutes with simple tools.
- Get a hygrometer: A small digital hygrometer gives you indoor temperature and relative humidity.
- Take a reading: Place the hygrometer in a central living area, away from kitchens, bathrooms, or humidifiers. Let it sit for a few hours for an accurate average reading.
- Use a dew point calculator: Enter your temperature and humidity into an online dew-point calculator. For example:
- Indoor temperature: 21 C
- Indoor RH: 50 percent
- Dew point: 10.2 C
If your window’s interior surface drops below that temperature on a cold night, condensation may form.
- Find your RH target: Most homes aim for 30 to 50 percent RH in winter, based on standards from the World Meteorological Organization. On very cold days, your target may need to be closer to 30 percent to avoid fogging.
Common Mistakes to Avoid
- Blaming the new windows: Older windows allow humid air to escape. New, airtight windows reduce drafts, which means moisture stays inside. The humidity was always there; it only becomes visible now.
- Testing humidity in the wrong location: Measuring air in a bathroom, kitchen, or damp basement will not reflect the home’s true average humidity.
- Ignoring everyday moisture sources: Long showers, boiling water, drying laundry indoors, many houseplants, or a humidifier set too high can all increase indoor humidity.
The Airtight Home Paradox
Modern homes are built to be airtight to save energy. While this reduces heat loss, it also traps moisture, cooking odors, and indoor pollutants. Older homes had natural ventilation through small leaks and gaps, but today’s construction relies on mechanical ventilation.
Balanced systems such as HRVs or ERVs have become common in new builds. These systems exchange stale indoor air with fresh outdoor air while retaining heat, helping maintain healthy humidity levels while still keeping the home efficient.
Frequently Asked Questions (FAQ)
1. What should I consider when doing a DIY condensation risk check?
The key things are to get an accurate reading with a hygrometer in a central location, use an online calculator to find your dew point, and then compare that dew point to how cold your windows get. Also, be aware of moisture sources.
2. How do I choose the best RH target?
The best RH target is a balance between comfort and safety. Aim for 30-50% RH. If you see condensation forming, lower your RH target (by running a dehumidifier or venting more) until it stops.
3. What are the advantages and disadvantages of different RH levels?
- Advantages (Balanced 30-50%): Comfortable, healthy air, and protects your home.
- Disadvantages (High RH > 50%): Can lead to condensation, mold, mildew, and dust mite growth.
- Disadvantages (Low RH < 30%): Can cause dry skin, scratchy throats, static electricity, and can even make you feel colder.
Conclusion
Condensation is not a sign of window failure. It is a signal that the home’s humidity level is too high for current conditions. Before upgrading your windows, take a few minutes to measure your indoor humidity and calculate your dew point. By knowing your RH target and managing moisture sources, you can enjoy clearer windows, a healthier home, and better long-term performance from any new window installation.