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The Dew Point and Insulation

We know that moisture, mold, and air quality are all related to one another. Moisture in walls can lead to mold growth, and that leads to low air quality. It turns out that insulation, specifically a lack of insulation where needed, can also contribute to a mold problem.

Here we define a term used by building scientists, dew point, and explain why it is essential to keep in mind when insulating a home.

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By Wayne Groszko, Renewable Energy Expert

Apr 7, 2025 · 5 min read

What Does Dew Point Mean?

Dew is a common term for the water droplets that form on surfaces in the morning or evening. The related term, dew point, refers to the conditions under which condensation (or dew) occurs. And dew is not what you want inside your home!

What Is the Difference Between Relative Humidity and Dew Point?

Relative humidity and dew point are two different ways of measuring the amount of water content in the air. The relative humidity is the percentage of water content in the air, relative to the maximum the air can hold at a given temperature. The relative humidity changes when the air temperature changes because warmer air can hold more water content than cooler air.

The dew point is an absolute measure of water content, independent of air temperature. When relative humidity reaches 100% and cannot hold more water content, this temperature is called the dew point.

How Do You Calculate Dew Point?

Dew point temperature can be calculated using Td = T - [(100 - RH)/5]. In degrees Celsius, Td is dew point temperature, T is observed temperature, and RH is the relative humidity. This formula was proposed in the February 2005 Bulletin of the American Meteorological Society by Mark G. Lawrence and is accurate for relative humidity values above 50%.

The dew point is the temperature, pressure, and humidity level that causes water vapor contained in the air to condense into liquid form.

condensation on glass — Water on a glass below the dew point. Photo Credit: Dane George

What Is An Example of Dew Point?

We see the dew point in action all the time. For example, take a cold drink on a hot, humid day, where water vapor condenses on the outside of the glass and drips down the sides. The glass's temperature is below the dew point for the humidity on that day, causing the glass to get wet on the outside.

Another way to think about it is that warmer air can hold more water than cold air. When warm air cools down, water vapor must condense to liquid form.

Why Does Dew Point Matter In a House?

Condensation can occur in a house where the interior temperature and humidity are high, but exterior conditions are cold. When a window, wall, or ceiling is not well insulated and becomes cold enough, condensation can occur on the interior surface, making that surface wet.

Windows are usually the coldest surface of the building envelope and one of the first places you'll see condensation and mold growth. However, in homes without adequate insulation, condensation can occur on any cold surface, potentially leading to large mold growth areas.

The dew point can also occur inside a wall or attic when moisture from humidity inside the building moves out through the walls and becomes cold enough.

Modern building codes require a vapor barrier to be installed behind the wall or ceiling finish to stop both air and water vapor from moving through the wall. However, some humidity always gets through. Where there are holes or gaps, humid air will bypass the vapor barrier and work its way through the insulation.

As moist air passes through blown-in or batt insulation, the temperature gets colder as it gets closer to the outside. It can eventually reach a point below the dew point. Water will condense inside the wall, leading to moisture damage and rot. You can avoid this by minimizing the humidity flow in the wall, insulating well, and allowing the wall's exterior side to breathe humidity out.

Homes have always been able to breathe, which in the past meant that they didn't have insulation, vapor barrier, or modern air sealing techniques like caulking and weatherstripping. This construction allowed the air and humidity to escape and dry out any condensation problems. The downside of this was vast amounts of energy required to heat uninsulated buildings, along with uncomfortable, drafty living conditions.

In modern construction and renovations, our homes are much more airtight. So building methods have adopted new ways to prevent condensation. For new construction, this means sufficient insulation, a vapor barrier on one side of the wall, a breathable membrane on the other side, and a ventilation system. All of these work together to prevent condensation. But be aware that when renovating older homes, problems can occur.

Homes with little or no insulation can experience condensation if you make the house very airtight without adding ventilation. Suppose you test your home's airtightness after renovation and find that it is less than three air changes per hour (ACH). In that case, you may need to install a heat recovery or energy recovery ventilator.

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heat recovery ventilator hrv — Heat Recovery Ventilator (HRV). Photo Credit: Wayne Groszko

What Are Solutions For Dew Point Issues Beyond Insulation?

HRVs and ERVs

Replace humid air inside your house with dry outdoor air and conserve the heat in the process! Don't have a ducted system? No problem! Consider installing a ductless HRV or ERV.

Energy Star Dehumidifier

In damp basements or crawlspaces, a dehumidifier might be essential. But keep in mind that they are heavy energy consumers. It's worth investigating other options to eliminate the source of moisture first.

Well-Sealed Air and Vapor Barriers

These essentially stop water vapor penetration so that humid air cannot penetrate a wall or ceiling cavity. However, a vapor barrier is only as good as its installation; all seams must be well sealed with tape and acoustical caulk. You can also lay a polyethylene vapor barrier over any unfinished basement floors that are a humidity source in a house.

Closed-Cell Insulation

Extruded polystyrene rigid board insulation and 2-pound spray foam (also called closed-cell spray foam) are dense enough to block air and water vapor. If installed continuously with no penetrations, they can prevent water vapor from entering concrete or a wall cavity (read more about spray foam options here).

Exhaust Fans

Bathroom exhaust fans are essential for removing humidity generated by bathing and showering. They can also help to increase air changes in the home throughout the day. Stovetop range hoods remove humidity generated while cooking. These must vent to the outdoors, not into an attic or other part of the house.

Dew point is an essential but often overlooked aspect of home design. If you see moisture building up on your window sills, ceilings, or walls, look into implementing one of the solutions detailed above.

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Article by

Wayne Groszko

Wayne Groszko is a consultant, researcher, and teacher in Energy Sustainability with 13 years of experience. He has taught at Dalhousie University and the Nova Scotia Community College, in the Faculties of Engineering, Environmental Science, and Energy Sustainability Engineering Technology. Wayne is also President of the Community Energy Cooperative of New Brunswick, and has worked as Renewable Energy Coordinator with the Ecology Action Centre in Nova Scotia. He holds a B.Sc. (Hon.) from the University of Calgary, and a Ph.D. from Dalhousie University.

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