Every homeowner should know about Dew Point

Although “comfort” is the end goal of our home environment, that word could have different meanings for different people.  However, we can use parameters like “temperature”, “relative humidity” and “air exchange” to define our version of comfort.  The first two, temperature and relative humidity, are integral to a third parameter, dew point. 

According to the National Weather Service, the dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%.  Practically, you can see dew point in action when you place ice in a glass of water and set it on your kitchen counter.  The reason that droplets of water form on the sides of the glass, is because water vapor from the air contacting the glass are condensing on the cold surface.  The surface of the glass is below the dew point.  Water droplets don’t form on the glass if the contents are room temperature or warm, because the surface will be above dew point temperature.  Dew point operates on a unique law of water vapor: Warmer air can carry more water, while cooler air can carry less. Here is a really good explanation of dew point: Understanding Dew Point.  Also, here is a fun little tool that allows you to play with the parameters of temperature, relative humidity and dew point to see how they affect one another and influence mold risk (although it’s a pity that the webpage may set off your internet safety program).  The thing that’s notable is that we’re going to be talking about 2 temperatures: the air temperature and the surface temperature (which is really the air temperature right next to the surface).  

Now, here’s where dew point is applicable to every homeowner: that condensing water has the capability to support mold and microbes.  We should make every effort to avoid allowing surfaces to get near dew point, because it’s a health liability.  In almost every climate, there is going to be a need to add “climatization” for part or all of the year, meaning you will want to heat or cool your home to make it comfortable.  This is fine, but securing the envelope (perimeter) of your home with 1) air barriers and 2) insulation is paramount to avoid approaching the dreaded dew point.

Instead of a single-walled glass or metal drinking cup holding ice water, think about a thermal mug.  In a thermal mug, you have two walls and a layer of insulation between them.  The insulation reduces the temperature difference between the inside and outside so that your hand doesn’t feel cold (or hot) when holding ice water or boiling liquids.  Insulation is great!  We’re all for insulating your home well.  But what is equally or more important, is air barriers.  If that thermal mug had leaks, then you would be getting ice cold or boiling hot liquid on your hands, not to mention the insulation in the mug would be compromised.  The same thing happens when homes have air leaks.  When the undesirable warm, humid air comes into your air conditioned house, the water vapor in the humid air condenses on cooler surfaces, supporting mold.  This is why it’s really important to air seal your home.  Yes, everyone has to come in and out of the home via doorways, but opening and closing a door for 5 seconds and letting in some humid air is not the same as the many small, constant leaks of humid air that many homes have.  If you have the opportunity and resources, Nate the House Whisperer has an Air Sealing Course that will help you seal virtually every type of home and construction situation.  Here’s why we put air sealing above insulation: it should always be done first.  According to Nate in this video at approximately minute 1:55, “If you are worried about insulation types but not worried about reducing the air leakage in your house and measuring where you got to (via blower door tests), your likelihood of failure and wasting a ton of money is really high.  Please, make sure that you are tightening your house.”

Then, after air sealing, insulation is of course important.  Insulation makes a temperature barrier that will help keep surfaces at a temperature higher than the dew point.  Nate the House Whisperer also has an insulation guide, as part of his The Home Comfort Book (available digitally and hardcover).  It’s really important that the homeowner is familiar with the why’s and how’s of insulation, before calling a contractor, because contractors often don’t know these things!   Throwing more insulation into your home when proper air sealing has not been done doesn’t help.  Insulation companies will do whatever you ask them to do, happily, because they’re getting paid.  They get paid for the insulation, but insulation alone often does not provide the results homeowners are looking for.  Again, air sealing has to come before insulation, and with air sealing comes the “before” and “after” blower door tests so that results can be measured.  

Getting back to dew point, every temperature and relative humidity condition has a corresponding dew point.  Proper air sealing and insulation helps you to make surfaces below the dew point temporary and visible–like on your glass of iced tea or the bathroom mirror.  Whenever you see condensation on windows, however, take note.  Nate’s experience tells him that when this happens, there is something also happening behind the walls.  This article (scroll down to “What other types of condensation can happen in homes?”) talks about the dangers of interstitial condensation, which is a fancy way of describing condensation inside a wall cavity.  It can happen in walls, but it’s also often a problem in flat and cathedral roofs.  Warm, humid air is lighter than cooler, drier air, and it will rise to the uppermost part of a room.  If the peak of a cathedral ceiling is not properly sealed (often the case), this air will leak out and condense in the rafters, causing mold and rot.  Bubbling or peeling drywall tape on such a ceiling is the same kind of red flag as condensation on the window–something is happening inside the ceiling too!  It’s time to get a construction expert with knowledge of air sealing involved, because fixing the ceiling with the same methods that it was constructed will not solve the problem.  

You might not have the motor skills to swing a hammer or climb tall ladders, but education is key to making your home comfy and healthy, and to be able to fix problems right the first time without costly mistakes.  This is why we also highly recommend the substantial free resources of buildingscience.com.  Just by typing “dew point” into the document search box, a number of documents come up, including specific ones like unvented roof systems.  This search also led me to a 3-part series on “How to Look at a House Like a Building Scientist” which is really the most helpful type of insight you can have!  Part 1: Air speaks to air leakage and measuring it via blower door testing.  “Controlling the air inside the building—by limiting air leakage—is critical for conditioning the air, and therefore affects comfort and energy use in buildings.  But in addition to carrying heat, air leakage often carries moisture with it: this can lead to a variety of durability problems.”  And health problems, we might add.  In Part 3: Temperature, Humidity, and HVAC Systems, the same building scientist encourages his audience to “think in dewpoint” because it “tells me how cold a surface needs to be for condensation problems (and therefore moisture problems) to start.”  He uses Munters Psychroapp (psychro is short for psychometrics, the study of thermodynamic properties of moist air), which is a little complex for the beginner; you can get a regular Dew Point Calculator app.  A practical application of knowing the dew point is knowing whether opening the window will make your house more humid, or more dry.  With a simple dewpoint app, you can input the temp and Relative Humidity (RH) of your indoor conditions and get the indoor dewpoint.  Then put the outdoor temp and RH in and get the outdoor dewpoint.  If the outdoor dewpoint is lower than the indoor dewpoint, then when you open the window you’ll get relatively drier air coming into your home. Win!  The author of this paper also used a chart of temperature and dewpoints at different heights in an attic to show why the insulation and joists were growing mold near the roof ridgeline.  An inexpensive pen-style humidity/temperature instrument like this one would help a homeowner to do the same.  When the dewpoint approaches the temperature of the space, there’s more than enough water to support mold.

Knowledge is key to fixing a lot of home problems without spending a lot.  When you have a condensation (dew point) problem, we can point you to start looking at the dew point, and then where air leakage might be occurring, and go from there!

Photo by Tamas Kolossa on Unsplash