The Energy Audit-My Moisture on Window Question

One of the questions I always ask while conducting an energy audit or assessment is if there is a build-up of frost or water vapor on any of the windows during the heating season.  Usually, the answer is no.  But I do occasionally get a yes, what is this piece of information telling me? Humidity or frost on a window is the topic for this week’s blog posting.

Frosty window

Over the last couple years, I have increased my focus on indoor humidity levels while conducting energy audit and assessments.  The control of bulk water and water vapor is the most important details to get right when building any home.  Water ending up where it doesn’t belong will damage a home, often without the homeowner knowing until it is too late. It can also affect indoor air quality and possibly cause health concerns.  The following is an excerpt from one of my “copy and paste” paragraphs used for my energy audit or assessment reports:

I recommend monitoring indoor humidity levels, especially during the winter months. Your home does not have an air exchanger, which can reduce humidity levels during the winter and improves indoor air quality. If a home has enough natural air exchanges, the humidity level will typically be low. I recommend purchasing a hygrometer or home weather station to verify humidity.  I recommend maintaining indoor humidity levels around 30% during the winter months.

Why do I key in on frost or moisture on windows?  The glass of a window during the heating season can act as a dehumidifier.  For condensation to form, the temperature of the glass needs to be below the dew point temperature of the room, which is dependent on the indoor humidity and air temperature along with the temperature outside.  The lower the outdoor temperature, the lower the glass temperature.  Many people think there is a problem with the windows in their home when moisture is present, actually it’s more likely there is a humidity source or ventilation problem in the home.

What if the homeowner tells me there is no frost on any of their windows? My thoughts are:

1. There is an air exchanger in the home.

An operating and maintained air exchanger will help to keep indoor humidity in check. If there is one present, I will find it during my inspection of the mechanical systems of the home.

2. There is a high rate of natural air leakage with outdoor air.

A high rate of nature air leakage will only be confirmed by a blower door test. I conduct more energy assessments, which do not include a blower door test, than an energy audit, which includes blower door testing and thermal imaging scans. Dirty fiberglass insulation, if present, at the rim joist or in the attic is an indication of air movements. I don’t know how much air is moving without the blower door test, but I would suspect a leaky home.

3. There are upgraded windows in the home.

Single pane windows will have a lower glass temperature than double pane windows.  Triple panes may not produce any moisture on the glass, even in homes with elevated humidity levels.  I can’t always rely on window moisture as a barometer to indoor humidity levels.  An inspection of the type of windows is also needed.

What if the homeowner tells me there is frost or moisture on the windows? My thoughts are:

1. There is no air exchanger or the unit is not used or not maintained.

Some of the homes I visit have no air exchanger, have an air exchanger, but it is off, or one that has not been maintained or cleaned. (See my last blog posting: Mechanicals-Heat Recovery Ventilators.) Depending on the age of the home, not having one may be a code violation. Balanced mechanical ventilation has been a code requirement in Minnesota for nearly 20 years. I was in a home several years ago where the occupants had lived in the home for over a decade and never new what the square box next to the furnace was. It had never been turned off or cleaned. My recommendation was to have an HVAC tech come out to clean and balance the air exchanger.

2. There is a source of moisture somewhere in the home.

The biggest sources of humidity I commonly find in a home is an open dirt crawl space and basements with leaky foundations. I was contracted on a weatherization project many years ago. While we were working the project, rain water from an average rain event flooded the basement to the point we needed rubber boots to complete the work. The water was over our ankles. The elderly homeowner had no other place to go, she lived in the space while we worked, and was there every time her basement flooded. Very sad. These major sources of humidity need to be addressed.

3. The windows are single pane or low quality.

As I said earlier, window quality, especially the number of window panes, has a lot to do with the formation of moisture on the windows.

4. The home is air tight.

A tight home, or one with low natural air exchanges will hold moisture within the home (and affect indoor air quality). This humidity can come from cooking, bathing, cleaning and breathing. A blower door test will confirm. An air exchanger may be suggested.

5. The home is new.

It’s normal to have very high humidity within a home for a period when it is new.  Concrete, wood framing, drywall, paint, and even some floor finishes will produce a lot of moisture which take up to a year to fully dry.  Some new homes during that first heating season may need to run both a dehumidifier and the air exchanger, and still might have elevated humidity levels.

What if the homeowner tells me I only have moisture on certain windows within the home?  I sometimes hear that this kitchen window or that bathroom window has frost.  Occasionally it’s a living room or bedroom window.  When I get these comments, I first look at the location.  Showering in a bathroom or boiling water in a kitchen will make a lot of moisture.  Are bath fans or kitchen fans used?  Do they vent to the outside? Do they move enough air?  I recently tested a bath fan that moved no air.  That bathroom had some mold growth in the corners of the exterior walls.

Window covering can also cause moisture to accumulate on windows.  This is the result of reduced air movement across the glass when the blinds are closed.  Insulated window coverings and blinds have always been a hard suggestion for me to make to a homeowner.  Insulated blinds can reduce heating costs and increase comfort, but they can also cause window damage and the good ones are not cheap. When I suggest insulated window blinds, I also outline the drawbacks.  This way, at least the homeowner knows the downsides.

I can learn a lot about a home with one question.  Sometimes this question leads to many more questions.  Information is key to a good energy audit or assessment.

If you haven’t already done so, follow me on Instagram.  I post several times a week on various topics about energy, building science, and construction projects I’m currently working on.  @northernbuiltpro

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