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Moisture and Humidity

The amount of water vapor in the air is called Absolute Humidity. A given area of air can hold different amounts of water vapor depending on the temperature of that air. The amount of moisture, relative to the temperature is measured as a percentage called Relative Humidity (Rh).

A range of 30% to 60% Rh is generally considered the most comfortable, dubbed the climatology 'Comfort Zone'. High humidity doesn't necessarily cause a threat to a given structure until you you either reach 100% Rh or hit something called dew point. When the air outside reaches 100% Rh, it starts dropping it as rain, snow, sleet, ice storms and fog.


Dew Point is defined as "the atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form." In essence, Dew Point causes (dew and) frost on your car, lawn, plants and everything else you see in the morning.

Reaching dew point isn't only outside, moisture can hit dew point on a surface that is cooler-enough than the air, such as the glass of lemonade waiting for you to finish mowing the lawn. Dew point can be hit on your windows, causing water to start pooling on your sill and begin mold growth. You may recall riding the school bus as a child and scraping frost off the inside windows? That was water vapor from all those exhaling children which hit dew point on the cold surface and then froze. The scary fact is that while many people wouldn't describe a school bus as being highly insulated, it still traps all that moisture. Now consider your highly insulated house; and you can imagine the amount of moisture you're putting into the air at home right now. That's without considering showers, cooking, watering pets and plants; and humidifiers. There's an article below on Indoor Air Pollution (IAP) and what the EPA has to say about it.

Dew point isn't the only concern- there's some amazing physics phenomena also in play here. One is the fact that moisture will always try to move from more, to less. While a moisture/vapor barrier works to prevent this; if there's no way for the moisture to get out it's likely to cause an issue. As a given area of air can only hold so much moisture (100% Rh) if you sit at 100% Rh, a small decrease in temperature will cause you to hit dew point. Giving the moisture a path to escape helps to keep the Rh within the comfort zone.

The difficulty with leaving holes in the walls for moisture to get out is heat loss. Ever since the 70s and the oil embargo, we've been on a binge of tighter houses and lowering heat loss. Unfortunately this tightening houses caused an unforeseen side effect: Sick Building Syndrome (SBS). Skip ahead to the section on Indoor Air Pollution for more on SBS.

Indoor Air Pollution

Have you ever held a job where- only when at work -it felt like you had a cold? Maybe even felt like the flu? Shortly after going home, or even just from your lunch-break it went away? You may have been experiencing a Building-Related-Illness (BRL) known as Sick Building Syndrome. With how tight our buildings are, even without symptoms, it's easy to see how this can be a problem. 

I'll ask this a different way, if a person stood before you and put a clear plastic bag over their head; how long would it be before that caused you to be concerned? Would it be immediate? Maybe after moisture started showing up as dew point was reached inside? Would it be after they started showing signs of oxygen deprivation? Or when they fell over? How long will it be before your own home, and the figurative- almost literal -plastic bag that it is causes you to be concerned?


For most the answer is dependent on visibility. Radon isn't scary to most because it's invisible- but mold is alarming. Carbon Monoxide's not very frightening; while seeing smoke in your house is, and smoke is Carbon Monoxide. Most people start seeking solutions to 'wet windows', sometimes getting new windows installed thinking that will solve their problem. Very few people seek solutions to future issues which all fall under the same concept: Healthy houses need to breathe.

Just as your body needs clean air in- bad air out, so does a building. When buildings are kept too tight they risk building up high levels of moisture, cooking and bathroom odors, gasses from plastics and cleaners, dust and dander, and other air pollutants. The EPA says that the air inside our home can be 2-to-5 times more polluted than the air outside. If you're curious to know more about the quality of the air in your home, we offer Air Sample Testing.

Air and Temperature

In the articles above I briefly mentioned a physics phenomenon in which moisture tries to move from more to less. The same can be said about heat. Oh sure, we can add new types of insulation, increases the R-value and we can slow that transfer down significantly; but we can never stop it. Heat will always move through, the only factor is the time it takes to get through. I also mentioned above what the EPA has to say about indoor air pollution. 



The approach to homes today is kind of like this:

Using the example from before, the example individual hair a plastic bag over their head. In this example, I glossed over 'Natural Ventilation' which, for argument sake, we'll poke a few holes in the plastic bags. From their perspective, keeping the bag warm is too costly, all the warmth escapes through the holes! To decrease heat loss, they add a second bag. Hmm, now the moisture's starting to build up but it's still too cold. Add another bag, and another, one more perhaps? There; now it's sealed up so tight, nothing can escape.

You might be thinking; "Well fine! Then how do I lower my heating bill, keep my house warm and get the pollutants out?" Well, there's a device called a Recovery Ventilator. They come in a couple of variants we'll go over a couple paragraphs down. A recovery ventilator is designed to take your polluted air and exhaust it outside, while taking the fresh air of outside and put it inside.


Admittedly, not everywhere has good outdoor air, but Maine's is pretty clean. Here's an automatically updated report on the quality of Maine's outdoor air according to the Maine Department of Environmental Protection. Before we continue on recovery ventilator's it's worth noting some people have allergies to things outside, and find relief when going inside, therefore they may think it's better inside. That's another example of that plastic bag we talked about. So moving forward, for individuals with allergies like that be aware that a High-Efficiency Particle Air (HEPA) filter can be added to a system to reduce these allergens before they get inside.

ERVs and HRVs

So depending on your climate, the ventilator you need may change. There's a lot of discussion on the matter, and it all boils down to this: a Heat Recovery Ventilator (HRV) exhausts everything that goes into it after the basic filter, and an Energy Recovery Ventilator (ERV) exhausts all the air, but retains a portion of the moisture, also after a basic filter. (A point to note is the basic filter is there for large particles, e.g. things that could get into the Core and block airflow; it is not there to filter small particles or 'clean' the air. Some units contain spaces for optional filters, otherwise more advanced filters can be installed in-line. For more details, give us a call 207-786-9400)

You might be thinking "but you said moisture can cause damage and mold!" and this hasn't changed. In our state, the outside air can get very, very dry when it's cold. As a result, HRVs here can make the air uncomfortable which usually results in occupants turning the unit off which certainly doesn't help the quality of the air.

So why does this help?

A recovery ventilator works on a pretty simple concept: continuous dilution. Another example, your house before a ventilator is kind of like a glass of murky water. Adding your ventilator pours say 1/4 glass of clean water into that, spilling some of the mix out. It's still murky, but give it time, add another 1/4 glass, and another. Eventually you'll see that the murkiness is gone.

The other side of the coin however is a question: at what rate are your pollutants introduced? If you drop a spoonful of mud in every time you pour fresh water in, you're not going to get anywhere. These pollutants like those mentioned in a previous article are cleaning products, hair products, building materials and more. That's where knowing rates of pollution and sizing the unit correctly for the space comes into play. I.e. increase the clean water in the previous example.

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