Humidity: The Pain of the South

This blog serves as a continuation of our previous blog about indoor air quality because humidity control is the 3rd pillar of good indoor air quality. I wanted to do a separate post altogether for humidity since there's so much more to discuss that a brief one or two paragraphs just wouldn't be of service. Living in the Jacksonville North Carolina area and being a HVAC contractor, we battle humidity control about 7 to 9 months of the year. The Air Conditioning Contractors of America Groups into geographical zone 3 for ambient conditions which encompasses much of Florida meaning that we deal with similar conditions. Anyone living here would agree with that.

So why is humidity control important to indoor air quality? Humidity that is too high supports bacteria, fungi, mold, and virus propagation. I don't think I have to tell you that we don't like those things. Low humidity is not often something we deal with here in eastern North Carolina, but humidity that is too low can cause nose bleeds, respiratory issues, and damage to wood furniture and floors. Basically there is a balancing act as we don't want to remove too much moisture from a home, but we do want to control it.

Anyone who reads my blogs know that I love a good baseline and measuring devices so let's baseline the ideal humidity levels. I'll start with saying that a target moisture level in a home is 50%-58% relative humidity. I hate the relative humidity scale as it is RELATIVE... Relative to what? Do we really understand what that means? A good HVAC service tech should, a meteorologist does, but the average homeowner doesn't. A better defined scale of good indoor moisture levels is air that has a dew point below 60 degrees. In the simplest of terms, this means that anything in that space at 61 degrees or higher will not form condensation/sweat. All modern hygrometers and IAQ sensors should show dew point as a metric when measuring moisture content. If they don't, you can easily pull up a converter online and convert any know temperature and relative humidity to dew point. This information comes from the psychrometric chart which can be convoluted although not impossible to read, but an online converter is much easier. The reason it's important to start looking at dew point rather than RH is you can have an RH of 65% (higher than what most people think is good) with an air temp of 68 and the dew point of that air is only 56 degrees. At 56 degrees dew point we serve all the target needs of slowing the progression of mold, bacteria, viruses, and fungi even though the "RH" is high.

What are the leading causes of high indoor humidity? The following is a list starting with the most common cause down to the least common...

1. Improperly maintained/operating HVAC equipment.

2. Oversized HVAC equipment

3. Leaky windows, doors, attic plains, and/or unsealed crawl spaces

4. Occupant habits

Notice I didn't mention insulation? Some of you are wondering if I know what I'm talking about right now, but think about it. The HVAC system can only remove moisture while it's operating and adding insulation makes a home more efficient meaning that the system will run less, and thus remove less moisture! That's all I will say about that for now as subject 2 from above will explain the rest.

1. If your home always had a comfortable humidity level and then all of sudden it didn't, this is likely due to lack of maintenance and degradation of the performance of the system. HVAC equipment running low on refrigerant or a dirty evaporator coil will lead to drastically increased indoor moisture. A system running slightly low on refrigerant can still cool a home and the system will be running more than it should, but the evaporator coil won't be getting cool enough to remove the rated amount of moisture. A system operating with a dirty evaporator coil will have extremely cold air coming out of the vents and will run longer, but since the surface of the coil is soiled it reduces the contact surface with the air passing through the coil and thus the coil will lack the ability to remove sufficient moisture. We see this on at least 20% of the service calls we respond to in a year which is why it made it to number 1 on our list. The last piece of operational issues affecting moisture control is system set up. Every system in existence has multiple fan speed selections allowing us to set the system up for more or less humidity removal. The slower the indoor airflow, the better the moisture removal is which is why proper load calculations and system commissioning is so important. This leads to category 2.

2. Oversized equipment is such a problem for our industry and it would be number 1 on the list, but it seems that in recent years at least half of the contractors in my area are doing a better job at sizing equipment. Remember earlier when we talked about longer run times lead to moisture removal? When an air conditioning system is oversized, it cools the space too quickly and shuts off leading to poor moisture control. Most modern systems take at least 15 minutes and up to 25 minutes of operation just to get the indoor coil below the indoor dew point and START removing moisture. A system that is only running 30 minutes and shutting down has barely started to condense moisture in the air before it turns off and almost no moisture control has taken place. We want longer run times while consuming as little energy as possible to create healthy homes which is why properly sized equipment is important and furthermore, multi stage may be necessary is some instances. Even a properly sized HVAC unit is calculated in a software that designs systems for the highest temps you will see in your area, for us in Jacksonville NC that's defined as a 91 degree day. That's only about 1% of the total hours in a year, so the remaining 99% of the time a single stage unit is still oversized even if not so by code and definition. Multi stage and inverter driven units allow systems to operate at lower energy consumption and lower capacities during non peak demand times so we can still get extended run times and moisture control.

3. Leaky homes are a common occurrence and often overlooked as the availability of competent labor to test and resolve this issue is very scarce. Building science professionals with certifications and field experience are trained to test homes for air leaks that allow moisture to come into the home and then solve those problems. Think about like this, if I cut a 1 foot by 1 foot hole in the side of your house and turn your ac on, the ac will cool the home and remove moisture while running. Eventually the ac will satisfy the thermostat and shut off. With the unit off all the hot and humid air from outside will continue coming in through that 1 sq ft hole negating every bit of progress the HVAC system made and we start the process over again! Now you're probably thinking "yeah if you cut a hole in my wall of course that will happen". Would it surprise you if I told you right this second that this already exist in your home and matter of fact your hole is much bigger than 1 sq ft? No it doesn't matter how new your home is, the hole is still bigger. Don't believe me? Go into your attic and look at 1 light or fan fixture as it pultrudes through to the conditioned space and tell me how much light you see. Check an exhaust fan. If you have a crawl space go under there, peal back a small piece of that fluffy AIR filled insulation, look up and tell me where the air barrier is! It's not there! We haven't changed much in terms of building homes in the last 50 years, we just got better at lying to homeowners and advertising. Like blinders on a horse, we don't fear what we don't see!

4. Occupant's habits can effect indoor humidity typically on a much less invasive scale that the forementioned issues, but still have an effect. Things like taking a shower and not using an exhaust fan to remove the excess moisture. Diving deep into a rabbit hole here but the exhaust fan will only replace that air with air outside, but the lesser of two evils in this scenario is that the moisture content of the air in a bathroom during bathing is much higher than the moisture content of the air outside 90% of the time. Another habit is leaving windows and doors open for fresh air. As mentioned in our last blog fresh air is a good thing, unless it's bringing a ton of moisture with it overtaxing the ability of the HVAC system and thus leading to issues. Kitchen exhaust fans are becoming a big problem. It sounds nice to have a 400 cfm kitchen exhaust fan to remove smoke from a home, but where is that 400 cubic feet PER MINUTE of air coming from? Your home, the air you've been conditioning for indoor quality and comfort. Nature loves when you do this because nature always like to balance things for you. When you turn that fan on the air it's removing gets replaced with outside air via the leaks we discussed in list item 3. You might be thinking that a 400 cfm can't make that big of a difference, but lets look at some quick easy math. if you have a home that is 1500 square feet with 8 foot ceilings, that's 12,000 cubic feet of air. You run a 400 cfm fan for 30 minutes and you have removed EVER SQUARE FOOT OF AIR IN THE HOME AND REPLACED IT WITH OUTSIDE AIR! How long do you think it's going to take your HVAC system to recover from that? I'm not talking about cooling the home to the thermostat set point because that's the easy part. How many cycles and hours will it take your system to remove all that humidity you just brought in?

This concludes this article, hopefully you have found something of use and as always, thank you for taking the time to read.