Last time we started discussing a simple HVAC system that can deliver comfortable and healthy homes for most houses in most climate zones - we call it BAD ASS HVAC.
A brief refresh, BAD ASS HVAC (and all HVAC systems for that matter) should be able to deliver the 6 Functions of HVAC:
We also talked about how important load matching is to providing comfort in homes. An HVAC system that can “load match” can put out just the right amount of heating or cooling that the house needs at any given moment, which requires right sized multiple stage equipment.
Finally, we discussed how MERV 11 filtration is the lowest level should be recommended. That is the lowest filter rating that does a good (but not great) job of removing the small particles (PM2.5) that go directly into our lungs.
Why is all this important? Remember the car analogy? Most residential HVAC systems can’t do any of these 6 functions well, while your car can do 5. We think that stinks, so we developed the BAD ASS concept to provide a simple path to solving it.
Most HVAC Equipment Is Replaced In a Hurry
Much of the root of home HVAC failure is that 87% of residential HVAC systems are replaced on an emergency basis. That means most systems are replaced under duress with little time to consider options. Typically the cheapest system from the first contractor who can show up goes in. (I just want heat!)
In our experience we have never seen the cheapest piece of equipment be able to effectively deliver the 6 Functions of HVAC. The only way to deliver the 6 functions is usually to replace the equipment again. That sucks.
Sadly it only takes a small amount of up front front education and planning to be able to deliver a much better equipment installation for our clients. Since the HVAC equipment investment should last 15-20 years, it’s important to get right. This series is trying to explore how to get it right more often. If you are a contractor, start offering BAD ASS HVAC as an option, if you are a homeowner, ask for BAD ASS HVAC.
This article is going to focus on the importance of dehumidification. Dehumidification of homes is a huge issue that affects air quality, your health, home durability, and much more that many realize. In our practice, we have found humidity to be remarkably hard to control.
Humidity & Health
Did you know we breathe 3000 gallons of air per day? Or that most high security prison inmates spend more time outdoors than we do? We and our families breathe at least a third of that air breathed inside our homes. Is it worth at least considering the quality of that air?
Ken Gehring of Themastor, who invented the ventilating dehumidifier, coined the term “green grass climate.” If your grass stays green most of the year without watering it, why? Could it be because it rains a lot? That’s moisture that needs to be controlled to provide a comfortable, healthy, and long lasting home.
If you’re thinking “I live in a dry climate, I don’t have to worry”, you may be right, the risk is much lower. That said Bill Hayward of Hayward Score had his home make him and his family sick. Mike MacFarland of Energy Docs had a client with a very sick child that solving mold and moisture issues in a bathroom helped enormously.
The more we learn, the more critical keeping a house warm and dry becomes.
Our practice, Energy Smart Home Performance, has over 40 Foobot air quality monitors in the field. Foobot measures temperature, humidity, dust (PM2.5), and chemical pollutants (tVOC). For more discussion on why we like those four measurements best, check out the Health Thief article from May 2018.
VOCs are volatile organic compounds, which have boiling points close to room temperature. That means we are likely to be breathing them at room temperature, and particularly if it's humid.
Many VOCs are essentially harmless like vinegar, cheese, or wine odors. But many others are harmful such as formaldehyde or chemicals in air fresheners, cosmetics, and cleaners. The first step is to reduce the number of these bad chemicals we bring into the house - look for “fragrance free” products as a start, but I don’t know of any fool proof way to keep VOCs out of your home.
The other challenge with VOCs is that they already exist in the building materials of your home, and in your furniture. While source control is best, it’s surprisingly difficult to achieve.
Which brings us to dealing with the VOCs that are already in your home. If we want to avoid breathing bad stuff, one of the best ways to do that is to keep relative humidity inside our homes in the 30-50% relative humidity range.
By watching these air quality monitors, we have repeatedly found that if we can’t control humidity, we can’t control chemical pollutants, which agrees with the research I have read.
Speaking of research, Richard Corsi is one of the best indoor air quality researchers I know. He seems to have been the PhD advisor of every air quality researcher I work with as well. Corsi found that high relative humidity is directly related to the release of VOCs.
This turns out to be true for lots of other factors as well including dust mites, bacteria, viruses, mold, and more. If you haven’t seen this ASHRAE chart, burn it into your mind now.
The blue “Optimum Zone” should be 30-50% relative humidity in our experience. Maintaining 40-50% RH in cold climates during winter is a sure way to create condensation, which then leads to mold, mildew, and rot. If ever you see condensation inside your windows in winter, keep in mind there is probably condensation inside your walls that you can’t see, feeding things you would prefer not to feed.
We target 30-40% RH in winter, enough to prevent nosebleeds and static shocks, but generally pretty safe for condensation. We’ll come back to this when we discuss humidifiers.
For now, note how the 30-50% range is very important to minimize a lot of bad things in your house. At a minimum stay under 60%.
The Elephant in the Room - The AC ain’t running!
AC tends to be the big dehumidifier in most homes. There’s a huge problem though, when April showers and May flowers are here, is the AC running? Or on those lovely fall days at the end of summer that are still warm, but the ground is still wet? On those 70-75 degree days is the AC going to be on? Probably not.
But there is a lot of humidity to remove, right? Absolutely.
That brings us to the elephant in the room: there is no way to control humidity with your air conditioner on days like this.
Days like that are getting more and more common, average dew points are on the rise as I explored in my The Coming Mold Explosion video. Here in Cleveland we have 3-4 months per year of conditions like this, many “green grass” climates have more!
Shoulder Seasons - The REAL Danger Zone
Remember those 40 Foobots we have in the field? They’ve taught us a lot. Here’s a Foobot dashboard chart from one client home where we performed insulation and air sealing upgrades, but have not yet touched the HVAC.
This chart is the entire month of September 2019, the home is in Cleveland Ohio. How much of the time was his humidity above 60%?
The answer? About half the time. There is a fairly high risk that something bad is happening in their home.
There’s a better metric to look at it though, called dew point. Dew point is the temperature that relative humidity hits 100%. It varies from moment to moment, but unlike relative humidity you only need to know one number, the dew point, where with relative humidity you have to know both the RH and the temperature. Relative humidity makes it hard to compare indoor and outdoor humidity levels, where dew point makes it easier.
You know what dew point is already though! If you have noticed condensation on a lemonade glass or beer can, that tells you the surface of the glass or can is below dew point. The air around that surface can’t hold any more moisture, so the moisture gets sucked out onto the glass or can. As the beverage and container warm up, they go above dew point and the moisture disappears. The same thing happens in homes, often in unseen places.
The generally agreed upon target is to stay below a 55 degree dew point. For more on that look up the EPA Moisture Control Guide by Lew Harriman. Let’s look at that same house, how much is it below 55F dew point?
The answer this time is almost never, the dew point was higher than we like to see the entire month of September 2019.
Why is this important? Check out this result from dpcalc.org (short for dew point calculator.)
This is a pretty typical shoulder season day. It’s 70-75 degrees out, and the dew point is a rather sticky 70 degrees. Still, it’s nice out! Windows are open, and moisture is pouring inside through the windows like The Blob.
Note the “Days to Mold” number of 6. If those conditions continue for 6 days straight, something bad is going to be growing somewhere inside your house. The good news is that those conditions don’t continue that long in most cases.
Or do they? It’s usually cooler in your basement or crawlspace, right? Let’s see what’s happening there on the same day:
Yikes! 2 days to mold! If your basement or crawlspace smell musty, this is probably what’s going on.
When we dry out that cool basement or crawlspace to a 55 dew point, what happens?
Look ma! No risk!
We highly recommend running a dehumidifier in basements and crawlspaces set to between 50-60% relative humidity.
Better still is to keep the windows closed and the indoors below 55 dew point. That’s a frustrating recommendation, but it’s been shown again and again to be true.
Every time you open the windows on a humid day, that humidity pours back inside like The Blob, and you have to pay to suck it back out when you close the windows again.
Now we understand the risk of not managing relative humidity (or dew point)...
What’s the solution?
The good news is that there is a technology that already exists inside many residential HVAC systems to be able to handle dehumidification when there’s no need for the air conditioner to run.
Next time, we’ll tackle the solutions to the dehumidification challenge. If you’re a contractor, be ready to send this article and the next one to your HVAC manufacturer tech reps. If you’re a homeowner be ready to give them to your HVAC contractor. Stay tuned!
Nate Adams is fiercely determined to get feedback on every project to learn more about what works and what doesn't. This blog shows that learning process.