Why is my house so humid?

I received an excellent question today, in a nutshell it was: “Why is my house so humid that condensation is dripping from the ceiling?”

The questioner reminded me of all the times I’ve heard this question, both on this blog and when I was in the field, helping track down issues in people’s homes. Clearly this is serious issue that needs more attention because it leads to problems with a home’s health, comfort and efficiency – exactly the things I focus on! (I also wrote extensively on this in a previous article: “Cathedral Ceilings – Mold and Moisture”)

Why is a humidity important?

The relative humidity (RH%) of air is a measure of how much water vapor is mixed in with the air. 0% means there’s no moisture in the air. 100% means the air is saturated with so much moisture that it would start raining as the moisture condenses into liquid water.

What isn’t obvious is that RH% depends on temperature. In the morning or at night, when the air is cooler, the RH% is highest. Often reaching close to 100% during the rainy seasons. As the air warms up, the RH% decreases, even though the air contains exactly the same amount of water!

For example, suppose it’s a clear, damp morning, there’s dew on the leaves and one your car in the driveway. You measure the temperature and humidity to be 60F and 100% RH%. Now the day warms up and you go out at noon. The temperature has risen to 80F and the humidity has dropped to 50% with exactly the same amount of water in the air.

This works in reverse, and this is critical, so please read this carefully!

It’s a beautiful afternoon. It’s not too warm but it is muggy, say it’s 75F and 70% humidity. You want to save energy and not run the air conditioner, so you open up the windows and let the air flow through. So now it’s 75F and 70% humidity inside the house. Plus, the humidity is invisibly getting into your carpets, clothes, books and everything else.

That evening the temperature drops and you’ve closed up the house. The water in the air is still there. As the temperature in the house drops and the house itself gets cooler, the RH% in the house gets higher. By the time it reaches 65F, the RH% in the house will have reached 100% producing conditions for condensation, which is water vapor collecting into liquid water that you see as water droplets forming on windows, the ceiling, rafters, etc.

Bathroom windows “sweat” due to the high RH% and lower window temperature

In a nutshell: water in the air condenses on cooler surfaces. The more moisture in the air, the higher the temperature at which condensation will occur. The colder the surface, the more likely condensation will form on it.

Why does condensation form on my ceiling and rafters?

There are a few reasons why you’ll often get condensation on ceilings and rafters before it shows up anywhere else:

  1. Humid air rises so it pools at the ceiling
  2. Ceilings and rafters change temperature more slowly than the air
  3. Cathedral ceilings often have less insulation and can cool at night
  4. Rafters conduct heat/cold from the roof down into the house

If you put a moisture meter at the ceiling level, you’ll find that the humidity is often much higher than at the floor level. Think of a helium balloon. Let it go and it goes straight up to the ceiling because helium is lighter than air. Water vapor (H2O) is lighter than air (a combination of nitrogen, oxygen and carbon dioxide) so it rises just like helium.

To summarize: condensation forms on the ceiling and rafters because the humidity is higher at the ceiling level and those parts of your house get cooler at night than other parts of your living space.

There are additional factors that can affect the formation of condensation on parts of the house. For example, when the air conditioner runs, it blows cold air on things. The air vents are pointed towards things in the house and cools them. It it’s pointed at the ceiling, it might cool off the sheetrock or rafters. Those items are heavy and stay cold for longer than other things. Cold things “attract” condensation (they actually cause condensation). So, just like your bathroom window gets foggy after a shower, the ceiling or rafters are getting ‘foggy’ with condensation. But they’re not transparent, so you don’t see it until the condensation gets so thick that it starts dripping on your head!

Why is there mold on my ceiling?

Ceiling panels in a humid room are being eaten away by mold

Often, before it starts raining in your living room, you’ll see mold, or black spots forming on your ceilings. This is especially common in bathrooms where showers lead to extremely high humidity levels every day. Why does this happen?

Mold likes water, food and heat. What do you get at the ceiling? Humid air, food (sheetrock, dust, etc.) and warmth!

If the moisture exists for long periods of time, it will attract mold spores which will thrive under those conditions. If the moisture goes away quickly, mold usually won’t form because mold really wants a steady supply of water in which to grow.

So if you can reduce the humidity levels in the house, the mold won’t want to grow. Even if the humidity goes up and condensation forms once or twice, if it dries out quickly, usually mold won’t grow. It’s that extended period of dampness that mold loves.

Now you know why condensation forms and what mold likes, how do you stop it?

What causes high humidity in the house?

We’ve covered the problem, but what’s the source? That’s the critical question.

During the summer, most of us run air conditioners. Air conditioners remove water from the air very effectively. In fact, if you look at the condensate lines when the AC runs, you will often see a steady trickle of water. That can add up to many gallons of water per day.

Suppose your house were like an air-tight balloon. As the AC ran, removing moisture from the air, the total moisture in the air would drop until the AC couldn’t remove any more. The air would be very dry. You’d never have high humidity problems.

But then take a shower. When you shower, the water saturates the air, pushing the humidity higher and higher. This is why it’s so critical to run a bath fan while you shower and for about a half hour after you shower – you have to flush that humidity out of the house.

Or perhaps you boil some water. That’s dumping moisture into the air. Or water a bunch of plants – where’s that water going?

There are many obvious sources of water inside the house that lead to increased humidity levels. You can control the obvious sources by running fans as needed to flush out the excess humidity.

You’re probably asking “if you’re in a sealed ballon (house) and your blow air out with fans, isn’t the ballon going to shrink? Where’s the air come from to keep air in?

This is where it get less obvious. The house isn’t an air-tight ballon. It’s a structure with windows, doors, chimneys, and all sorts of nooks and crannies where air can come in the house. So every time you run a fan, or the clothes dryer, or open a door or window, any air that goes out gets replaced by outdoor air which carries water vapor along with it.

Or, maybe you just like fresh air and open the house when it’s cool out, like in the morning or evening. Nothing like that cool, fresh air, right? We just have to remember that that fresh air is often laden with moisture during the summer. Airing the house out can make the house very humid indeed.

There are other, much less obvious sources of household moisture. You’ve probably noticed that basements are often “dank” and humid, smelling of mold. Why is that?

Why are basements often humid and moldy?

Basements, are underground. When it rains, where does water go? Into the ground! Where are natural springs? Underground! Where is ground-water? Underground!

Moisture problems often start in the basement

Your basement is built in a pile of dirt that’s filled with water.

The foundation walls of most homes are very water permeable – that is, water vapor easily flows through cinder blocks and concrete and even rocks. Water vapor is extremely powerful and tiny – it gets through the smallest of cracks, even things you can’t see. In fact, it’s one of the tiniest molecules in nature. Water goes everywhere!

So, your house is built into the wet dirt, and water vapor easily moves through the foundation walls and into the house. Since water vapor is lighter than air, it floats up through the house, increasing the humidity levels inside the house. In many older homes, water coming in through the basement can be the largest source of water entering the home.

Making basements worse is that they’re usually closed up from the rest of the house. Water vapor that comes through the floor and walls builds up down there, leading to high humidity levels. That leads to musty, moldy basements. But remember, much of the moisture still flows up into the house, helping to cause those moldy spots on your ceiling!

A short aside on wet basements: if your basement has been good most of the time but periodically gets wet, check your gutters. Thousands of gallons of water comes down the downspouts during rain, and you have to get that water far away from your foundation walls. Clean gutters and downspouts are the first step. Then you need to direct the water away from the house, preferably draining at least six feet from the foundation walls. And sloped away from the house to the water doesn’t pool at the foundation. If you have this problem, fix it. You’ll thank me when your basement stays dry and mold stops growing everywhere in the house!

This doesn’t help! Get that water far away from the house

How else can humidity enter a house?

Here’s the biggie, and least obvious – through your air conditioning and heating system ductwork!

If your house gets excessively humid during the summer, and dry during the winter and maybe gets very dusty even though it seems tightly sealed up, you probably have faulty ductwork.

I’ve seen this in every type of home, from multi-million dollar mansions to mobile homes. Ductwork is the weak link in a home’s heating and cooling system. Until recently, there were no standards for ductwork. Fortunately, in many areas, ducts have to be tested to be relatively air-tight. But the vast, vast majority of homes’ ducts have never been tested. Even ducts that were fine a few years ago can develop serious leaks, leading to horrible conditions in the home.

A detached duct like this can double your cooling bills and cause health problems

If you’ve looked at all the obvious places, your basement is dry, you religiously use your bath fans when you shower and nobody can figure out why your house is still humid in the summer, test your ducts!

I had one client with a very high-end home, built by a top builder. He said “my floors warp when it rains. My family get nosebleeds in the winter. The art is getting ruined. I’ve installed a dehumidifier and it does nothing. My air conditioning bills are outrageous and my builder has done everything and charged thousands of dollars and nothing worked! HELP!”

It took less than an hour to find the problems, and he had two different problems.

Problem 1: A return air duct had fallen off
Problem 2: The air handler was mounted in the attic and the filter port didn’t have a cover

The “return air duct” is one of the ducts that runs from a grill in your house that sucks air from the house, into the duct, through a filter and back into the AC, to cool it. When the return duct falls off, the air gets sucked from wherever the duct is, in this case, the hot, humid attic! This easily doubles the load on the air conditioner, since now it has to cool 120+ degree attic air. The attic is ventilated somewhat so at night, all the outdoor humidity fills the attic and gets sucked in and blown into the house.

It also sucks in whatever nasty stuff is in the air in the attic, probably fiberglass dust and mouse droppings. Which it promptly blows into your living space. YUCK!

Problem 2, the filter port, is less obvious but almost as important. Look at this photo:

What is getting sucked in that doesn’t get filtered?

The “open filter port” is extremely common and damaging to your health, comfort and efficiency. Your AC system is supposed to circulate air from inside your house, draw it back through the filter, cool it down and redistribute it around your home. When working properly, it cleans and dehumidifies your air every time it runs.

If you don’t have an air-tight filter port on your system, or worse, no cover like this, the giant fan in the system is going to suck air from the attic/basement/garage where the filter is. This air won’t get filtered and it will contain all the humidity and dust/impurities that are in the air.

Symptoms of air filtering and duct system failure are:

  1. The house gets humid very quickly on muggy days
  2. Running the AC doesn’t seem to dehumidify the air
  3. The house gets dry during the winter (if you have forced air heat)
  4. Air conditioning bills seem very high
  5. Air conditioner struggles to keep the house cool
  6. Air coming out of the vents isn’t very cold
  7. House gets dusty fast
  8. Even when the system is turned off, you get drafts from the vents
  9. Sometimes bad odors will come from the vents
  10. Air vents are dirty/dusty

These systems can be very easy to find if they’re caused by issues like illustrated in the photos. Or very difficult, if they’re hidden away in an attic or even inside the walls. The best you can do without specialized equipment is to inspect the parts you can easily access. But there’s one other trick you can use to determine if there’s a problem…

Testing your ducts for air leaks

If you want to determine if there are big problems with your duct system, you can try this. It’s not definitive, but it can definitely help.

  • Close up the house. All windows, doors or other things that allow air in
  • Turn OFF the air conditioner/central heating
  • Turn on every exhaust fan – bath fans, cooking fans etc.
  • Turn on the clothes dryer

This is “depressurizing” the house. That is, it’s trying to blow as much air out as possible so that you’ll be able to feel the areas where air is leaking in. The test works best in winter, when it’s very cold. But during a hot day in summer it can also work.

Now, go around the house and feel every air vent. If the ductwork is tight, you should feel nothing. However, if there are air leaks, you can feel a draft. The bigger the draft, the larger the leak. Sometimes, you can feel a much larger draft from some vents than other. That probably indicates that the problem is close to that vent or the duct that supplies it.

If you get a slight draft out of all the vents, then there’s probably a problem near the main air handler because the air is flowing in there and coming out everywhere.

The test isn’t precise but it’s a start and will help you narrow things down.

Get a professional duct test

If you haven’t yet been able to track down your problem, get a professional duct test. They will seal your system and blow air through with a large fan. They will also be able to measure the air leakage in the ducts, which will tell them whether you have a big problem (detached duct) or a smaller one (small leaks). A professional should be able to track down the problem and fix it.


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