If you’ve ever owned a home, you’ve probably had mold at some point. Mold on the wall. Mold in the shower. Mold in the basement. Maybe mold in the attic, like shown in this picture.

Mold spores are all around, floating in the air, on surfaces, in your house plants, wherever you look, you’ll find mold spores. This is perfectly natural!

On the other hand, when mold grows uncontrollably, like shown above, it can cause horrible damage and have serious health ramifications if you have sensitivities.

So what causes mold to go from benign to growing like crazy? 

The answer lies in humidity. Mold needs moisture in which to grow. That’s why you often find mold in showers and basement. But not just any moisture, but high levels of moisture. “Ok,” you say, I understand showers and basements. “I can run the bath fan for a while after a shower and flush out the humidity” (yes, you should run the bath fan for about 20 minutes after a shower!) “And I put a dehumidifier in the basement to help that problem. But what about the darned attic?

Let me tell you about attics! But first, I have to tell you about condensation…this won’t be painful, I promise.

What is condensation? It’s simple! – it’s just the water vapor in the air finding a something cool enough to for the water vapor to turn into liquid water. Warm air can hold more water vapor that cold air. That’s why the window in the bathroom gets covered with condensation when you take a shower. The air in the shower is hot, and saturated with water vapor. Everything else in the bathroom is cooler, but the window (in the winter), that’s really cold. So that’s where you see the condensation. 

“Ok”, you ask, “but what about the attic!?” 

Here’s the deal. During the winter, do you heat your attic? Hopefully not. There’s probably a lot of insulation on the attic floor, trying to keep the heat in the house and the attic cold. Aha! The attic is cold! And what about the roof? It’s really cold, especially on the north side of the house which doesn’t get any sun.

So, any moisture that gets into the attic is going to condense on one of these cold surfaces. Where the moisture condenses is a big clue as to where the moisture is coming from.

For example, in the picture at the top of this post, it’s immediately obvious that the moisture is coming up that roof cavity and condensing on the cold roof in the attic. In fact, that wood is saturated with water – literally dripping! That means there’s a lot of moisture coming up that cavity.

How’s that happening? Well, I can tell you with 100% certainty that the moisture is NOT coming from the outside. Why? Because the outside air is cold and dry. If it were going to condense, it would have condensed outside, not on your roof. So rule that out.

If not from the outside, it must be coming from the inside. AHA! Remember, warm air holds much more moisture than cold air and inside the house, there are showers, animals, cooking, breathing, house plants and so on. All putting moisture in the air. That moisture rises up through the house and follows the ceiling upwards. And what’s above the ceiling? The attic. 

But how does that moisture get into the attic? You’ve got walls and ceilings and sheetrock. What gives?

Water is a tiny, tiny molecule. When in its vapor form, it’s still tiny – vastly smaller than liquid water. Think about a balloon. Even the tiniest hole and the air comes rushing out. It’s the same with your house. That recessed light you installed? That’s filled with holes. Ceiling Fan? Holes! Electrical outlets? Holes! Door frames? Holes. 

There are holes everywhere!!! And any moisture in your home is going to find those holes. In fact, during the winter, literally gallons of water is moving from the inside of your house into these cavities – walls, attics, etc.

In small quantities, the moisture harmlessly flushes out and is dried out by the winter air. No problem. But get a big enough hole, like around recessed lights, and you’re fighting a losing battle. Seriously. Recessed lights are one of the biggest problems I encounter because they’re never installed air-tight. NEVER.

And so, almost every roof that I look at that is near recessed lights, is covered with mold. There’s probably millions of dollars of rotten roofs caused by recessed lights. This is made worse by the stupid practice of installing ridge vents on homes in an attempt to add ventilation. 

This is a controversial statement, so let me explain further. What’s the purpose of a ridge vent? It’s to give air a place to rise up and out of the attic. Well, if air is leaving the attic, where does it come from?

In many cases, there’s no place in the attic for air to come in and replace the air that’s going out the ridge vent. So what happens? That ridge vent sucks the air out of the house – exactly what you do not want to happen!!! So the better the ridge vent is working, the more moisture is getting sucked from your house into the attic and the more mold problems you’ll get. This is why so many contractors actually make attic mold problems worse. They’re just solving the problem the way they were taught – add more ventilation!

The Solution

Before you panic, there are solutions.

First, remember, moisture problems and mold occurs where warm moist air comes in contact with cooler surfaces. So how do you avoid the problems?

1. Don’t let warm, moist air out of the house
2. Don’t let warm, moist air touch cold surfaces
3. If warm, moist air does get into a cold space, get it out of there fast.

Got that? It’s that simple. If you follow these rules, you cannot have moisture and mold problems.

So how do you do this?

1. If you don’t have recessed lights, don’t add them! 
2. If you already have recessed lights, seal them airtight. This requires some handy-work. Seal the circle around the perimeter between the light fixture and the ceiling. This can be done with spackle and high temperature silicone caulk. All the holes in the fixture? A dab of caulk or some foil tape can largely seal those.
3. If you have other holes in your ceiling, seal them up the same way. Pretend you’re building a submarine. Seal all the holes like your life depends on it. Read this article on cathedral ceilings.
4. If you’re building a house and you want cathedral ceilings, read this article. Then have at least 4” of closed cell spray foam applied directly to the underside of your roof. You’ll be shocked at the price, but consider this – if you use fiberglass and your roof rots out, you’ll spend at least $10,000 to redo your roof. Spray foam will seem cheap in comparison.
5. If you’re ventilating your attic, do NOT just add a ridge vent. You have to install a ridge vent AND matching soffit vents. Or, do it the old fashioned way, and install gable vents on each end of the attic so that the air can flow through. You can’t just have one vent. Air has to flow in and it has to flow out.

When you’re all done absorbing this, read my other articles. This is probably the most far reaching and potentially complicated topic I’ll write about, so there’s lots of material!

Basement insulation and moisture

Bathroom venting

Cathedral ceilings

Good luck!