Author’s Note: I am setting up a series of building investigations that I have done over the years as puzzles, presenting successive pieces as an interesting way to tell the story. As with any story, you can read the end first if you want, but that approach cuts down on the drama (admittedly not a bad thing for some folks…)

“How do we know if someone really knows what the cause is? I’ve been dealing with mold for 20 years. We had mold in our previous house. My health has cost us so much financially already. Maybe the solution won’t be too expensive. Maybe it will be. I just want to know what the cause is and what to do about it so we can finally have this whole problem behind us.”

I lifted this quote directly from an email I received a couple of years ago, and sadly it’s not that unusual: homeowners frustrated with persistent or recurring mold after a wide variety of building practitioners of various stripes have rolled through, scratched their heads, and shrugged.

Let’s work through this puzzle pretty much how it played out.

Puzzle Piece #1:

Client: The mold is oddly only in certain closets and always down low, fading and thinning as you move from the baseboard up to about two or three feet (see Image #2, below).

Pete: The only practical way of getting rid of the mold is to manage moisture. I can help, but almost certainly I need to come and assess your home for moisture problems.

(Interlude as homeowner digests the cost and checks my qualifications…)

Puzzle Piece #2:

I drive up, and after introducing myself, immediately start walking the site looking at and taking pictures of bulk water management (see Images #3, 4, and 5) on the outside of the home. (It had rained long and hard the night before I showed up.).

Client: Why are you starting on the outside? The mold is in here.

Puzzle Piece #3:

Pete: It’s pretty damp inside your home, given that you are still intermittently heating, you don’t have a whole-house ventilation system, and your home is closed up (mid-April, 10:30 a.m.).

• Outdoors:           59°F (dry bulb); 52°F (wet bulb); relative humidity 64%
• Basement:           56°F (dry bulb); 49°F (wet bulb); relative humidity 60%
• Kitchen:             65.5°F (dry bulb); 55°F (wet bulb); relative humidity 56%
• Upstairs (MBR):   68°F (dry bulb); 56.5°F (wet bulb); relative humidity 46%

Client: Huh, it doesn’t feel that damp to me; are those numbers high?

(Author’s note: we humans are pretty good at sensing dry bulb temperature, but notoriously lousy at sensing relative humidity).

Puzzle Piece #4:

Pete: Nice dual boiler (wood/fuel oil). How much stove wood do you normally store inside the basement for your boiler?

Client: We try to keep those 4 racks in the basement (see Image #6) as full as possible in the winter.

(Author’s note: one cord of wood can release as many as 800 pints of water as it dries inside a heated building).

Puzzle Piece #5:

Pete: Do you use your stove and oven quite a bit?

Client: Oh yes, we both like to cook and bake.

The oven has a range hood fan, but it’s a recirculation unit. No ventilation there. And burning 1 therm of gas releases about 1 gallon of water for unvented gas appliances or about 2.5 pints of water to cook a typical family dinner.

Puzzle Piece #6:

Pete: I just checked all the bath exhaust fans. The downstairs unit works fine (see Image #8), as well as the one in the upstairs bath, but the master suite bath exhaust fan does not work at all.

Client: You are making me nervous because I see a pattern developing here…

Puzzle Piece #7:

Pete: OK, it’s finally time to look at the mold.

Client: It’s funny, but the mold is in these three closets. It’s mostly these two on the first floor and then a bit of mold in the upstairs hall linen closet.

Pete: Huh — all three are along the central partition of your house. This is a modular home, isn’t it? (See Images #9, #10, and #11).

Puzzle solved?

Pete: Your home has high levels of moisture pretty much on all levels. In the winter, cold air is leaking in at your basement rim joist and being pulled up the home’s marriage wall, which — with just that fiberglass insulation — forms a highway for air to leak from basement to attic.

The first-floor mold pattern is caused by the cooler basement air bringing the surface temperature of your gypsum wallboard down below the dew point so that condensation forms. But as that cold air travels up, it gets warmed and the mold fades.

I think the mold in the upstairs linen closet (which is much less pronounced) is caused when it’s really cold and your wood boiler is going full tilt with the combustion air it needs pulling cold air down from the attic on occasion.

Client: Oh my word; this all sounds really expensive to fix.

Pete: It’s not nearly as bad as it sounds, really. Complete the action list below. Frankly, many (if not most) of these things you could do yourself.

The mold problem has been eliminated

About a year and a half later:

Client: “We did everything you recommended to address the mold here. The mold problem has definitely been eliminated. We have not seen any more or new mold anywhere in the house. And it just feels better overall. It’s hard to describe how it feels better exactly but it does. I guess a word to describe it might be that it feels fresher in the house.

Pete: Sounds like money well spent, all things considered.

In addition to acting as GBA’s technical director, Peter Yost is the Vice President for Technical Services at BuildingGreen in Brattleboro, Vermont. He has been building, researching, teaching, writing, and consulting on high-performance homes for more than twenty years. An experienced trainer and consultant, he’s been recognized as NAHB Educator of the Year. Do you have a building science puzzle? Contact Pete here. You can also sign up for BuildingGreen’s email newsletter to get a free report on avoiding toxic insulation, as well as regular posts from Peter.