A three-season cabin built in the 1940s became a year-round dwelling two years ago, but owner Marty Pfeif has discovered an alarming problem: a bumper crop of mold in the attic.
In a post at the Q&A forum, Pfeif ticks off the particulars, including no apparent attempts at air-sealing, “shake and rake” R-19 insulation on the attic floor and some batting against the walls, no vapor barrier, and a ridge vent but no gable vents.
In addition, he’s recently learned that an exhaust duct for the bathroom fan has fallen apart, allowing moist air to spill directly into the attic for the last couple of years.
“This week I found mold in the attic, and lots of it!” he writes.
“I’m spraying the mold with bleach, and have ordered ,” Pfeif continues. “I have removed a 12×5-foot section of insulation, and I am in the process of air-sealing this section.”
When he’s done, he’d like to have a more usable space, either for storage or possibly for use a loft.
“The house is only 600 square feet,” he says. “It has vaulted ceilings everywhere but the bedroom and bathroom. I have no idea if mold is growing in this part of the roof. I plan to hire a new contractor, but this time I want to know what should be done, instead of blindly trusting that they will do the job correctly.”
Air leaks are the problem
The cause of the problem is no mystery to GBA senior editor Martin Holladay, who writes, “The most likely cause of the mold is air leaks in your ceiling. These air leaks allow humid interior air (conditioned air) to enter your attic. The moisture in the air condenses on cold attic surfaces, encouraging mold growth.”
The ridge vent, he adds, is just making the situation worse because it lowers the air pressure in the attic, thereby pulling air through cracks in the ceiling and adding even more moisture.
Holladay’s recommended fix is to temporarily remove the insulation and seal the air leaks. But, he adds, don’t worry about adding a vapor barrier, such as polyethylene plastic, because the problem is air leakage rather than vapor diffusion. “The moisture is piggybacking on exfiltrating air — it’s not diffusing through your drywall,” Holladay wrote (before Pfeif revealed that his ceiling lacked drywall). For more information on the difference between air leakage and vapor diffusion, see the links in the “Related Articles” sidebar.
How to seal the air leaks
Pfeif has bought some canned foam to seal the leaks, but is this the right stuff? Maybe not.
“As you move the insulation out of the way and air-seal the cracks, you’ll find that spray foam in a can is useful for big obvious leaks like around ceiling lights and sometimes drywall cracks, but you’ll typically use way more caulk which is easier to tool into place,” says Andy Chappell-Dick.
Keith H suggests that Pfeif get pro-grade spray foam equipment (one source is ). “The consumer cans are terrible for applying foam and dry up fast,” Keith says. “You can also get a super long gun if you want for reaching under the roof. A properly stored pro can on a pro gun lasts months.”
He adds that while caulk is a common suggestion, his experience has been that caulk shrinks over time. “Foam shrinks, too,” he add, “but is easier to apply in generous quantities.”
As the online discussion developed, Pfeif shared the fact that his ceiling consists of tongue-and-groove boards. Holladay suggests that drywall can be an effective air barrier, and it might be the simplest approach in this situation.
“The easiest approach is to install drywall on the interior side of the tongue-and-groove boards,” says Holladay. “Of course, that means you won’t see the boards. If you love the boards, and want them to be visible, you can try to remove them carefully so that they can be reinstalled later. As long as they weren’t installed with ring-shank nails, this is probably doable.
“After the boards are removed,” Holladay continues, “install drywall with taped joints. Then re-install the boards on the interior side of the drywall (assuming you like the look).”
Skip the bleach
Pfeif has been attacking the mold with bleach. That’s a recommendation that pops up frequently, but it’s not without its risks, and it’s unnecessary.
“The bleach is working, but my eyes are very itchy, and my cheeks feel irritated,” he complains. “While wearing the the smell of bleach became intolerable. I don’t think I had the mask sealed, and the fumes were trapped under it. I have been fit tested and trained on how to apply the mask, but the 3M mask I purchased at the store was stiff and difficult to mold. I feel like it did more harm than good.”
Chappell-Dick says that misting moldy areas with bleach won’t fix the problem and may even make it worse. Also, an N95 mask is for particulates, not exposure to chemicals.
“I don’t know enough about chemical exposure protection to say what would protect you from bleach but an N95 won’t,” he says. “I suggest you try calling 3M or another vendor of respirators to get assistance with what type of respirator cartridge is needed. Again, I would not continue the bleach.”
Who said anything about bleach? Holladay ask. “Skip the bleach,” he says. “If you need to clean up mold, ordinary soap and water (for example, dish detergent or laundry detergent) is all you need — plus some way of keeping everything dry.”
And as for Pfeif’s comment that a friend has offered to power-wash the attic once all of the insulation has been removed, forget it. Malcolm Taylor put it this way: “Pressure washing the attic is an insane idea.”
Don’t make the solution more complicated
Pfeif has pondered bleach, pressure washing, new gable events, and vapor barriers, but Chappell-Dick reminds him not to overthink the solution. It’s really all about the air sealing.
“People like the look of a board ceiling, especially in a cabin, but by itself it’s not compatible with a heated house,” he writes. “It must have an air barrier added. Draping sheet plastic over the joists in the attic is an awkward solution at best — hard to seal and prone to compromise. Plus, you don’t need any kind of ‘vapor’ or ‘moisture’ barrier on your ceiling — just an air barrier, for reasons that Martin also discussed.
“So it may be a tough decision,” Chappel-Dick continued, “but really your only workable solution is to put up drywall on the ceiling, either just covering up the boards, or tear off the boards and reinstall them after the drywall is taped (although FWIW this introduces a thousand nail holes into the drywall…)”
Throw out any old insulation that looks moldy and replace it. With an effective air barrier in place, any remaining mold spores will remain dormant and harmless.
One footnote: Pfeif should think twice about using that attic for storage or as a loft, Chappell-Dick says. “That’s Yosemite Sam shooting himself in both feet,” he writes. “A usable attic is a troubled attic. With sufficient insulation, an attic is a terrible place to store stuff — dusty, hot, accessible only through a very tightly sealed door. As for the loft idea, you’d have to completely change how the space is insulated and air-sealed; now the roof becomes your thermal and air barrier, which can be a huge challenge in an old building.”
Our expert’s opinion
Here’s GBA technical director Peter Yost’s take:
A common problem with turning cabins in heating climates into year-round homes is that people often insulate first when the required order of updates should actually go like this:
(1) Improve bulk water management.
(2) Air seal.
(3) Do the above while maintaining/designating directional drying.
(4) Manage interior and household sources of moisture.
You could argue that #4 could be moved up one, two, or even three spots. I am OK with that, but #4 must be on the list, and insulation should be dead last.
For seasonal cabins which now need to be airtight, you need to bite the bullet and guide your air sealing with a blower door. It doesn’t matter if you or a contractor is doing the work: the blower door will tell you if or when you have managed to improve the building’s airtightness. In my experience, for buildings in cold climates, airtightness should be in the range of 3 to 4 air changes per hour (at 50 pascals) to manage wintertime condensation.
Do you need to clean up the mold before you run a blower door? Good question. For most of us, mold spores are just particulate irritants, annoying but not injurious. If you are worried about too much contact with the mold spores, clean them up with soap and water as Martin described. You are physically removing the spores as opposed to trying to “kill” them with bleach.
And one more thing on item #4: I get called quite a bit about interior moisture problems in the winter in cold climates just after energy retrofits. Prior to insulating, the home was tolerating quite a bit of interior or occupant moisture loading. But those same loads can become a problem after the energy retrofit. Here is the list — pretty much in order of priority and prevalence — of the interior sources of moisture I find that need to be better managed after the energy retrofit:
- Cordwood stored in a basement.
- Exposed dirt or unsealed crawl space floors.
- Unvented gas appliances (heaters, stovetops, ovens).
- Lack of, or poorly installed, kitchen range hood.
- Lack of, or poorly installed, bath exhaust.
- Clothes dryer vented to the interior during the winter.
- Greenhouses attached to living spaces.
Along with buying at least one decent hygrometer so you know what your overall interior moisture is (see this GBA article: Measuring and Understanding Humidity), check off all of the moisture sources listed above, and you will be in good shape.