Building Science vs. B.S.

This video is a promo for my upcoming video series, Building Science vs. B.S.

You hear B.S. every day….


houses need to breathe
too much insulation causes roofs to rot
furnaces dry out your house
cold water boils faster than warm water
green building is always more expensive
ridge vents always help cool your roof and reduce moisture


Unfortunately, B.S. like this gets spread by supposed experts, further perpetuating these damaging myths.

The problem is, people are really bad at reaching proper scientific conclusions. Our brains are wired to look for patterns, and research has shown that we create patterns where none really exist. For example….

  • Did you ever buy a new car, then suddenly notice that model everywhere?
  • If you’re a woman who has been pregnant, didn’t it seem like there were lots more pregnant women around?

It’s called “perceptual vigilance” and it leads to all sorts of incorrect conclusions. It causes people to think something in their environment has changed, while in fact, it is only their brain that changed!

There’s another favorite term among scientists: “correlation is not causation.” This is a related cousin to perceptual vigilance. What they’re saying is that just because two events appear to be correlated, it doesn’t mean that one caused another.

Insulation  does not cause roofs to rot!

Let’s start with a simple piece of B.S., usually spread by poorly informed contractors.

The scenario

A contractor remodels your bathroom in the winter. They install some lights and a fancy new bath fan. Since you’re “green”, you suggest that he* adds insulation to your attic.

Argh! Mold in the attic!

After a few months, you go up to your attic to store some boxes and find mold everywhere! WTF! you think as you beat a hasty retreat. You call your contractor and yell at them about all your mold taking over your attic as you think about having to replace your roof and everything you’ve been storing in the attic.

“Oh yea, I’ve seen that before”, says the contractor, “It’s because you wanted me to add that insulation. Didn’t I tell you? Too much insulation causes mold.”

After much pleading, the contractor comes back, helps get rid of the mold, and pulls out most of the insulation. “There”, he says confidently, “that will get rid of the mold problem!”

The summer goes by and most of the attic is mold free. That autumn you call back the contractor and tell him that he must have been right. Insulation causes mold.


What really happened…

There were two two primary reasons you didn’t have mold before and mold developed afterwards

  1. Before the renovation, the bathroom was better air sealed. When the contractor installed the new fan and lights, he did a poor job of sealing the bathroom so lots of moisture was allowed into the attic. Plus, maybe he vented the bath fan directly into the attic – that is almost a guaranteed catastrophe.
  2. Before you added insulation, more of your home’s heat was escaping into the attic, keeping it warmer. Mold grows where there’s too much moisture. Water vapor condenses more on colder surfaces. So when you added insulation, it got colder in the attic (which it should) and the coldest area was under the roof deck.

In addition, you’re much more likely to get attic mold during the winter because the attic is cold when it’s properly insulated. During the spring and summer, the attic may be too warm for the moisture to condense. The high temperatures can literally bake the moisture out of the roof. Same for the sunny south side of the roof (for those of us in the Northern hemisphere).

In the above scenario, chances are, when winter rolls around, you’ll get mold again because the attic will get cold enough for serious condensation issues. But, if you are leaking enough heat to the attic, you might not even have problems then.

Sloppy construction causes mold

Insulation did not cause the mold — sloppy contracting work leading to high moisture levels combined with a cooler attic caused the mold. There are plenty of attics that have been super insulated that have absolutely no mold problems.

How do you fix it properly?

Bathroom areas are notorious for problems like this because showers contain air saturated with moisture. Air leaks from the bathroom allow moisture into the walls or up into the attic. So it is imperative that you takes extreme measures to air seal bathrooms and properly flush out the moisture.

  1. Ensure the bath fan is properly vented out the roof with insulated, sealed duct. Not out the soffit. Not out the gable vent. Out the roof using a high quality roof cap using 4″ insulated duct sealed with high quality tape.
  2. Seal all holes around fans, light fixtures, wires, etc. in the attic so that moisture can’t leak out of the bathroom and into the attic. Often, you will need to seal the top of the wall framing because there are big gaps where moisture can flow through. If you see discolored insulation anywhere, that’s a sign of an air leak. Seal it!
  3. Best is to add a dehumidistat to automatically run the bath fan when the humidity in the bathroom is too high (>60%). This removes the human element of needing to guess when to run the fan.

Taking these steps – venting the humidity properly and air sealing the bathroom from the attic, will eliminate that source of moisture, greatly reducing moisture and the related mold problems.

Keep in mind that you might have several bathrooms and other openings that allow moisture into the attic, so I strongly urge you to do a full evaluation of the attic, looking for any place that is not air-sealed. Recessed lights, attic hatches and pull-down ladders are particularly notorious for these issues. See The Energy Guardian for really good products for sealing and insulating hatches and ladders.


*Contractors are mostly male, so I’m using male terminology rather than the awkward, but  politically correct: he/her etc. The mere fact that I have to justify the use of a pronoun annoys me to no end….

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