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Why is there mold in my attic?

Written By Tom

Published on 05/30/2017 11:02 AM


Why is there mold in my attic??

Let's face it! It's a pretty rare occasion nowadays that people find themselves in their attics. In fact, it's not unusual to hear a seller of a property they have owned for 20 years state that they've never even peeked up there! During the heydays of the 50's and 60's, this cramped space was often used to store that old collection of warped 78 RPM records that everyone had and a few stacks of National Geographic magazines tied together with twine, along with various other items of which nobody seemed to know their point of origin.  It also housed that thin layer of asbestos-laden vermiculite insulation that we used to play in. (that explains a lot!) This space was entered a few times a year in order to retrieve and return the plywood Santa that sat in the front yard every December.

Fast forward to the 70's, more specifically, 1973 when America met its first energy crunch with a vengeance. (Thanks, OPEC!) Along with spikes in the costs to heat and power our homes, not to mention fuel our vehicles, our building industry began researching and discovering new ways to contain the end-products of this fuel, namely heat and electricity, in order to increase the energy efficiency of all the stuff our baby-boomer generation had accumulated.

Add to all that, creeping suburbia continued to grow like a weed with its unrelenting boom, and subdivisions were popping up with reckless abandon as the great migration from our cities continued.

Building standards

As has been the case throughout my 30+ years in the building industry, building standards have always seemed to be several years behind best practice, or even good practice in many instances! Especially when it came to energy efficiency. We seem to have this "learn-as-you-go" mindset.  We standardize something, wait a decade and see how it worked out. Then come up with other standards in hopes of fixing/altering/correcting what we initially did to eliminate the negative impact that whatever it was created.

Over the decades, we learned about venting our attics in order to "sweep the air" through these spaces. We learned about installing more insulation to our attic floors, which we usually did in the form of piling on additional fiberglass batts. And those insulation batts sat ceremoniously in place all the while as the warm air from the houses continued to rise between the sections.

Air sealing

One place we were late to the dance, and continue to be so in many instances from actual practice in the field; air sealing our attic floors. As warm always moves towards cold, and warm air rises, our attics are in the perfect target zone. Air sealing is the process of providing a seal to prevent the passage of air or vapor; in this case, from our house into our attic. Lacking this seal, every small hole or gap through the attic floor is allowing the warm air that you paid for, not to mention the water vapor your family creates, to rise into the attic. We call this thermal bypass. Thermal bypass is described as air, heat, and moisture that moves in and out of a building through openings in the building’s envelope.

Add this to our 1970's version of attic venting and insulation improvements and we've created a perfect storm that actually pulls heated air out of our houses, sending it right into the attic. This heated air then condenses on the underside of the roof framing and becomes a perfect environment for mold growth.

The fix?

When we have mold in our attic, there are actually two things that need to happen.

  1. Eliminate the mold that exists
  2. Eliminate the conditions that allowed it's presence

Is it the Black Mold?

This is the question I am asked 99.5% of the time when I discover mold in an attic. Mold is black or blue or greenish blue or white or purplish-green and a host of other color variations. There are literally thousands of different molds that are black! Some are toxic, some are allergenic, some are both and some are neither. Don't be scared off by reaction words. That's not to say we don't still have to deal with it. We do!

Should I have the mold tested?

You can test the mold if you want, though the fact is, whatever 28-letter scientific word your mold test determines you have, the resulting recommendation is going to be the same. Eliminate the mold that exists;  eliminate the conditions that allowed it's presence. So in my opinion, testing is a waste of money. If it makes you feel better though, test away! Bottom line; we have mold! We want it gone! Let's get rid of it, regardless of what brand it is!

Insulation stops heat, not air

All the insulation you can pile in the attic will not stop the air that moves around it. Insulation slows the transference of heat only. We must stop the movement of air and vapor!

How do we eliminate the conditions that caused it?

I mentioned air sealing and thermal bypass previously. In order to stop this migration of warm air and vapor, we need to start over with the way that we think about our attics. Usually, this requires removing the insulation that is currently on the attic floor and having an "air sealing protocol" implemented.

  • During the air sealing process, a contractor (usually a skinny one) will crawl around the attic floor sealing every gap, hole or crack that is communicating with the living space below. This will stop the air from moving into the attic and provide a continuous thermal boundary.
  • Next, reinsulating (with something other than fiberglass batts((my opinion)), whose stated R-value is only achieved under perfect laboratory conditions and never, in my experience, in an actual attic.)
  • Sometimes increasing the soffit and ridge ventilation will also be helpful
  • Let's not forget any kind of exhaust fan, such as bath and kitchen vent fans. If it has the word "fan" in it, it should go directly to the outside; not into the attic, not near the gable vent, not tied up near the ridge vent, but to the outside.

If you take anything from this article, the mold isn't there because you haven't piled enough fiberglass batts on top. The mold is there because warm air and vapor are moving through your attic floor.

Oh, and who can fix these things? Building Performance Contractors, who are specially trained in understanding air and vapor movement through houses and equipped with the knowledge to rectify these conditions. Visit the Building Performance Institute (BPI) site for a list of local contractors carrying this designation.  http://www.bpihomeowner.org/find-a-contractor

That's all for now! Stay safe!