I want to tell you a story. A story about a guy in Philly, in the 30s, before insulation, before attic ventilation.
So, this guy has a drafty house and it is cold. But he has an idea, he puts some stuff in his ceiling, above the plaster. Yes, this is before gypsum—some of you old guys remember plaster, right?
So, he puts this stuff above the plaster ceiling to help keep heat in the house in the winter. The inside of the house gets warmer, his wife gets warmer, the kids get warmer, he gets warmer, his wife is happy, so he is happy.
A few months go by and he is so excited about his idea that he climbs into the attic to admire his work, and he notices some black and green discoloration on the underside of the roof.
What is this? Today, we know it is mold.
Why did this happen? This is a little tricky to answer.
You see, when he put this insulation stuff above the plaster, the rate the heat escaped from the house went down; that means the higher parts of the building, the roof deck, and the roof rafters didn’t get heated as much as before, and when it’s cold outside they are colder.
All of this time the family went about their normal business indoors: Eating, bathing, cooking, breathing, etc.—creating moisture. Everything they do creates moisture.
Because the house is drafty and warm, moist air rises, the air passes through the ceiling and the insulation and carries moisture with it. This warm, moist interior air finds its way into the attic, but the heat doesn’t anymore because of this insulation stuff that he put above the plaster.
Now the roof deck is colder than it used to be and when the warm, moist air finds the cold surface it creates condensation on the wood roof deck. Then, when wood is wet enough for long enough we get the birth of a microorganism. You see, this is the building science version of the birds and the bees.
The difference is that this offspring doesn’t listen, can make you sick, and in rare circumstances can kill you…
This is why we started venting our attics – to remove moisture. When we started adding insulation to our ceilings, roofs got colder and the moisture that would travel into the attic had to be vented before it caused condensation. We changed the building envelope by adding insulation to the ceiling, which caused roofs to get colder, which lead to condensation and increased the risk of mold and mildew growth.
You see, mold spores are always present in the air, on our bodies, and construction materials, so it is impossible to have a mold-free house, but we can deny mold the conditions for active growth. Mold requires oxygen, food, water, and spores to become active.
Spores are always present—so is oxygen—and we build with all types of organic materials that can provide food, so water is the only component that can be controlled to effectively minimize the risk of mold growth. We are talking about bulk liquid water here, not just moisture vapor in the air. That’s not the issue, unless it condenses.
When we design and create tight building envelopes, we can then control the interior environment—temperature and humidity—from the inside out, with a proper air management system. We can minimize the potential for condensation and significantly reduce the risk of mold and mildew. And by maintaining interior relative humidity between approximately 40 and 50 percent, not even dust mites can grow.
Visual signs of mold and mildew are the manifestation of a problem, they are a symptom that can be identified when there is a problem, but, generally the problem is not the mold, the problem is the cause of the mold: Warm, moist air contacting a cold surface, resulting in condensation. 80 years ago, ventilation solved this problem by exhausting the warm, moist air to the outside.
But, just as insulation options have evolved over the last century, so has our understanding of building science, building envelope design, and methods to control interior environments.
Now, when it comes to the building envelope, we know that we have to protect our structures first from bulk liquid, then from air leakage, then from heat transfer,
and lastly from vapor. And it has to be in that order.
Why? Because this is the order in which the elements damage building materials and affects interior comfort.
It is also the same order of importance for minimizing
the risk of mold and mildew.
Bulk liquid is most damaging—a roof leak; a plumbing leak—these water events have to be addressed immediately and corrective action must be taken; fix the leak and dry out the wet materials.
But the other elements are often overlooked.
Air sealing and reducing air leakage should be high on your priority list when it comes to managing mold and mildew risk. Why? Because air carries moisture—actually, air movement is the number one way that water vapor moves through structures.
By tightening structures and lowering natural air changes per hour, we create environments with less air leakage, which means less water vapor movement through assemblies, and we separate the interior environment from the exterior environment.
Then we use insulation to reduce heat transfer through the building envelope and make our structures more energy-efficient.
Once we have a water-resistive, air-sealed, insulated structure, we use an air management system – the HVAC system – to take care of the indoor air quality from the inside out. We distribute the conditioned air throughout the structure, which means we can more effectively control and condition the interior of the building. Driving at our overall goal of creating comfortable interior environments by controlling interior temperature and humidity.
When we condition structures, like in the middle of summer, we are pumping air at around 55 degrees Fahrenheit through the ductwork, and, like some sadistic plot against homeowners, the ductwork is sitting in the hottest part of the building – the attic, and, to make matters worse, ductwork has the lowest R-value requirement of any component in the building code. Really? How did this happen?
Taking all of this into account, we know that structures need to be water-resistive; they need to be air sealed; they need insulation and our mechanical systems, including the ductwork, should be inside the building envelope. In years past, all of these could be accomplished, but it required labor intensive approach to air sealing, using conditioned square footage in the building for mechanical systems or a change in the ceiling design to create a drop-down chase system. Oftentimes, this didn’t happen.
Today, we can achieve a lot of these design features by incorporating spray foam insulation into the building. The building code has caught up with building science in some ways and we can now create unvented attic assemblies in accordance with IRC 806.5, and the number one benefit of using this advanced design concept is that it moves the mechanical system and ductwork inside the building envelope.
So, next time you are told by a prospect, builder or code official, that attic ventilation is required, you can share these concepts, point them in the direction of IRC 806.5 and help them catch up to good building science practice.