Northern Thermal Comfort
2379 Rux Rd.
Arbor Vitae, WI 54568

The Insulation Lab

Unforeseen Consequences - Mold

Jeff Aderholdt - Sunday, February 14, 2016

There was a time when we didn't worry about energy efficiency. Energy was cheap. The technology to increase efficiency was expensive. It just wasn't cost effective, and therefore it wasn't a high priority. But then it changed. Starting in the mid-70s, energy costs started to rise dramatically. We started trading in our gas-guzzling American cars for more efficient Japanese imports. Interest increased in alternative forms of energy such as solar and wind. Some even built stills to make there own ethanol.

This concern over energy efficiency was not limited to our cars. Our homes were becoming more and more expensive to heat. Thus started a forty year trial and error project to stop our houses from hemorrhaging energy. The bleeding needed to be stopped, or at least lessened. We started adding insulation. We started plugging holes. All of this was new to us. We never saw some of the consequences that would result from our efforts. One of these consequences was, and is, mold. Some of the techniques we used, and still use, create a perfect breeding ground for mold. This danger is more severe in the extremely cold climates. Southern, warm and humid climates are also at risk, but that is not my expertise. Though the science can be extrapolated for use in such climates, I will leave that discussion to those who know it better. My focus continues to be on cold climate issues. For mold to grow, you need three things: temperature, moisture and food.

How to grow mold (Simplified)

Temperature: Though it's true that mold can live below freezing, for mold to grow, the temperature needs to be above freezing. The warmer the temperature, the better the growth. It thrives over 60°F.

Food: Mold can not do photosynthesis, so it cannot manufacture it's own sugars. Instead, it feeds off of decaying organic matter. In construction, that is most of what we build with. Even if we use non-organic materials, much of the dust in the air is organic.

Moisture: Once we get over 60% sustained relative humidity, mold can grow. The higher the relative humidity, the better it grows.

But aren't we also creating a mold growth “Danger Zone” using conventional framing and insulation? Absolutely! Any situation where we have a temperature change from a comfortable indoor temperature and humidity, to, or below freezing, there will be an area of mold growth potential. Totally eliminating “The Zone” is a virtual impossibility. Though we can't eliminate it, we can do a lot to minimize it.

Application to Construction

Above freezing, 60% sustained relative humidity, (since most situations do not create a sustained relative humidity, but rather an intermittent seasonal spike, we need to be concerned when these spikes reach and hold 70% and higher.), and organic matter for food. If we create a situation where all of these criteria are met, mold can and usually will grow. But how does this apply to construction techniques?


Two of my most read articles have been “Flash and Batt – Risky in Cold Climates”, and “Moisture, Psychrometrics and Relative Humidity – Their Effect on Structure and Air Quality”. In both of these articles I discussed moisture control and the dangers of some common construction practices. In both of those articles, I focused on when and where we reach the dew point, 100% relative humidity, and start to experience condensation. I included the simplified psychrometric chart showing the dew point, and the cross section of a flash and batt wall showing where when the dew point occurred. But if the focus is on mold prevention, the dew point is not when we need to be concerned. The danger zone starts much sooner. Using the simplified psychrometric chart, based on a house at 70°F with 40% relative humidity, we can determine that, at about at 54°F and going to the point of freezing, at 32°F, we have a “Mold Growth Potential” zone. We can now adjust our “Flash and Batt” diagram to reflect how large our “Danger Zone” really is!

Mold growth Minimization

The first step to minimize the mold growth “Danger Zone” is to prevent moisture from getting into the structure in the first place. In cold climate construction, we do this by using a vapor barrier and air sealing at the inside of our thermal envelope. The less moisture that gets into the walls and ceilings, the lower our relative humidity will be. The lower the relative humidity, the smaller the “Danger Zone”. Now, there are those that promote getting rid of the vapor barrier all together. The theory is “since moisture will get in there anyway, let's give it an easy way out” At this time, I don't agree. To me, is sounds like “don't plug the leak in the boat because that water leaking in has to get back out”. The more you seal the leak, the less that needs to get out! The less that needs to get out, the faster it can get out. Still, the moisture that does enter does need a way out, and that brings us to the second step.

The second step is designing the structure to allow the trapped vapor to escape to the exterior. In a conventional vented attic, that not a problem, since the air movement carries away any excess moisture. As for walls, conventional construction allows for vapor diffusion through the exterior of the walls. The result is that, whatever moisture does get past the primary vapor / air-tight barrier has a way to get out. So, while the vapor / air-tight barrier makes the “Danger Zone” smaller, the exterior vapor diffusion shortens the duration of “The Zone”. At times, some have thought it a good idea to make the exterior of walls vapor tight, with disastrous results. (For more information, see “Smart Vapor Barriers – Are They Really that Smart?”).

The third step is to use insulation materials that help to manage moisture. This is an area where dense-pack cellulose has an advantage over fiberglass. Dense-pack cellulose adds an additional layer of air movement resistance, and thereby controls vapor movement. Also, since cellulose is able to absorb and store moisture, it provides a “moisture buffer” over the winter, helping to keep the “Danger Zone” small. (For more information, see “Dense-Pack Cellulose – Cellulose Reborn”).

The fourth step is to minimize the organic matter in the zone. The largest amount of construction material that is at risk of being in “The Zone” is the exterior sheeting. The solution is to keep it too cold or too warm to fall in “The Zone”. At times, people will notice some frost forming on the exterior sheeting during cold weather and raise concerns. Though excessive ice may be a sign of too much air leakage, some frost is normal, as it is a result of moisture leaving the walls. The fact that it is frost is a good thing since the moisture is below freezing, and thereby too cold for mold to grow. As the weather warms or the sun hits it, and the frost melts, the moisture vaporizes and dissipates, reducing the time in “The Zone”.

The other way to keep the exterior sheeting out of the “Danger Zone” is to keep it warm. To keep the sheeting warm enough to stay out of “The Zone” takes a lot of additional insulation. In sub-arctic areas, such as the Alaskan interior, this is done by building walls where 2/3 of the insulation is outside the sheeting (See “The Arctic Wall”. Although they promote an elimination of the vapor barrier, as a mentioned earlier, right now that doesn't make sense to me. Still, it's good information!).


This brings me to an area of concern. It has become common for homes to be built with a layer of sheet foam to the exterior of the siding. From the start, this made me a little uncomfortable. The rationale is the layer of foam adds insulation to the weakest point, over the studs, plates, and headers. Also the reasoning was that since the sheeting is now warmer, there is less condensation potential. It was said that it is like moving your house to a 30°F warmer climate. This all seemed to make sense! The problem is that by installing foam sheeting like this, we are putting the sheeting inside the mold growth “Danger Zone”! Remember, the potential for mold growth starts well before the dew point. Because of this, I do not recommend that exterior foam be installed, unless it is 2/3 of the total wall insulation. If sheet foam is added, think about adding it to the inside or think of other creative ways to increase insulation at the weakest points. Currently, I am working with one of my contractors on some experimental techniques. But that's for another article.

And our forty year trial and error project into energy conservation continues. We shouldn't be surprised if we need to make technical and philosophical adjustments along the way. In a lot of ways, we are still in the beta test, working out bugs and learning from our mistakes and successes. If we are doing things the same as we did twenty years ago, we are probably doing it wrong.

At NTC Insulation, we are always looking for better ways to make your home both energy efficient, as well as sustainable. Are you building in our service area? Give us a call. If you are outside of our service area, ask us about our consultation services (It's amazing how much can be done over the internet!). If you are a professional or involved in energy science education, talk to us about speaking to your group or class. Put our knowledge and experience to work for you.

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