How to Avoid Mold Part 2 – Control Temperature

by Erik North on August 26, 2013


An Energy Auditing Blog article on reducing mold risk by reducing the homeowner’s ‘water sports’ (water related activities in the house) was published at GBA last week. Allison Bailes, the self-described energy guy with the funny name, rightly pointed out that temperature plays a key role as well.

Temperature Makes Humidity Relative

Relative humidity is exactly that…relative. Relative to the current temperature and pressure conditions. Moisture exists in the air as water vapor, one of the gases in the atmospheric mix. As the temperature rises, water vapor can make up a larger portion of the gaseous mixture.

Relative humidity is the amount of water vapor in the air as a percent of the maximum possible humidity at that temperature. If you lower the temperature with the same moisture, the relative humidity rises.

And when humidity reaches 100%, well, $%^& gets real.

100% Humidity Means Temperature Problems

When air is saturated with moisture and the temperature drops further, it can no longer hold the moisture. The moisture is wrung out of the air mix as condensation. It’s the reason you see morning dew on grass. The temperature plunges overnight and can’t hold the same moisture it could 30 degrees ago. This temperature where water vapor condenses out of the air is the dew point.

Allison’s point was that under normal humidity conditions, cold surfaces can still grow mold. The temperature dip increases relative humidity and the likelihood of condensation. It recalled one of my favorite audit photos:


Lower Temps Means More Mold

This picture showed some poorly installed insulation near the house’s shower. The high humidity would rapidly cool as it neared the uninsulated section of ceiling. The moisture condensed out of the saturated air and eventual grew that nice patch of mold.

What Does This Mean for Buildings?

What does this mean for buildings? Controlling moisture is one factor in curbing mold growth but controlling surface temperatures is another. The house’s air barrier needs to be wholly intact to keep interior air from contacting cold sheathing material. Properly installed insulation will warm potential condensation surfaces, keeping them over the dew point. Buildings sheathed with foam board insulation warm the interior walls, ameliorating condensation-based mold issues.

Mold creation has a few different factors. One is temperature. So control your surface temperatures and control the mold.

{ 4 comments… read them below or add one }

tedkidd August 26, 2013 at 10:37 am

Nice post Erik.

“Mold creation has a few different factors. One is temperature. So control your surface temperatures and control the mold.”

Are you willing to take the next step and suggest that thermostat setback is a really bad idea because it makes both RH control and dew point surface temperature control a serious issue?


Erik North August 26, 2013 at 11:08 am


Thanks for the comment.

Do thermostat setbacks make RH control a serious problem? Dropping the temperature would raise relative humidity, sure. But I think we’ve both seen enough houses to realize that real problems arise from poor thermal control of solid surfaces, flaws in the air barrier that allow moist air to contact condensation surfaces and high interior moisture loads.




tedkidd August 26, 2013 at 2:31 pm

Erik, I don’t think we know that.

I think some may assume the “where” based upon what they want to sell, but are they conveniently cherry picking causes they like and disregarding those that create contradictions to their belief schema’s or self-interest? How honest is that?

What we KNOW is the surfaces are below dew point. And your argument is “avoid mold, temperature control.” Yet you are suggesting temperature control at the thermostat is not important? This is confusing to me, and I suspect to others.

What I know is that people with humidifiers attempt to keep 30-50% at 68-72f thermostat temperature. As you move toward outdoors the surfaces will get colder. As you move down, particularly if there is a lot of stack, you will get colder (though that air may also be drier having not crossed the humidifier). At some point will you hit 50f dew point?

If they drop air temperatures in their homes 8-15f for setback chasing the $25 a year savings suggested by Michael Blasnik’s studies (, what happens to surface temperatures? Would it be reasonable to suggest those temperatures will drop 8f? If not, where are these “laws of thermodynamic’s savings” going to come from?

Will surface temperatures that were 55° at 70° air temp possibly drop to 47° when air temps drop 8° ? Remember our 50.51° dew point?

Either temperature control is important, or it isn’t. Are Thermostat’s a SIGNIFICANT tool for the control of temperature in houses? I think this is one where you can’t have your cake and eat it too.

I agree, we need temperature control to avoid mold. Stop playing thermostat games and you’ll see a lot of mold problems go away.

Fix the house, and the $25 savings from thermostat games will go away too. It will be replaced by $350 in savings from having a better house.


Erik North August 26, 2013 at 10:31 pm


Well, I think we can agree that fixing structural issues to improve comfort and savings is more important than fiddling around with a thermostat.

And of course the thermostat is a significant control of interior temperature. Where we are disagreeing (and continue to do so) is that the thermostat is a primary driver of mold problems. It may exacerbate existing issues but wouldn’t summon those conditions out of whole cloth.

I’ve never seen a mold issue where the cause wasn’t poor thermal control of solid surfaces, flaws in the air barrier or high moisture loads.

Fix the cause of the mold problem and the homeowner can fiddle with the thermostat to their heart’s content.


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