re: 60% RH guideline, I
re: 60% RH guideline, I recommended 50% when I was at the Calif. Air Resources Board IAQ Program, based on a large field study in NY homes by Larry Kinney for NYSERDA. They found surface condensation (and perhaps mold too ?) above 50%. Also, Arizona studies found dust mites in carpet on slab foundations, even at very low RH, thanks to micro-climate in the carpet. I recall that at least one EPA webpage ca. 2000 also recommended 50% RH target.

For sustainable, resilient, climate adapted design, I now recommend adding a safety factor for projected climate change. For example, the NE region is expected to get hotter and more humid in the summer, and much of California will experience heat waves of Moist Tropical type (and other types) more frequently. The Moist Tropical type has a greater risk of heat-related deaths and will also incrase the peak demand on the power grid. This means we need to do a better job of passive cooling and moisture control.

I’m not an expert, just a homeowner with an interest in keeping my living situation as ideal as possible, and I must admit that your above quote is easier said than done. From what I understand, and I could be mistaken, even if you manage to seal everything, water vapor can enter and leave your home through diffusion, depending on the difference between indoor and outdoor saturation.

In fact, I noticed that the dew point in my home (which is in FL, very humid outside!) rises at a much faster rate than the temperature. That is, if I do not touch the AC or my dehumidifier in the warm, moist season, the inside of my home maintains a relatively stable temperature, climbing ever so slowly; yet the humidity rises eerily fast until the indoor dew point matches the outdoor dew point!

Temperature is much easier for me to deal with than water vapor.

@Charles, I’ll weigh in here. As long as you have a good air barrier somewhere in the assembly, there’s nothing wrong with a design that allows drying in both directions. Of course, a lot depends on the location of the air barrier.

Also keep in mind that “drying in both directions” doesn’t mean the absence of a vapor retarder. In mixed-humid climates zones, drying in both directions is the preferred approach.

Charles, it’s certainly

Charles, it’s certainly possible to have an assembly that doesn’t allow water vapor to flow all the way through, and such an assembly can work. See Lstiburek’s article, The Perfect Wall:

https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall

If you look at the vapor profiles in the diagrams, you’ll see that those walls don’t all vapor to flow through. Earlier this year he followed up the perfect wall article with one called Flow-Through Assemblies. Here’s the link:

https://buildingscience.com/documents/building-science-insights/bsi-091-flow-through-assemblies

He even says this design could be the “more perfect perfect wall.” Both articles are essential for anyone who wants to understand this issue.

Charles, I specialize in

Charles,
I specialize in construction with autoclaved aerated concrete (AAC). To meet Passive House, I add external rigid mineral wool, followed by an air gap and siding. The interior is plastered directly on the AAC. This wall assembly can dry to the interior or the exterior–with the exception of several square feet in bathrooms when I tile directly on the AAC. Ceiling and roof assembly is generally drywall with cellulose insulation below a ventilated roof, which can also dry in both directions. AAC is also relatively airtight, so there is no issue of nothing in

Allison, with all of the talk about humidity & mold I think it is appropriate to ask this question. I read recently in a building magazine profile about a builder who says he builds “green homes” that are able to dry from all directions so you’ll never have issues with mold, etc. Is that what we want? For a home to be able to dry from all directions? To me that almost sounds like no one (or no thing) is in control.

Brett, thanks for your

Brett, thanks for your comment. I’m certainly not always right. But people who work with mold tell me they find mold in ducts with humidifiers. The first comment here by Lee Ramey is one example: “The majority of the time, if I find a working humidifier I find visible suspect mold in the humidifier unit itself and about 2 feet beyond it in the supply plenum.”

If heating systems ran continuously, it would indeed be difficult to grow mold inside a duct system. But they cycle on and off. The more oversized they are, the more they cycle and the more time they spend off. When underinsulated supply ducts run through an attic (and many do here in the South), they cool off quickly when the system goes off. When the ducts cool off and the moist air is still in there, the conditions for mold growth exist. If there’s a delay on the humidifier in addition to the blower, that can make it worse. Also, not all parts of the duct interior get washed with hot air when the system comes back on. I think there’s plenty of opportunity for the moisture from the humidifier to find some cool spots and a food source and start growing mold.

Allison, I think you’ve connected some dots incorrectly. In your article you say that humidifiers can create mold-friendly conditions in the duct work when the heated air passes across cold surfaces.
Then in the comments, you reference Bill Rose’s book which states that you need conditions in excess of .8 (80% RH?) for a month to grow mold.
At normal furnace plenum temperatures, you are generally adding humidity to 100°F – 120°F air. Based on a psychrometric chart, to maintain 80% RH at 100°F for a month, a humidifier would need to consistently raise the moisture in the air to above 230 grains/lb of dry air. I can only imagine what this would do to a home when the air cools back down to room temperatures.
The simpler explanation is that the 80% RH is reached during the summertime when the A/C is running. Cold air reaches dew point easier than warm air does. Maybe the mold is only noticed during the heating season.

Thanks Allison,

Here’s the paper:
“Indoor Humidity and Human Health–Part I: Literature Review of Health Effects of Humidity-Influenced Indoor Pollutants”

See document page 204 or search “Sterling”

Cheers,
RB