I live in Austin, TX and we have a few months of high humidity and high outside temps. I’m getting ready to design a new home and I have been battling with this issue. My girlfriend of over 20+ years (stopped counting) likes it around 68 to sleep…me not so much. Our new house will have 2 zones, a his and hers zone if you will (rooms on opposite ends of the house), and likely 2 geothermal HVAC systems in the 3000 sq. ft. home. After reading this helpful article, I am leaning to doing an Insulated Concrete Form (ICF) house. Will that alleviate these potential risks, along with a dehumidifier? Also, due to PTSD, I can’t sleep with any noise or I wake up…is there a way to design ducts/returns/equipment to be completely silent? It is so annoying, I can’t even sleep with a ceiling fan on.

Here in Austin, they love flex ducts (I don’t like them – I have allergies up the ying yang), and most installers seem to cut corners on HVAC design and installation, or charge an arm and a leg.

Any suggestions will be greatly appreciated. I have just gotten so much conflicting answers, it is hard to sort.

David, your points, as always
David, your points, as always, are well taken. My dehumidifier hour per day usage is supplemental during cooling months (one or two hours as most) and gets some help with electricity usage by running thermostat comfortably higher. A 70 pint dehumidifier draws around 600 watts.
As to vapor drive, I spend my first insulation dollars on sealing the “shell of the egg”. I do not want hot moist outside air meeting cool inside air anywhere if possible.
Great topic Allison. Thank you.

It’s worth pointing out that
It’s worth pointing out that a dehumidifier will further blow up the energy budget. Moreover, dehumidifiers have less capacity and higher operating cost at lower ambient temperatures. OTOH, anyone who sets their t’stat in the 60’s apparently doesn’t care about energy efficiency. But they should care about mold and house rot. And as Allison noted above, a dehumidifier does nothing to mitigate those risks. If anything, it will exacerbate the problem by increasing vapor drive.

Ivan, yeah, that sounds like

Ivan, yeah, that sounds like a recipe for a lot of condensation inside the walls.  With the plastic vapour barrier behind the drywall, none of the water vapour has a chance to diffuse through the drywall and provide a little bit of drying.  It all gets stuck in there, and with the house at 17° C, they’ve got a really cold surface to collect it.

Dave, I was hoping someone

Dave, I was hoping someone from St. Louis would see this article and know about Ted Drewes!  Yes, I went to Wash. U. but can’t say I got a good education there.  Not because of the school, though.  I wasn’t ready for grad school then so I dropped out after about six weeks.  That’s why I was working at a dry cleaners’ in December ’83.  Eight years later, I did go back to grad school at Florida.

Nathan, yes, a dehumidifier

Nathan, yes, a dehumidifier will solve some of problems (e.g., sweating supply registers), but it doesn’t help with mold growing inside the walls because of outdoor air leaking into the cavity.  That requires air sealing the exterior sheathing, which is why I wrote this in the last section:

To avoid a wet, rotting house, you need a really good air barrier on the outside of your walls to keep that humid air out.

Unfortunately, that’s hard to do on existing homes.

Now this is spooky cool. I
Now this is spooky cool. I was about to add the exact same comment. I install dedicated dehumidifiers in all of my coastal NC homes to supplement the normal HVAC system, keeping RH >50% at all times. It runs occasionally during the cooling months, but really works during the swing or shoulder months when AC is not used or seldom used. In our house, we are comfortably running AC at 77 or 78 degrees (dry heat, Arizona thing), thus taking some load off of the AC system.
As to sealing the exterior, any air movement within the walls is counterproductive and dangerous from a mold perspective. My blower door numbers are in the .5ACH50 range. For air quality, we add a 6″ fresh air duct with filter and temperature/humidity controlled blower set to 120cfm. This fresh air is added into the conditioned attic space to mix with conditioned air prior to entry into living spaces.