Yep, learned this the hard way as a young boy scout sleeping under the stars!

Thanks, that is very helpful.

In general, it is a bad idea to cool the interior of a home (or any building) below the outdoor air dewpoint, especially if it is a conventionally stick framed structure. Overcooling invites uncontrolled moisture formation within exterior (and sometimes even interior) wall and upper story floor support assemblies as well as condensation / insulation failures of oh-so-common attic ductwork.

Given rising dewpoints owing to climate change, my stance on setpoint vs dewpoint has come under increasing pressure as I enter my 4th decade living in the humid southeast.

I recommend cooling setpoints no lower than 75*F, ideally 77-78*F and strive to provide HVAC systems and building science testing / upgrades that drive comfort at those temperatures…in a nutshell that means much better humidity control via both HVAC equipment selection and improving how the structure performs as to infiltration.

72*F is my absolute lowest allowable cooling setpoint, and that preferably only intermittently, such as at night. Any customer / client that wants to cool their home to sixty-anything degrees becomes an ex-customer / client ASAP…let someone else die taking that mold litigation hill!

If a structure must be routinely cooled below outdoor dewpoint there are assemblies that tolerate that…my personal favorite is ICF (Insulated Concrete Form) walls with closed cell spray foam insulation under roof sheathing with everything blower door verified substantially airtight before drywall goes up. In other words bury the dewpoint temperature within vapor impermeable exterior wall and ceiling / attic assemblies verified to be substantially airtight.

The issue of “frost point” is quite relevant to heat pump operation during cold(er) weather. Those of us in the southeast have used heat pumps for decades, but advances in heat pump technology, sky-high nat gas prices, and the push to decarbonize homes has suddenly made much of the rest of the planet much more interested in heat pumps.

A key element of both the efficiency and reliability of heat pumps is managing frequency and duration of defrost cycles. That in turn depends very much on ambient outdoor frost point. A poorly selected or installed heat pump can get hammered by icing during cold rainy weather – defrost capability overwhelmed by rainfall.

BrianR, you mean like this:

Why Doesn’t Frost Form in the Carport?

RoyC in New England we occasionally get frost on roofs (houses & cars) overnight in the fall even when the measured air temperature never dropped below freezing. Perhaps not quite the same as your “frost point” but kinda similar due in part to radiational (super) cooling.