Can an Unconditioned House Survive a Humid Climate Summer?
Imagine that you’ve built a high-performance home somewhere in the southeastern US. It’s airtight. It’s well-insulated. The mechanical systems were designed properly and keep the indoor conditions just where they need to be. Now, you have an opportunity to spend the summer somewhere exciting. You’re going to Seattle! Or Halifax or Ann Arbor. It doesn’t matter where you go. What matters is how you leave the house for the summer. Can you just turn off all the mechanical systems? Or will your beautiful unconditioned house fall apart without air conditioning?
Enclosure, mechanical systems, and occupants
A house is a system. It’s a complex arrangement of sub-systems that, when designed, installed, commissioned, and maintained properly, provides a safe, healthy, comfortable, durable, and efficient place to live. In building science, we focus on three parts in particular: the building enclosure, the mechanical systems, and the occupants.
When the building enclosure is incomplete, you end up with a house that can fail on all of the end results you want. When the mechanical systems aren’t right for the house, you get problems with comfort, noise, moisture, and efficiency. And when the folks in the house do things like set the thermostat to 68 °F or leave the doors open, you again get trouble.
So, the question here is: Can you take the occupants away, turn off all the mechanicals, and come back to a house that’s still in good shape? What this means is that you’re relying on the building enclosure to keep things from falling apart. Will it work?
Moisture is the key
With no people in the house, there’s no reason to keep the interior at 75 °F. But an air conditioner has two primary jobs: to lower the temperature (sensible cooling) and to reduce humidity (latent cooling). Summer design temperatures for most of the Southeast are in the low to mid 90s Fahrenheit. With no AC running, the temperature in an unconditioned house might rise to 90 °F or so.
Higher temperatures indoors can result in some problems. Also, some materials don’t do well if the temperature stays elevated. Sealants, adhesives, vinyl, wax, and some other things might degrade in the heat.
And then there’s mold and microbial growth. You may have an airtight, high-performance home, but it won’t stay dry indoors without the mechanical systems removing moisture. Just turn off your AC for a day and see what happens to the indoor humidity. This is the number one reason you can’t leave a house unconditioned for an extended period in the summer.
How does moisture get into a closed-up, unoccupied house?
Dr. Joseph Lstiburek is famous for many things. One of those is getting people to understand that air leakage is more important than diffusion when it comes to water vapor moving across the building enclosure. The image below shows his latest work on this topic. As you can see, a 1 square inch hole with a 4 pascal pressure difference moves nearly 100 times as much water vapor as the amount of vapor diffusion through a whole sheet of drywall (4′ x 8′) over the course of a typical spring, summer, and fall. It would be worse in Florida or Houston because of the higher outdoor humidity.
![More moisture moves through a wall via air leakage than by vapor diffusion. [Source: Building Science Corporation]](https://www.energyvanguard.com/wp-content/uploads/2026/08/moisture-transport-air-leakage-vs-vapor-diffusion-scaled.avif)
If you leave with the house at 75 °F and 50 percent relative humidity, that amount of infiltration will bring into the now unconditioned house 15 to 20 pounds of water per day. This assumes typical Southeast dew points in the 70s Fahrenheit. As the humidity in the house goes up as it sits there unconditioned day after day, the rate of water vapor entry will go down. That happens because the indoor humidity level approaches the outdoor level, reducing the difference between indoor and outdoor vapor pressures.
Add to that the smaller amount of water that comes through the building enclosure via diffusion and the water from the ground entering via capillary action. The result is you have an initial barrage of water making its way from outdoors to indoors and then a house that stays wet.
Three types of houses
I began by asking what happens with a high-performance home left unconditioned in a hot, humid summer. But that’s a tiny fraction of all the houses that are out there. Here, though, let’s divide them all into three groups.
High-performance homes – These homes have low air leakage rates, good insulation, good windows, and good mechanical systems. And yet they still can add about 20 pounds of water per day to the indoors.
Mid-range homes – I’m thinking of standard houses built since the 1970s here. They’re insulated. They’re somewhat airtight, often in the range of 7 to 15 ACH50. The air conditioner is oversized—which short cycles and removes less humidity—and they probably don’t have a dehumidifier.
Old homes – They have lots of air leakage, usually higher than 15 ACH50. They may have insulation in the attic, but it was added later. The walls are often not insulated.
The truth is that they all suffer from moisture problems if you leave them unconditioned in the summer. Moisture enters these homes in a variety of ways. It elevates the humidity and loads porous materials with water, too. Mold and other microbial growth can go wild in these conditions. The difference will be in how the moisture problems develop.
So, will the house survive?
The first answer here is almost certainly yes. A few months of extra heat and moisture in the house won’t cause it to collapse. Yes, it will accumulate moisture while it sits unconditioned. The likelihood of microbial growth will be high. But when the people come back and start conditioning it again, it can dry out.
But some houses are on the edge of moisture problems spiraling out of control when they’re occupied and conditioned. Turning off the air conditioners and dehumidifiers for a few hot, humid months can push them over the edge. The homeowners come back from their summer in Portugal, walk into the house, and immediately leave to find a hotel room.
What’s the best way to protect an unoccupied house?
To keep the microbial growth down as well as avoid other problems, you need to do two things. First, don’t turn the air conditioner off. Instead, set it back. You don’t need to keep the indoor temperature as cool as when you’re home. But it’s good not to let it go above the mid-80s Fahrenheit. That keeps the house from getting too hot. It also prevents refrigerant problems that can occur when a system doesn’t get used for months. And it does a little dehumidification, but probably not enough.
That’s why you also need to run a dehumidifier (or two or three). The dehumidifier, unlike the AC, operates based on the indoor humidity level. It also will warm up the house, which is another reason to keep the AC turned on rather than shutting it off completely.
The nitty-gritty
Houses are for people. But that doesn’t mean they can’t fare well when no one’s home. And in a humid climate, that means you’ve got to provide some kind of conditioning when the occupants are away for an extended period in hot, humid weather. You can’t just leave a house unconditioned, but that doesn’t mean it has to be fully conditioned. For an empty house in a humid climate in summer, it’s all about controlling moisture. And the best way to do that is to run a dehumidifier with the air conditioner still running but set at 80 or 82 degrees Fahrenheit.
Tip of the hat to newsletter reader John Matejka for the suggestion to write on this topic. He wants to build a small guest house near his cabin but would like to know if there’s a way to leave it unconditioned for months at a time.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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What if the electricity fails for more than a few days while you’re gone? In that case, first you should hope that incoming moisture through capillary action is minimal. Second you should hope that if a sump pump is important to your drainage system, it has a backup. If you know that these two are satisfied, it might be better to come to that in an airtight house. If your drainage situation is marginal or bad, you’re probably better off leaving some windows open so at least the house won’t be even more humid than ambient. Most sheds aren’t air conditioned. They aren’t pleasant to enter in summer, but they’re not just rotting from lack of AC.
This is what happens inside stud framed wall cavities every day – and it doesn’t even have to try to pass through the drywall. We do have clients on the Florida coast – without a whole house dehumidifier – whose primary residence is in Tennessee. When they are away from the Florida home they simply leave small cracked window openings on each floor and they say they have never had any musty smell or mold issues.
that vapor diffusion 4×8 versus 1″x1″ hole concept seems to suggest that when ridding a building of moisture, moving air is more prudent than relying on a vapor diffusion port.
Good article, Allison. You helped clarify that humidity happens in the Southeast, and if you don’t want your house to accumulate moisture and mold inside, you must either dehumidify or air-condition your home in hot humid months, regardless of whether it’s a new high performance home or an old energy hog. Seems like we’re coming to terms with the reality that it’s not efficiency that causes mold. It’s humid outdoor air! (Mostly) OK, I buy it. We can’t ignore physics. Where it’s humid outside most of the year, we need to dehumidify indoors (if we want clean and comfortable indoor spaces). But there’s one obvious question about this topic that seems to be ignored. Before the advent of AC in homes, most homes (even in the Southeast!) didn’t have air conditioning until sometime in the 60’s. That’s within my lifetime. What did people do to prevent indoor mold accumulation back in the good old days?
And check whether the controllers are hard wired or if some lazy contractor installed one powered by a battery which will stop working just after you go away on your trip.
From sad experience, the outcome of this is lots of mold.
What about a home built that was built 80 or more years ago, never designed for mechanical cooling and totally dependent on natural infiltration, exfiltration and air movement for moisture control? These home were built with materials that different responses to temperature and moisture that currently used materials and systems.
This is a lesson that school districts here in TX learn every year. They have serious mold problems in most buildings because they turn the AC off the last day of school and don’t turn it on until school starts. And then they end up trying to clean the mold out of everything and often replacing carpet and it’s still a smell throughout the year. And then over Christmas and new years they also like to turn the heat off then are surprised when a water line or a chill water line breaks and floods a building.
There is this fallacy that management thinks they can save money by not cooling and heating a building when in fact it ends up costing far more in building repair and unusable buildings.
And thank you for rubbing it in on how nice the weather is there in GA. My friend is just south of you in Fayetteville and sends screen shots of that cold summer weather and green stuff on the ground while we are at around 40 days of 102*-109* and 4-8 hours a day are over the design temp for here as well. Our usual summer weather which is not computed into the design temp sadly enough. But we have a cold front and it’s not supposed to hit 100* today.
Here is my quick analysis of this situation. Hot humid climates have typical summer days with nighttime lows around 75 F and daytime highs around 95 F. The dewpoint is usually around 70 F throughout the day. If you have a house with no cooling and no internal humidity sources, it will like hover around 85 F dry-bulb and 70 dewpoint temperature after a few days or weeks. That is roughly 60% RH. Do you really think that would cause mold problems? What am I missing?
Roy: I believe your assumption of “no internal humidity sources” is wrong. At the initial conditions, the rate of water vapor entry via infiltration even in a 1 ACH50 house could be 15 to 20 pounds of water per day.
I am not talking about your initial condition. I am talking about what happens after a few day and for the rest of a hot summer with no AC. At that time, there is no net humidity gain since the indoor humidity will be roughly the same as outdoor humidity. The internal humidity loads that I am talking about are showers, cooking, etc. which probably don’t occur when the house is unoccupied.
Allison, I did some double checking on your “initial” condition. If I assume 1 ACH for a 2000 ft2 house with 8 ft ceilings and outdoor conditions of 95 Fdb and 70 Fdp, I get 18 lb/hr of water entering the house due to infiltration which is in the ballpark of your calculation. However, we must remember that there is also exfiltration going on, and at this infiltration/exfiltration rate, with the indoor air assumed to be 75 Fdb and 50%RH, there is also 11 lb/hr of water leaving the house. Thus, the net “initial” rate of water gain is about 8 lb/hr. But eventually (within days), the average indoor humidity will equal the average outdoor humidity if there are no internal moisture sources present. This is why I claim the indoor dewpoint would end up around 70 F.