Modern Home, Modern Problem: Managing Indoor Humidity

Have a new home and experiencing an unusual amount of indoor humidity? As a strong advocate for energy-efficient homes, I’d bet there’s a high chance your new, efficient home might not have been detailed properly if you're here searching for solutions. The following article most specifically addresses homes that do not have vented crawlspaces and vented attics in hot, humid climates.

Understanding the Issue

Your HVAC system handles two types of thermal loads: sensible loads and latent loads. Think of the sensible load as the temperature displayed on your thermostat. If it’s 75°F indoors and you set your thermostat to 72°F, your HVAC kicks in to lower the indoor temperature to the desired level—this is addressing the sensible load.

Now, think of the latent load as the humidity level in your home. If your HVAC system is running, it passes moist air over a cool coil, causing condensation. This dehumidifies the air, reducing the latent load.

Here's the problem: HVAC systems are primarily designed to handle the sensible load—not the latent load. Many systems aren’t designed to engage once the relative humidity creeps up too high. This can be problematic for new homes for two main reasons:

1. Tighter Construction: Modern homes are built more airtight using building wraps, tapes, caulks, and fluid-applied membranes to minimize air infiltration. With fewer air exchanges, the HVAC system doesn’t need to run as often.

2. Improved Insulation: Enhanced insulation values mean that conditioned air stays inside for longer periods. As a result, there are fewer calls for the HVAC system to run.

Why Is Your New Home So Humid?

Because of these tighter construction methods and better insulation, your HVAC system runs less frequently. Meanwhile, daily activities—breathing, cooking, showering, doing laundry—generate significant moisture inside the home. This issue is particularly noticeable during summer nights when there’s less temperature difference between indoors and outdoors, no solar heat gain, and everyone is at home generating moisture. The HVAC hardly runs, but the indoor humidity levels continue to rise.

What Can You Do?

1. Use Spot Ventilation
   Make sure to use spot ventilation when cooking, doing laundry, showering, etc. This is crucial for maintaining lower humidity levels in your home.

2. Check Your HVAC System
   Your HVAC system might be oversized, which means it cools your home too quickly and doesn’t run long enough to properly dehumidify. Ideally, on a 92-degree day in North Carolina, your HVAC system should run nearly continuously. Ensure your system was installed according to the correct sizing procedures (Manual J for load calculation, Manual S for equipment selection, and Manual D for duct design). Don’t know how to find a competent HVAC company? Check out our guide to learn more.

3. Adjust Fan Speed
   If the fan in your air handler is set too high, it may cool the home too quickly, preventing adequate dehumidification. If possible, adjust the fan speed to a lower setting to allow the evaporator coil more time to dehumidify the air.

4. Set Fan to Auto
   Set your thermostat fan to “Auto.” This ensures that the fan stops when the thermostat reaches the desired temperature. Allowing the fan to run continuously can blow air over the wet coil, reintroducing moisture back into the air.

5. Consider a Dehumidifier
   Adding a dehumidifier can help control humidity levels. Mold can start forming when relative humidity (RH) at surfaces hits 70%, and dust mites thrive in carpets at these levels of humidity as well. Even if the overall indoor RH is below 70%, cool air blown directly onto a surface can increase localized RH, leading to mold growth. It is recommended to keep relative humidity in a home between 25 percent and 60 percent.

6. Ventilate or Air Seal
   If you’ve tried the above steps and are still experiencing issues, you may need to improve ventilation or air sealing. A blower door test can help determine your home’s air tightness. Depending on the results, you may need to either add ventilation or improve air sealing. If ventilation is necessary, consider installing an Energy Recovery Ventilator (ERV), which can filter and condition outdoor air before it enters your home. Here is one important aspect of ventilating in the south per Joe Lstiburek, a pre-eminent engineer and building scientist, “Air which is brought in from the exterior to supply ventilation needs and make-up air needs should be conditioned to ‘dew point 55’. In other words, this air should be cooled to at least 55 degrees Fahrenheit in order to dehumidify it. At dew point 55, the temperature of the air is 55 degrees Fahrenheit and its relative humidity is 100 percent. Once this air is warmed up to 75 degrees Fahrenheit, the temperature of typical air conditioned spaces, its relative humidity will be approximately 50 percent. This air now mixes with the air in the space diluting/reducing the conditioned space moisture levels/relative humidity. The rate of dilution or mixing is determined by meeting the 60 percent relative humidity limit within the conditioned space.” (Lstiburek, 1993)

Why It Matters

Understanding how your home's design and HVAC system affect indoor humidity is crucial for maintaining comfort and preventing potential damage. High indoor humidity levels can lead to mold growth, poor indoor air quality, and damage to building materials. While modern energy-efficient homes are built to be airtight and well-insulated, these features can also inadvertently cause humidity issues if not properly managed.

Addressing indoor humidity isn't just about comfort; it’s about safeguarding your home’s integrity and your family's health. If you're experiencing ongoing humidity issues despite your best efforts, it might be time to consider a more comprehensive inspection to uncover hidden problems. Consider contacting Kurtz Residential and describing your issue. With our expertise in residential inspections and a focus on detailed, personalized service, we can help identify the underlying causes of your humidity problems and recommend effective solutions to ensure your home remains comfortable and healthy.

Sources

Lstiburek, J. (1993). Humidity Control in the Humid South. Building Science Corporation. Retrieved from https://buildingscience.com/documents/confpapers/cp-9302-humidity-control-in-the-humid-south/view.