It’s summer time in the deep south and that means heat. Not the “dry heat” that some say you don’t feel as much. Down here we’re talking sweltering, humid, liquid-crab-boil heat that taxes your air-conditioner to keep the inside of your home comfortable when it’s 99 outside – temperature and humidity. This is the time of year when proper attic ventilation is critical to your comfort and your health. Comfort is obvious, but health? Yes, a poorly ventilated attic may promote growth of all sorts of biological entities, which can certainly affect your health.
First, we are going to stay focused in this issue on the attic. For the majority of homes in this area, that means an insulated ceiling just above your cool, dry living space (more on that later), an air-conditioner and distribution ducts running throughout your attic, and wood framing with asphalt shingles capping it all off. Around the exterior bottom of your roof, you probably have some slotted metal vents, and either spinning turbines or ridge vents along the very top of your roof. All of these items can be considered part of the “Building Envelope”, which is a building science term (yes, we are no longer building homes the way our grandparents did) for the physical separator between the conditioned (inside) and unconditioned (outside) of a building and includes components that provide resistance to air, water, heat, light and noise transfer. This also includes walls, door and windows, and the foundation, but we are only addressing the attic in this discussion. Getting a little more technical, the term “Thermal Envelope” is anything that controls the flow of heat into and out of the building. Here we are talking about the insulated ceiling above your living space.
Next, let’s look at the wonderful property of physics that makes attic ventilation work – temperature differential. It creates weather (at least the wind) and it gets the air moving in our attics. Simply put, hot air rises, makes room for cooler air, which gets sucked in to fill the void. In our attics, this means the hot air gets exhausted out the turbines or ridge vents, and cooler air gets pulled in through the vents around the bottom of the roof. Modern building codes prescribe the specifics of how much ventilation capacity is required, based on the size of the house, and as long as you have a reasonable balance between intake at the bottom and exhaust at the top, air is going to move through the attic. Sounds simple, right? The problem is our attics have only gotten more complicated, due to poor home design, subdivision restrictions and in some cases, just plain bad construction practices. Many of our roofs have very little ridge for the ridge vents to work, so turbines are a better option (if your neighborhood doesn’t prohibit them). Forget those louvered gable vents – they look nice but are generally inefficient. Some home designs inhibit the free flow of air, due to differing ceiling heights, second story or bonus rooms, or improperly installed insulation. But balance is critical, because if your hot air is getting exhausted at the top, the attic is going to find air to make up the difference, even if it has to pull it out of your living space. In that scenario you can be introducing cool, dry air into a hot, humid environment and promoting condensation.
Finally, we must not forget mother nature. She is constant, relentless, Jurassic-Park-T-Rex tenacious about getting what she wants. And mother nature wants balance. This means balance between pressure, temperature, humidity, etc. This means that after you spend your money to get a cooler, dryer living space, mother nature will do everything possible, through any means (opening) possible to get the inside of your home just like the outside. So … if you do not have proper ventilation in your attic, you end up with mother nature’s worst – hot, humid air just sitting there in your attic. Hot humid air mere fractions of an inch away from the parts of your living space you’re are making cooler and dryer. So now we are talking opportunities for condensation and mold growth. Depending on conditions, this can mean dripping air-conditioning ducts, mold growth within your attic insulation, mold growth on the ceilings in your rooms. In the deep south, our air-conditioners work to lower the temperature and humidity within our homes, and generally create a negative air pressure condition while they are operating. And to be efficient at de-humidifying our homes they need to operate a lot. All this creates those temperature, humidity and pressure differentials that mother nature hates.
Every home is different, and one solution does not work for all conditions. If you have seen indications of condensation or mold growth, have concerns about the type and/or capacity of your attic ventilation, or your home’s air-conditioning performance is not meeting your expectations, a qualified component professional should be consulted.