What is a Building Envelope, and Why Should Homeowners Care?

Many people think of their homes as a collection of different systems, and they’re absolutely correct. Your electricity, plumbing, and HVAC equipment represent separate systems that “power” different aspects of your home.

However, there’s another way to think about your home – not just as a collection of systems but as single system in which different components work in tandem to produce a clean, efficient, and comfortable living experience. To start thinking of your home in this way, it’s important to understand the concept of the “building envelope,” which is defined as the continuous thermal and pressure boundary that surrounds a structure.

This diagram from Energy Star shows the anatomy of a home envelope:

Energy Star Home Envelope Diagram

Imagine you’re driving your car near a construction site on a windy day. You see a large cloud of dust on the horizon. You’ve got no choice but to drive straight through it, but you’re not worried. After all, you’re in your car. You won’t have to breathe in any of that nasty dust.

Except you forgot that your back windows were cracked open.

Pretty soon you’re coughing and sneezing and trying to rub the dirt out of your eyes. You’re used to thinking of your car as a closed envelope, which is why you weren’t worried about the dust. But when there’s a hole in the envelope, pollution can get inside, not to mention lots of hot or cold air, wind, and precipitation.

The point is that it’s useful to think of your living space in much the same way: an envelope that functions best when it’s closed tightly. It should also include a controlled, deliberate method for bringing in fresh air from the outside.

Tightening your home’s building envelope improves energy efficiency.

When it’s freezing outside, you probably don’t open your living room windows. Doing so would cause lots of heat to escape, forcing your furnace to work harder. Keeping your windows closed tightens your home’s envelope, retaining more heat indoors and lowering your winter energy bill.

But what if there are holes in your home’s envelope – small, mostly invisible holes that you can’t close as easily as a window? Unfortunately, leaky home envelopes are very much the norm. Many of the Atlanta-area homes we service contain small air leaks that, cumulatively, amount to leaving a window open all year! These leaks can wreak serious havoc on heating and cooling costs, especially when temperatures hit their highest and lowest extremes.

Ways to mitigate and eliminate air leaks around your home include:

  • Air sealing: A home energy audit will reveal all of the leaks in your home envelope, most of which are found around doors and windows, pipes, ductwork, and wiring penetrations. Caulking these areas seals the envelope and creates a continuous thermal and pressure boundary.
  • Air sealing again: After sealing the leakiest parts of your home, we often perform a second, blower door-guided air sealing job. The reason? After sealing your home the first time, the blower door will reveal new places where air is leaking out! It’s also a quality control check that ensures we’ve met your goals for a healthier, more comfortable, energy efficient environment.
  • Optimizing attics and crawl spaces: By air sealing all wiring penetrations in your attic and sealing and encapsulating your crawlspace (more on that in a moment), you can further tighten your home envelope and maintain greater control over indoor temperatures all year round.

Minimize the entry of dirty, unhealthy air.

Your home envelope isn’t just a thermal barrier. It’s an air barrier, too. Depending on where you live, outdoor air quality can range from consistently fresh and clean to smoggy, stinky, or saturated with seasonal allergens. Outdoor air, of course, can have a profound effect on indoor air quality (IAQ), but it’s not just the air around your home that’s getting inside. It’s the air beneath your home, too.

If your air handler lives in a basement or crawlspace, a lot of the air inside that basement or crawl space is also the air you and your family are breathing. Why does that matter? Because many unfinished basements and vented crawl spaces don’t harbor controlled air or contain moisture barriers. Conditions in these areas often range from damp to completely wet – the perfect environment for mold and pests to thrive.

Needless to say, you don’t want harmful mold spores penetrating your home envelope, but that’s exactly what happens in many homes with wet crawl spaces. Here are some possible solutions for avoiding this common problem, mitigating entry of other environmental air pollutants into your home, and improving your home’s IAQ:

  • Use the right air cleaning system: High efficiency air filters outperform typical pleated air filters without restricting air flow.
  • Install UV lights: By installing UV lights over your system’s indoor coil, you can kill bacteria and mold before your HVAC system circulates it throughout your home. UV lights in your ductwork can also eliminate odors, bacteria, and other indoor pollutants before you have to breathe them.
  • Install an Energy Recovery Ventilator (ERV): An ERV or fresh air damper brings outdoor air into your home and filters it before it starts circulating. It’s like opening a window but without allowing outdoor pollutants to pierce your home envelope.
  • Install a Whole House Dehumidifier with Outside Fresh Air: An effective, less expensive option to the ERV is a whole house dehumidifier. It works by bringing in outside air, filtering it, and dehumidifying the air before circulating it inside your home. You will be much more comfortable at higher temperatures, a result of less moisture in the air. And with controlled humidity, it’s all but impossible for mold to grow inside your living space.
  • Seal and encapsulate your crawl space: A dry crawl space is a wonderful thing. By installing a sealed vapor barrier over the crawlspace floor and spraying your exterior walls with Icynene closed cell spray foam, you can eliminate crawl space mold and make the crawl space uninhabitable for pests. Of course, if bulk water is entering the crawlspace, a proper drainage system should be installed first. And if your crawl space contains any mold, you must first kill and clean it prior to encapsulation. Otherwise, you’ll do more harm than good!

Home performance testing can help you identify the most important improvements and prioritize them – so you get the most bang for your buck and more effectively improve comfort.

Home comfort: The ultimate fringe benefit

IAQ is just one aspect of home comfort. Additional benefits of tightening your home envelope include:

  • More consistent temperatures throughout your home
  • Better indoor humidity regulation throughout the year
  • No more cold floors in the winter (or hot, muggy floors in the summer)
  • Cleaner, more comfortable crawl spaces and attics – going inside them won’t seem so scary anymore!

Remember: Your home is a system. Your home’s building envelope is the protective layer that surrounds that system, retaining the air inside of it and keeping it clear of pollutants. Having holes in your home’s envelope damages that system and prevents it from operating at peak performance, diminishing overall comfort for you and your family.

So plug those holes and start optimizing your home envelope! The sooner you build a tight envelope around your home, the sooner you’ll lower your energy bills and improve your IAQ.