The Surprising Secrets of Tight Homes

When I built my first home for a customer back in 2000, I had a lot to learn about building energy-efficient homes. Over the years, I’ve become something of an air sealing obsessive, working more as an energy auditor than a builder these days. Conducting blower door tests on new homes and older ones, I’ve seen it all – from leaky drafty messes to impressively airtight fortresses.

You see, I live in Climate Zone 7, with around 10,000 heating degree days and usually fewer than 500 cooling degree days. In my neck of the woods, builders can get away with using good old polyethylene sheeting as both an air and vapor control layer. Most new homes I test achieve a 2-2.5 air changes per hour at 50 Pascals (ACH50) – which is pretty darn tight. Our houses survive because we have a requirement for balanced mechanical ventilation. Take that poly away, though, and most builders will struggle to pass the blower door test.

Not every home needs to be Passive House tight at 0.6 ACH50, though. In my experience, getting air tightness to around 2 ACH50 for the majority of new construction is just about perfect. So how can you achieve that kind of performance? Well, let me share a few of my top air sealing tips.

Sealing from the Bottom Up

When planning an air sealing strategy, I always start at the foundation and work my way up. Homes in my climate, especially during the heating months, tend to leak both low and high. The stack effect draws in cold air at the lower levels and pushes out warm air at the higher levels – and the bigger the temperature difference between inside and out, the stronger that effect.

Sealing the bottom plate is crucial. I’ve talked to many builders who only use an adhesive like PL400 to connect the treated sill plate to the concrete. Not only is that a poor air seal, it’s also not an effective way to prevent moisture wicking up into the wall framing. Instead, I like to install a sill seal between the concrete and the bottom plate. Then I seal the exterior with a tape or liquid flashing that bonds to both the wall sheathing and the foundation material below – whether that’s concrete, wood, or foam insulation.

On the interior, I’ll seal the connection between the concrete wall, slab, or subfloor and the wall framing using a bead of acoustical caulking, right before the vapor control layer or drywall goes in. This helps create a continuous air barrier from the foundation all the way up.

For an older home like mine, a 1952 Cape, I used Henry Blueskin VP100 self-adhered membrane to seal the foundation-to-wall connection on the exterior. I increased the adhesion by priming the concrete with Henry Blueskin Prep first. It’s been holding strong for over a year now – I still can’t peel that Blueskin off the concrete. On the interior, I just used a big bead of acoustical caulk.

Sealing the Top

The next key air sealing detail is maintaining the continuity of the air barrier at the top of the exterior wall. If you’re building the “perfect wall” with continuous exterior insulation and rafter tails added after, the air control layer can simply follow the roof sheathing up. But most homes these days have rafter tails that are part of the roof structure, which means the air control layer needs to move to the interior ceiling.

You can accomplish this by either taping or mechanically fastening a water-resistive barrier (WRB) to the top of the exterior wall and sealing it to the interior ceiling air barrier. That interior air control might be airtight drywall, or it could be a smart vapor retarder like Siga’s Majrex Pro, Clima’s Intello, or CertainTeed’s MemBrain.

In my climate, I’m not afraid to use 6-mil poly on the ceilings as long as the space above is an unconditioned, vented attic. But with cathedral ceilings or unvented attic assemblies, I’d want to use one of those other vapor-open air barrier options.

The connection where the wall meets the ceiling, along with any penetrations through the interior ceiling, are where I tend to see the most air leakage issues during blower door testing. That’s where the stack effect can really create problems, pulling warm, moist air out into the attic space. Best case, you just lose some heat. Worst case, you end up with attic rain – literal moisture dripping from the ceiling inside the home. Not good.

Plugging the Holes

Of course, the biggest holes in the air control layer are the ones we put there ourselves – windows and doors. Controlling water is the top priority here, but you also need to ensure a continuous air seal. When using an exterior air control method, extend the tapes or fluid-applied sealants into the window or door opening so the air barrier remains continuous after installation. Then use a minimal expanding spray foam or backer rod and caulk from the interior to complete the air seal.

Penetrations for things like electrical wires, water spigots, and HVAC vents can also punch holes in the air barrier. Tapes, gaskets, fluid-applied sealants, and caulks can all be used to seal these. My first choice is usually a good quality tape, but if there might be movement (like with a PVC vent), I’d go with a gasket. For electrical wires, I like the “one hole, one wire” approach – it’s hard to get a good air seal around multiple wires in the same hole.

Interior penetrations like electrical boxes, plumbing pipes, and HVAC ducts also need to be properly sealed, whether they’re in walls or ceilings. Again, the “one hole, one wire” principle applies. I typically use canned spray foam, but caulks and other sealants can work too. Airtight electrical boxes with foam gaskets can help, but you still need to seal where the wires enter.

Comfort, Efficiency…and Health?

The improvements I’ve seen in air tightness, from my own 1952 home testing at 12.5 ACH50 to the average new home today closer to 2 ACH50, are pretty remarkable. In fact, I’ve even tested a few homes at an incredible 1 ACH50.

But while tighter homes are certainly more energy efficient and comfortable, the connection to health isn’t so straightforward. It’s important to remember that excessively tight homes can also be unhealthy if they aren’t properly ventilated or if they contain high levels of off-gassing materials.

That’s why, when building a tight home, you also need to focus on indoor air quality. Proper mechanical ventilation is a must, as is carefully considering the materials you use – eliminating things like foam insulation, cheap carpeting, and formaldehyde-laden building products. The goal should be a healthy, durable home, not just an airtight one.

As we continue to push the limits of home energy efficiency, it’s critical that we don’t lose sight of the bigger picture. Tight is right, but only if it’s done the right way. With a little planning and the right approach, you can create homes that are comfortable, efficient…and truly healthy too.