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Passive House Design: Sealing Leaks for Air Tightness

Mark A. Miller is a practicing architect/builder/developer living in Chicago who designs projects around the country. His studio, Mark A. Miller Architects + Builders, designs and builds high-performing, energy-efficient homes that speak to the soul. Mark recently co-founded the Passive House Alliance Chicago and is lecturing about the Passive House standard throughout the Midwest. You can learn more about his unique approach to designing thoughtful homes at his websites: Zen + Architecture and Passive House Midwest. 

I wanted to start to go through some of the techniques that help one achieve the Passive House standard. We spoke last time about the difference between “passive solar design” and “Passive House design” as there seems to be much confusion. These are two different approaches to achieving better levels of energy efficiency. Today I would like to focus our dialogue on one of the key techniques in Passive House design—air tightness.

Have you heard the expression “It’s like having screen windows in a submarine”? In most American homes to date, the concept of air tightness has been marginal at best. The key concept here is to keep the unconditioned air (the outside air) from coming in to our conditioned air (you know, the expensive stuff our furnaces and air conditioners have produced for us) and killing its effectiveness. The more my expensive interior conditioned air stays “in”, the less my expensive air handling equipment will run to make more of it.  You lose it, you have to replace it.  So let’s keep it around as long as possible, is the Passive House mantra.

air leakage diagram 
Air can leak into and out of your home through various places. Illustration Courtesy Mark Miller.

So, let’s look at our standard American home’s building components and get enlightened to all of these “built-in” opportunities for outside air to come in to our conditioned spaces. Here’s a list:

Wall caps at dryer vents: these are usually made of thin sheet metal, sometimes with a damper, which you hear flapping in a winter breeze, just to remind you of all the cold creeping into your house

• Similarly, bathroom and kitchen hood exaust fan wall caps. Same issue. (As a side note, I am working on a new product that will upgrade this element to become completely air tight. If you might be interested, email me at

Electrical outlets on exterior walls: leaky wall assemblies will allow cold air to get inside a home’s conduit. You can feel it pouring through the outlet cover. I have seen insulated gaskets on the market to help with this, but the best idea is to not run any piping in your exterior walls.  How do you get outlets there if you need them? Think creatively.  You can have a thicker baseboard and put them in there.  You can put them in the floor. “Think outside the box.” Wait, think consciously not to disturb the permiter of your box, may be a better saying here.

Sill plates: the intersection of the framing of your walls sitting on your foundation wall is a huge culprit for leakiness. Critical attention must be paid here to air seal all materials in this area. The use of “Sill Sealer” under your sill plate helps; foams and gaskets are available, and the quality of the sheathing installation to the framing all have an effect. In very old homes, we find insulating the floor cavity from the perimeter of the home, to five feet in, helps stop cold air from transferring around the house.

Hose bib piping: air seal around these penetrations

Recessed lights: mainly in second floor or attic/roof ceilings, allow cold attic air to come into the conditioned space. Some companies are now making products which do a better job air sealing this component, but pay attention to meticulous installation to achieve full benefit. Another interesting thing I discovered during the construction of a home is that air from the home can travel through these lights and into the conduit feeding the light. If that conduit goes into a cold attic space, the moisture in the air will condense and you can have water dripping out of your light fixture. Ask your electrician to install some fire-resistant material to prevent this from occurring, yet be removable should one need to pull wiring in the future. 

Attic hatches: Did you know your home can lose up to 10 percent of its heat through this device! These are usually uninsulated. Heat rises and will find the path of least resistance out, through your hatch. We found a neat product called the “Energy Guardian” by ESS Energy Products. It’s a five-inch-thick insulated frame that you install around the hatch opening in the attic. It comes with a matching five-inch-thick foam cap which gives your hatch around an R-35, which is much better than “0”.  Make sure the attic floor is equally insulated around this area so there is not a break in your insulation layer.

Plumbing vent through the roof: Every home has a plumbing vent stack penetration in its roof. These are usually larger pipes of four to six inches in diameter.  It’s important that all pipe penetrations are properly sealed for air tightness, not only at the roof, but where it passes through any material.  Passive House practices having no vent piping in the outside walls.

Because there are so many opportunities for air to penetrate the building envelope, it is critical to re-evaluate every situation discussed, to improve common details for optimum energy saving performance.