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Building Science and My Cat

Paul Kando

Could my cat know better how the laws of thermodynamics I wrote about last week apply to keeping warm in winter than the men who built my once- leaky old house? Mid-morning, in the sun-room, he curls into a ball, fur fluffed. An hour later, he stretches out, his fur smooth as silk. By noon he is stretched out on the cool kitchen floor. He purrs contentedly even when I, with no fur, reach for a sweater to ward off the chill of another cold and windy April day. We share a warm-blooded coexistence, so I might as well settle down to learn.

Insulation under and around slab
Slab on a Passivhaus House
photo credit:Chris Corson, EcoCor LLC

A cat’s body temperature is about 101ºF, mine 3º lower. We both seem comfortable when the room is between 68 and 70ºF – some 30º cooler. That’s because, like a house, we have built in heating systems: our metabolisms. We would overheat if we couldn’t radiate our excess heat to our surroundings. Not hot, not cold, that’s the idea. Pick the best spot to keep comfortable. Curl up or stretch out to vary your surface area. To vary the amount of insulating air, fluff up your fur just so if you are a cat, put on or doff layers of clothing if you are a human.

Our comfort depends on our surroundings. We absorb heat from a stove, or radiate it to a colder window. Air blows at us or away from us. We absorb or lose moisture. These are the building blocks of being comfortable. We can adapt to the seasons. Some even move back and forth between Maine and Florida.

A house cannot adapt like this. What it can do, is keep conditions surrounding us – temperature, air movement, relative humidity, air quality – stable and comfortable. Just like an animal’s fur coat – or body-hugging long johns and sweaters – a well built house wraps its occupants in an airtight, superinsulated envelope. Like a cat that curls up in the sun, a well-built house is oriented to maximize solar gain. Like a late winter nest snug around newborn squirrel, skunk and bear, a well built house keeps its occupants comfortably dry and warm.

A Passivhaus (PH) does this most cost effectively, because it optimizes the house as a system, down to the smallest detail, before anything happens on the building site. Modeling by computer what needs to be bought, how construction must proceed in order to ensure a predetermined performance and ruling out waste and mistakes in advance, is more efficient and less expensive than “winging it” or making and correcting errors on the building site.

On these pages, I describe what I consider the best PH system features I have observed in Maine. However the key to PH construction is the system optimization. In other words, you cannot build a PH by merely combining features as if, like most conventional houses, it were just an assembly of components. Furthermore, since PH details are left up to each designer-builder, they may vary from house to house, even though overall systemic performance will not.

A PH surrounds its occupants from all sides, including from below. In my opinion, the most cost-effective PH foundation is a super-insulated concrete slab. Since the house will need no bulky conventional heating system, why have a hard-to-keep-dry Maine cellar? A floating slab may feature 12" of rigid polystyrene (EPS) insulation over a crushed rock bed, in effect serving as the form into which 8" of steel-reinforced concrete is poured. In this arrangement the slab’s edges are also superinsulated. The EPS layer is extended 48" around the perimeter, slightly slanted away from the slab, to channel rainwater away from the building and protect the foundation and the surrounding ground from freezing, down to the 4' frost line.

Concrete is not a good vapor barrier. Therefore a continuous polyethylene vapor barrier is added, meticulously sealed with a specially designed tape around all pipe and utility penetrations. This continuous barrier will also be taped to the air/vapor barrier of the walls, forming a continuous airtight cocoon around the heated building envelope. The total thermal resistance of such a slab foundation is about R-54, compared with about R-3 for a conventional 4" concrete slab, the edges of which are often left uninsulated.

If a PH is built over a garage or unheated basement, the basement ceiling is air-sealed and superinsulated, with it’s vapor barrier sealed to the walls, as above. The air-tightness of this continuous barrier is so important that most PH builders test it with a blower door during construction.

In retrofitting an existing foundation, the first task is to keep the basement dry, by preventing moisture infiltration and removing water that cannot be kept out. Beyond these basics, some of the techniques described above may be emulated. For example, insulate around the perimeter of a concrete slab, frost-protect a foundation wall with a foam board slightly sloping away from the house, add a polyethylene sheet to a cellar or crawl space floor, seal the foundation to the rim-joist, air-seal the sub-floor above unheated basements and crawl spaces, and superinsulate basement and crawl space ceilings. For best results, plan improvements based on a thorough, independent, energy audit. In general, don’t just fix leaks, insulation, ceilings, walls, basements and attics: improve your home as a system!