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Passive House Design-The Heating System

We talk about Passive House quite a lot and will likely continue since it’s one of our favorite subjects.

I’m excited to report that Tessa, our designer, has three Passive Houses in Design Development right now and we receive questions daily from folks cruising the web. The Passive House ‘buzz’ that is starting to happen is very energizing for all of us at The Artisans Group!

I have a favorite question that gets asked in response to when a person discovers that a Passive House requires no heating system. No heating system is so revolutionary a concept, and fairly strange to comprehend when viewed in the light of how most homes have been and are built today.

So, the question is:

“If there is no heating system required for a Passive House design, will I be wearing a sweater most of the winter just to take the chill off?”

The simple answer is no, in fact you will require fewer sweaters than before. Let me explain. The Passive House concept is based around a Major Comfort Criteria, this comfort criterion dictates that no exterior surface be less than 64 degrees. This may seem like a simple concept but it is common for the exterior surfaces in most of our homes to be a lot cooler than that (think of how cold a window can be)! When we design to the Major Comfort Criteria, we end up with a much more comfortable and long lasting home overall, avoiding issues such as convection, stratification and radiant asymmetry.

There are a lot of ways to feel cold in your home, the one we most commonly think of is drafts. Convective currents or drafts happen as heated air rises, as it cools it ‘falls’, this happens throughout all homes in all spaces to some degree but it is most notable in front of windows because the temperature difference is the greatest in front of a window. The warm air above the window cools more rapidly because the window is a poor insulator, so the warm air ‘falls’ more quickly, creating the age-old dastardly draft. In a Passive House the windows are significantly more efficient, with thermally broken frames and much improved U-values, mitigating the currents that cause drafts.

Another way to feel cold and have potential for condensation issues (have you ever stacked boxes in a closet and gotten mold or mildew on them?) is stratification. This is where cold damp air settles in the room and why our feet get cold first (warm air naturally rises). Not to mention that the cold damp air settling to the floor of the room will condensate on any warmer surface it encounters. Condensation and mold issues greatly reduce the lifespan of a building, also creating poor environments for human health. Yet again this is mitigated in Passive Houses by windows with improved efficiencies and assemblies with thermal-bridge-free detailing.

Yet another way to not enjoy the full potential of our thermostat set to 68 degrees is the radiant effect or radiant asymmetry. When electromagnetic waves come in contact with an object, the waves transfer heat to that object. This transfer requires ‘line of sight’; it’s best to think of this in terms of a camp-fire. If you are sitting in front of a camp fire all toasty and warm and someone moves in front of you, instantly your heat source is cut off and your sensation of direct heat is lost. That’s radiation. In a room where the heat is set to 68, and the air temperature is an even 68, one can still feel cold through radiance towards a cold item in the room, such as a window.

So in answer to the question posed above, the point of Passive House design is NOT to sacrifice comfort and make us all live in iceboxes, in fact it is the exact opposite. We now understand why a Passive Houses thermostat set to 68 degrees will feel warmer than a typical house. In fact, in Germany where there are thousands of these homes it is a very popular selling point. Passive Houses use higher performing windows and reduce thermal bridges, combine this with super insulation and although there is not an oversized furnace humming away racking up the energy bills, there is ample heat for the space provided through the fresh air delivery system. The whole point is to minimize losses to maximize gains- not sacrifice comfort.

Pretty cool, really.