Question

If there is no heating system required for a passive house design, will I be wearing a sweater most of the winter to keep the chill off?

Asked by Zeta
Olympia, WA

Answer

Tessa Smith

Answered by Tessa Smith

Olympia, WA

The Artisans Group

March 12, 2010

The simple answer is no; in fact you will require fewer sweaters than before (OK, you may need to keep a few for your friends' houses, but not for your own!). Let me explain. The Passive House concept is based around Major Comfort Criteria; these comfort criteria dictate 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, then cools as 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 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 naturally wanting to rise). Not to mention that the cold, damp air settling to the floor of the room will condense 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 campfire. If you are sitting in front of a campfire 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. Passive Houses simply have improved comfort (we now understand why a Passive House's 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 and racking up 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.

Tagged In: heating cooling, passive house

Do you have a question about greening your home? GreenHomeGuide invites you to Ask A Pro. Let our network of experienced green building professionals – architects, designers, contractors, electricians, energy experts, landscapers, tile & stone specialists, and more – help you find the right solution.