This is our version of FAQs
Y did you name this blog and web site EdgewaterHaus? We will be building our home on Edgewater Lane. We will design and build the house to meet the German-based Passive House standard, which requires a nearly airtight building shell, maximizes solar heat gain, and relies on mechanical ventilation to ensure proper indoor air quality. The German word for “house” is “haus,” hence EdgewaterHaus.
Y did you select the PassivHaus green building standard? That’s easy – it’s arguably the most challenging, and more importantly, one of the few green building standards with an objective post-construction performance criteria. Other major green building standards are based on energy model predictions, proof that green materials were purchased and used, and other pre-construction decisions. They are agnostic as to how the home is actually built. Imagine buying something as complex as a car, knowing the tires were checked for proper inflation, the chassis lubed, the radio sounds great, but no one ever took the assembled car for a test drive before putting it on the dealer’s lot for sale. Well, that’s how we build homes today. Homes are just as complex as cars. We test individual home mechanical and electronic components, but we rarely test the performance of the ENTIRE building after it’s built. PassivHaus focuses exclusively on energy efficiency for the entire building. The final exam is taken after construction is completed, and the house gets either a pass or fail grade.
Y the suspense: so what is the PassiveHaus standard and how is it applied? There are only three but very specific criteria: (1) a maximum heating/cooling load per square foot; (2) a total energy load per square foot; (3) a minimum “tightness” of the home’s building envelope, i.e., the exterior walls and ceiling of the house. Similar to some other green building standards, the first two elements are based on a proprietary energy model prediction prior to construction. It is the third element which differentiates PassiveHaus as it is performed after construction is completed. An independent 3rd party technician conducts a blower door test, where all the exterior doors and windows are first closed, one of the entry doors is removed and a large fan is temporarily sealed into the opening. Turning on the fan pressurizes the inside of the house compared to the outside, and an instrument is used to measure the “tightness” of the building envelope. So even if the model predicted the home design would “pass,” the best quality materials were used, and the contractor assembled the walls and ceilings with the proper thickness of insulation, the house could still fail if the contractor did not properly seal air leaks or avoid thermal bridging. For you building geeks, the specific standards are 4.75 KBtu/sf/yr for heating and cooling; 38.1KBtu/sf/yr for total energy load; and 0.6 air changes/hr at 50 pascals. For you non-building geeks, that’s a VERY tight house, some 90% more energy efficient that the typical US home.
Y are air sealing and thermal bridging so important to building a tight house? Proper air sealing and minimizing thermal bridging is both art and science; it’s the devil in the construction details. Air sealing means putting up an air barrier like TYVEK and caulking all openings in the building envelop to prevent air infiltration. In a typical US home, put your hand in front of an electrical wall plate on a gusty day and feel the air coming in. It also means ensuring proper gaskets to seal against air convection around exterior doors and windows.
Thermal bridging refers to the conduction (remember your high school physics of the three heat transfer mechanism: convection, conduction, and radiation) of heat through materials. Many homes in colder climates are built with 2 x 6 walls instead of 2 X 4 walls. Fiberglass insulation is about R-3.5 per inch. So the extra 2″ depth in the 2 x 6 walls provides R-19 insulated wall rather than just R-13. Great.
Well, not quite. In a typical home, the wall studs are spaced every 16 inches, and doubled around doors and windows. Complicated roof lines and large windows mean even more studs in the wall. Then there’s the wood bottom plate below the studs, wood top plate above the studs, wood headers above doors and windows, along with wood fire-blocking and backers for attaching railings etc. I’ve read studies showing that as much as 25 percent of the wall is actually solid wood. Most homes are framed with pine, a softwood that has an R-value of about 1.25 per inch (wood is an good electrical insulator but a poor thermal insulator). That means that heat can transfer through the R-1.25 per inch wood by conduction much faster than through the fiberglass R-3.5 per inch insulation. Thermal bridging refers to this conduction of heat from the warm inside to the cold outside via the wood. Studies show that a 2 X 6 wall insulated with R-19 fiberglass batts provides an effective wall assembly (studs + cavity insulation) of about R-14. Not so good.
A tight building envelop requires building the exterior walls to avoid this thermal bridging.
Y not just build to code? Building codes in the U.S. are MINIMUM standards to ensure safety, fire prevention, and environmental performance. Not many people aspire to buy the minimal costly shiny new car on the lot. Most of us want better than the minimum. The current U.S. building code (the International Energy Conservation Code), which must be adopted locally, was revised in 2009 to establish a maximum 7 air changes per hour at 50 pascals. Even where the code is enacted, I rarely read of jurisdictions that enforce that. That’s a huge difference from the Passive House standard of 0.6 air changes per hour!
Y spend more for super energy efficiency building shell instead of other “green” alternatives like geothermal, solar voltaic, or solar thermal? That’s both a philosophical and economic question. The Passive House philosophy is to first invest in constructing a very tight building envelop, thus reducing the need for costly mechanical systems. The building envelop should last for the life of the structure without need for any maintenance or repair. Mechanical system require annual maintenance and repair. The economic analyses I have read support the passive house approach as being a better life-cycle investment for new construction. That may not be the case for remodeling an existing home because of complications with adding more insulation to walls/ceilings, and upgrading to better windows.
Y also pursue LEED along with Passive House? While I believe Passive House is a much more effective standard that LEED, none-the-less, the broader sweep of LEED is attractive. I particularly like the LEED aspects about water conservation, sustainable materials.
[more to come]