What is the best wall construction for Passive House?

A recent article in the Home Energy Magazine analyzes the embodied energy of different wall structures for Passive House construction in cold climates. Basically, it’s great to have super insulated homes, but home much extra energy does it take to build them? Said another way, how many years will it take before the embodied energy it took to build the walls becomes less than the energy those walls saved.

They compared the following wall structures

  •  TJI frame with blown-in fiberglass insulation, built in Urbana, Illinois.
  • Insulating concrete form (ICF) with exterior expanded polystyrene (EPS), built in southern Wisconsin.
  • Structural insulated panel (SIP) filled with urethane foam with an interior 2 x 4 wall filled with blown-in cellulose, built in Belfast, Maine.
  • Advanced 2 x 12 stud framing filled with open-cell spray foam and insulated on the exterior with either EPS or vacuum insulated panels (VIPs), built in Bemidji, Minnesota.
  • Double 2 x 4 stud wall insulated with blown-in cellulose, built in Duluth, Minnesota.

The energy payback time for the wall assemblies ranged from immediately for the double-stud wall to 4.4 years for the mass wall—not a big chunk of a building’s expected lifetime. Because of the HFC blowing agent, the advanced frame with spray foam envelope has a carbon payback of 23 years.

Although the double-stud wall comes out smelling of roses in these comparisons, as long as you avoid specifying insulation made with an HFC blowing agent and minimize the use of energy-intensive materials, such as concrete and OSB, all of these envelopes would have a good energy and carbon payback.

Here is the double studd wall with cellulose:

The double studd walls stop thermal bridging while the cellulose insulation has very low embodied energy.

The double studd walls stop thermal bridging while the cellulose insulation has very low embodied energy.

As a New York Passive House builder the big question for me is how does this affect Passive Houses built in existing Brownstone buildings. The double stud and cellulose can easily be applied on the inside of the Brownstone brick walls. But there are two problems with this.

One problem is space. Brownstones cost a lot of money and to loose an extra several inches of floor space is a big deal.

The second problem is deterioration of the brick walls. Those brick walls have survived wonderfully for the past 100 years thanks to the nice warm heat from the building. Once you install the double stud walls you isolate the brick on the outside of the thermal envelope and the bricks are susceptible to freezing.

When the mortar in a brick wall freezes it expands. When it thaws it contracts. Over the years this wears away all the mortar and the wall falls apart. How long this takes is still a bit in the air since all Passive Houses in NYC and Brooklyn are only a couple years old.

One solution to both these issues is to build a less thick wall. You gain space and a little heat is lost to the outside, stopping the bricks from freezing. Clearly this is not ideal given the lost energy.

If anyone has solutions to these issues I am very interested to hear them.

Das Haus: First NYC Passive House in BK

For those of us who live in historic homes we know that our period dwellings bring us both joy and frustration. The frustration is largely attributed to the endless repairs that classic Brooklyn Brownstones require and their not so efficient envelope.

Eco Brooklyn has renovated many brownstones and knows first hand how challenging it can be to air seal and insulate an building while still keeping it’s traditional character.

With the advent of new energy efficient building techniques Eco Brooklyn is part of a new trend in Brownstone renovation: instead of following traditional guidelines to fixing up a house, some Brooklyn homeowners are transforming their townhouse into a Passive House – a German technique that can reduce a homes energy consumption up to 90%.

This past week, the Eco Brooklyn interns took the metro North train up to White Plains for the Das Haus Symposium. There were a number of speakers, some coming all the way from Germany to talk about projects, ideas and products that have either already migrated to the US or are on their way. The Passive House concept was a topic of interest.

The Passive House standard focuses on 5 main strategies:

  1. Insulate strategically
  2. Stop Thermal Bridging
  3. Achieve air tightness
  4. Install high-performing windows for thermal comfort
  5. Reduce mechanical systems with heat recovery ventilation

Jordan Goldman, the engineering principal at Zero Energy Design was a speaker at last week’s symposium. He is a Passive house consultant who recently finished a passive house restoration at 23 Park Place in Park Slope. The completion of this project marked the first certified Passive House in New York City!

The original structure at 23 Park Place was built in 1899 and had been owned by a few artists until it was abandoned a few years ago. After the new owners purchased the dilapidated property they decided to do a Passive House retrofit on the existing structure. Julie Torres Moskovitz from Fabrica718 was the lead architect on the project. She enlisted Jordan Goldman as the engineering consultant on the design.

Since this property was not land marked the retrofit became a complete makeover for the structure. For instance, all the fireplaces and chimneys were replaced to increase the overall air tightness of the space.

As noted before, air tightness and a system of interior and exterior air exchange are the key stone elements to creating  a cohesive thermal envelope ensuring maximum energy reduction.

23 Park Place met the air tightness requirements of a passive house, and far surpassed the requirements of NYC. 23 Park Place is not only 15 times tighter than a current building norm is achieved the highest air tightness level in all of New York City-  .38!

In addition to the insulation, comprised on 23 inch thick walls and three pane windows

Passive House calls for all the joists and meeting points to be sealed to create a continuous thermal envelope.

Although after so much emphasis on the insulation, you must be wondering how could anyone possible endure such stuffy conditions. The answer to this seemingly uncomfortable air is the energy recovery ventilator or E.R.V.

Essentially, the inside air is pulled through the ventilator, the heat is then transferred to a membrane, the air is cooled and then exits as exhaust. The fresh air outside is simultaneously being pulled in and warmed by the membrane. This system, which is referred to as “counterflow” maintains a constant temperate within the thermal envelop.

The Passive House energy use standards are far more stringent then those used by the US Green Building council, which issue certifications for LEED and the Energy Star program. It is considered excellent if a LEED certified structure can reduce energy consumption by 30% and Energy Star homes typically save about 15 to 20%.  With a Passive House there can be up to a 90% reduction in heating and cooling.

Now that’s a paradigm!

Fortunately there are a number of Passive House projects underway in New York City, many of which are located right in Brooklyn. As a New York Passive House builder we hope to see an increase in the demand for Passive House design in the upcoming years. It costs within the range of normal construction yet greatly decreases a building’s impact on the environment.