Historic Preservation & Passive House Working Together In NYC
By: Ken Levenson, A.I.A.
February 01, 2011
Too often energy efficiency and historic building preservation are seen in opposition. This is a false dichotomy — we can make our historic buildings very energy efficient while retaining their essential historic fabric. We can even make the historic buildings better and longer lasting in the process. The two goals can and should support each other.
I am currently completing an historic townhouse renovation in Brooklyn Heights to meet the Passive House retrofit standard, a voluntary construction standard that results in a 90% reduction in heating and cooling energy demand, and as much as a 70% reduction in overall energy usage.
A Passive House renovation not only results in energy savings proportionate with our climate challenge but provides a home that is more comfortable and healthy and affordable for its occupants. There are no drafts. The air quality is better inside the house than outside air. Monthly utility bills can be measured in the tens of dollars, not hundreds. (And yes, the windows can be opened.) Passive House, while brand new in America has succeeded in Europe with housing construction and renovation at all income levels and building types — now totaling over 20,000 buildings, and growing exponentially.
What Does It Take To Make A Passive House?
Passive House and other very energy efficient renovations require careful consideration of building science — including issues of insulation, air conditioning, air tightness, thermal bridging, condensation and vapor migration. Most renovations now include insulation and air conditioning — yet the serious implications of these installations are typically not considered. Often, instead, ideas of breathability have misdirected concern, effort and money. [i]
Dispelling such urban legends and digging into the real building science is the starting point for aligning historic preservation and energy efficiency. Case in point: the Brooklyn Heights Association, The New York Landmarks Conservancy and the Historic Districts Council were unified in opposition to our Brooklyn Heights Passive House proposal — testifying against it at our Landmarks hearing. [ii]
In testimony and in side discussions, the opposition often contained variations of three myths:
- Historic building walls need to breathe air — therefore, making them air tight is unacceptably risky.
- These townhouses are well built for our climate, thereby making air tightness unnecessary.
- Historic townhouses are already energy efficient, particularly in relation to post WWII construction — so these buildings should not be burdened with further efficiency requirements.
We use building science to set matters straight. Working up from the bottom of the myth list:
- Yes, 19th Century townhouses are more efficient than the glaringly inefficient glass boxes of the last sixty years. We can do vastly better and should.
- And yes, New York City's historic townhouses are quite pleasant for stretches of the year. But they are frigid in winter and stifling in July and August. The windows leak (even the new ones), the walls leak and lack insulation, and ventilation is spotty at best — making drafts insufferable in winter; and with windows wide open in the summer, an interior coated with city soot, dust and pollen. The historic building while culturally invaluable can and should be made functional for the 21st Century.
- Breathability is the most unhelpful myth — and ultimately a case of mistaken identity. Breathing indiscriminately is as dangerous for buildings as it is for people — what you inhale matters. Air flow across the wall enclosure is a building's biggest liability. Air flow can carry tremendous amounts of water with it, easily saturating wall assemblies and potentially leading to freeze/thaw wall destruction and failure, mold and rot. Air tightness protects the insulated wall assembly from water damage.
Air tightness has other benefits too — protecting the occupants' comfort and health — and is essential for energy efficiency. With our Brooklyn Heights Passive House renovation the full effect of air tightness was made dramatically clear this past December: The structure had been made air tight but was still completely uninsulated — with no heat source other than the workmen, work lights and the sunlight. With temperatures in the teens the inside was still comfortable — the workmen in fact had to take off their coats and sweaters. Sitting on site they'd marvel at how a house without any insulation could be so warm in such cold weather. The answer is air tightness.
While air tightness is a clear benefit, the decision exactly how to insulate these old buildings is likely the most critical one to make. We will insulate these old buildings, as comfort, economics and the climate challenge demand it, but because we are insulating on the inside face of the exterior walls, they do become colder and less able to dry. The more insulation we add, at the interior the greater chance of destructive masonry freeze/thaw cycles. Luckily freeze/thaw is dependent on several bad things happening simultaneously — including moisture saturation and extreme cold — so that with proper water shedding from functioning cornices and sills, air tightness and carefully calibrated insulation levels, we can responsibly insulate these buildings. Fortunately, given the relatively moderate climate here and the optimal surface to volume ratio of townhouses and multi-family buildings dominating New York City, the insulation levels can be very moderate, yet still achieve Passive House level optimization.
Breathability, often mistakenly attributed to air flow, is really about water vapor flow. Historic building walls should breathe water vapor (not air). Due to humidity and pressure differentials between outside and inside, water vapor moves into wall assemblies — generally moving outward in the winter and inward in the summer. [iii] Once there is water in the wall it must be able to get out making vapor barriers often not only unhelpful but detrimental to the wall construction.
Today, everyone installs air conditioning without giving much thought to its effect on the building and this is risky. Remember, in the summer, water vapor typically moves from the outside toward the inside — accelerated by solar forces. Air conditioning amplifies this vapor movement. So if a vapor barrier has been installed on the interior portion of the exterior wall, the wall can saturate, resulting in mold growth. Therefore when we air condition it is critical to address the wall's drying capability — and typically another argument for eliminating vapor barriers in the wall assembly.
Last but not least, Passive House retrofits introduce a new piece of equipment to historic buildings: the continuous high efficiency fresh-air ventilator. Supplying filtered fresh air to living spaces and exhausting from bathrooms and kitchens the ventilation system insures a comfortable and healthy interior environment. The ventilator can help maintain lower interior humidity levels in the summer and higher humidity levels in the winter. This moisture control helps keep the structural components in a steady-state environment, fighting moisture related problems and maximizing longevity — a real benefit to the historic building structure.
In conclusion, with careful consideration of building science issues, renovating our historic townhouse to Passive House standards should be a win-win proposition — for the occupants, for our city, and for the climate — with preservation and energy efficiency working together.
[i] Breathability, a concept mired in circular reasoning, claims old building enclosures were built with a good deal of air leakage and therefore need to leak air to function properly.
[ii] The specific question before the Commission was whether it was appropriate to install simulated double-hung windows — allowing us to maintain the essential look and feel of historic windows while providing air tightness. The Commission overwhelming approved our proposal.
[iii] But the amount of water is a tiny fraction of what air-flow can carry into the wall. Wall failures have not been found to result from excessive vapor diffusion into the assembly but due to air leaks carrying bulk amounts of water.