Balancing strategies in older homes
By Nancy Barba, AIA, LEED AP
What is an historic house?
Given that many people associate the term “historic” with Williamsburg and 18th century houses, it may surprise you that soon, houses built in 1969 or earlier could also be considered historic. Houses that are over 50 years old and retain their integrity are generally eligible for listing in the National Register of Historic Places. Houses can also be on the National Register if they are within a designated Historic District.
The National Park Service examines a property’s age, integrity and significance to consider its eligibility for the National Register. This includes determining if the property is old enough to be considered historic, whether it still looks much the way it did in the past and if the property is associated with important historic events.
But just because a house is not individually listed on the National Register, or within a National Register Historic District, doesn’t mean that it’s not historic. It just means that the federal agency that designates properties won’t recognize it when it comes to federal funding and that a local historic preservation ordinance may not regulate your house (unless it falls within a local historic district).
This article will address best practices in balancing energy saving and user comfort objectives with historic properties regardless of their designation.
Getting started: The energy audit
If you are considering energy upgrades to your historic home, consider having an energy auditor and architect who work with historic properties look holistically at your house and make recommendations before you begin.
Retaining well-built homes is far preferable to the alternatives, such as demolition and the landfill. When deciding whether to renovate or rebuild, the important questions to consider include: "Does your house possess good bones?" and "Can it be renovated or rehabilitated to become more efficient?" Tearing down and replacing a house with new construction is not the best environmental choice.
In 2016, the Preservation Green Lab—the National Trust for Historic Preservation’s sustainability think tank—conducted a study on the environmental value of re-using buildings versus constructing new. It focused on pre-1940 single-family housing stock.
The study found that it takes 38 to 50 years for a new single-family residential building that is 30 percent more efficient (than an average performing existing building) to offset the climate impacts related to new construction. This means that you could construct a new house that’s 30% more efficient than a nearby identical existing house and it could take up to a half-century for the new house to have a better environmental impact than the existing house. Or, as architect Carl Elefante stated in the now popularized phrase, “The greenest building is the one that is already built.”
Historic comfort elements
Prior to 1940 and the advent of mechanical systems replacing natural ventilation, many houses were built with seasonality in mind. For summer cooling, houses had east, south or west side porches, cross ventilation, chimney effect of gravity vents, cupolas or stairs with venting skylights, exterior and interior shutters, screen doors, window screens, and ceiling, attic, and window fans. Over time, some of these features may have been removed and therefore must be reinstalled or created anew.
Of course, in the heating season, your house is called upon to reverse its modes of operation and keep the cold air out. Some of the same elements that helped keep your house cool—like attic insulation—will also help with containing the warmth. But most will need to be retooled to address winter conditions. Cold weather measures—air sealing, storm windows and doors and coniferous tree windbreaks—all contribute to maintaining comfort and reducing energy usage. The following are typical recommendations for improving a house’s energy performance with a preservation lens.
One of the first items to consider is upgrading the house’s mechanical system. An old furnace or boiler is often the biggest energy user in an old house. And while a new high-efficiency heating system can be costly initially, the payback is immediate and ongoing. Also plan on upgrading the hot water heater with a system that runs off the boiler.
Historic houses with limited room for wall insulation are often well-suited to geothermal systems. Alternatively, many homeowners are considering mini-splits or electric heat pumps coupled with solar PV arrays. Again, the initial cost can appear high, but with tax credits the payback time is often compelling.
Photovoltaic arrays can be challenging on historic houses but may be achievable if the solar orientation provides a south-facing roof plane that is not visible from the public way. On properties with more land area, the solar panels can be ground-mounted and obscured from the public view to retain the historic character of the property. Some local historic review boards are open to allowing solar panels on roofs that are visible, but many are still unwilling to accept this option.
In Maine, most homes can remain comfortable without air conditioning, except for a couple of weeks during the summer months when conditioning can help one survive the most humid days. Considering the cost and payback, ceiling or window fans and select room-sized, portable air conditioners may suffice while using significantly less energy than a full air conditioning system.
Other cost and energy saving measures can include changing out light fixtures to LED lamps to save on electrical use. And simply adding a timer to the bathroom fan will take care of unwanted moisture without undermining the goal of retaining as much heat as possible.
Claiming attic and basement spaces
In creating a more energy-efficient home, you may also consider denser use patterns. Achieving more space within the same footprint might mean insulating an attic or basement space, which is usually more efficient than building an addition. It’s important, however, to make sure that water is not a problem in the basement, as this promotes mold growth. So, handling any water issues first is important and is best achieved with property drainage and waterproofing from the outside.
Except in special situations, we tend to discourage the use of spray foam in historic houses as it is not reversible, meaning that it is difficult to remove in the future. And covering stone foundation walls completely means obscuring the stone and mortar, so if there is a future problem it could take much longer to detect. Spray foam also forever alters the appearance of the stone foundation walls.
If one is not trying to create added living space, fully insulating the basement ceiling may be a better, reversible option. The homeowner can then remove outdated modes of insulation in the future in favor of improved technological advances.
It is almost always preferable to retain the original trim and clapboards on an historic house, so blowing in insulation rather than insulating on the outside is a good way to avoid damaging original materials. Blowing cellulose into existing wall cavities is an art, to be sure, but there are many contractors who have been doing it for years. In fact, there are now inexpensive ways to check with infrared cameras to make sure that all voids have been filled without disturbing the existing interior plaster or sheathing on outside walls.
Of all the improvements, insulating the roof or attic floor offers the best value on an historic house. Open attic spaces lend themselves well to blown in cellulose and can greatly improve conditions in summer and winter.
Air-sealing is highly recommended and can be done selectively with caulking at open joints. Again, spray foam insulation in between attic rafters is not reversible, especially when it becomes buried in a closed-in ceiling or wall.
Every house needs to be evaluated according to its layout to determine the best insulation options. Some odd shaped roofs may be best addressed with spray foam insulation solutions. But if a home does not need its attic space conditioned, the best option is a reversible solution like dense-packed cellulose, mineral wool or rockwool. Blow in this loose product to as much depth as possible on the attic floor. About fourteen inches of depth will provide the code required R-49 in cellulose and R-56 in rockwool. This solution requires soffit vents or end wall louvers to draw air in, keeping the roof surface cool and averting ice-damming.
Repair or rehabilitate windows
In 2012, the Preservation Green Lab studied the payback for window replacements, concluding that “selecting options that retain and retrofit existing windows is the most cost-effective way to achieve energy savings and to lower a home’s carbon footprint.”
High-performance enhancements can be as simple as having a millwork contractor rehabilitate the sash and add integral, mostly concealed, weather-stripping. With double hung windows, the greatest air infiltration (draftiness) is felt (and measured) through the meeting rails. Pairing weatherstripping with a sash lock—and remembering to cinch the sash together by locking it—cuts down air infiltration significantly.
Other enhancements include installing interior or exterior storm sashes. Adding storms can achieve an added R-value of just under 1.0 at the windows, and double-glazed storms typically add R-1.75. Combined with the original window sash, the overall R-value of 2.75 allows a window system that approaches that of new window R-values at lower cost, with less environmental impact from the production of new windows.
Storm windows are important for single-glazed windows, as condensation from interior frosting can be damaging to the wood over time. The Green Lab also advocates for using interior cellular shades as a first, least-cost step to improve efficiency.
Contrary to public perception, it is possible for historic houses to achieve net-zero. It makes sense when you think about the extensive use of glass (with an R-value of 4 or less) in new construction. Many old houses start with wall R-values of 4 or more, and once you’ve added more efficient heating systems - the same systems available for energy production as in new construction - you are on your way to a more efficient home.
A house with new systems and the best air sealing and insulation possible is now primed for further considerations, such as producing your own energy by photovoltaics, wind power or geothermal.
Finally, the one device I wish we would have installed in our historic house: an occupancy sensor for the lights in our teenager’s room. If we had started when she was a pre-teen, we would have saved enough in electricity use to plant a small carbon-offset forest.
This article re-printed from the Fall/Winter 2019 issue of Green & Healthy Maine HOMES. Subscribe today!