New Building Envelope Retrofit Strategies

Building America’s research yields retrofit solutions for existing homes.

New homes have never been more energy efficient, thanks to stricter building codes and greater awareness of the link between energy efficiency and occupant comfort. But what about America’s aging housing stock? 

According to the most recent American Community Survey , more than half of all owner-occupied homes in this country were built before 1980. The percentage is even higher for multifamily buildings. Many of these older homes and units have 2-by-4 walls, leaky windows, minimal insulation, and little to no air sealing.

Older, energy-inefficient homes can be restored to peak performance with a thorough retrofitting, which includes new, rigid insulation, aerosol sealant and upgraded windows. Credit: Andrea Astes/iStock

There is tremendous opportunity in retrofitting existing homes, apartments, and townhouses. Residential buildings account for 35 percent of total U.S. energy consumption; nearly half of that energy is used to heat and cool homes. 

Upgrading existing building envelopes by adding insulation, air sealing, and/or replacing windows can significantly reduce heat loss and gain through a building. But envelope retrofits present distinct challenges. 

For owners, they can be disruptive and costly. Multifamily buildings with rental units present even more of a challenge, as many retrofits must be scheduled when tenants move out and the spaces are unoccupied. Utility programs often offer incentives, but too few people take advantage of them, and the number of homes that receive upgrades represents just a fraction of the total building stock.

For this reason, the U.S. Department of Energy (DOE), through its Building America program, has been supporting research to develop and validate envelope retrofit strategies that are affordable, easy to implement, and broadly applicable.

Energy Savings From Re-Siding

Every year, around 1 million U.S. homes are re-sided. “That’s a lot of houses, and yet the vast majority are not having their performance improved at all,” says Christine Liaukus, architect and principal investigator for a series of Building America projects focused on “opportunistic retdrofits.” 

Such retrofits take advantage of an upgrade such as re-siding to add insulation or boost weather resistance. A team headed by Liaukus and the New Jersey Institute of Technology (NJIT) first tested this concept with a project called “Re-side Tight,” in which contractors were trained to add a weather-resistive barrier to 15 homes that were being re-sided. 

In 2020 and 2021, the team took the concept a step further with a Building America project called “Re-side Right,” installing one inch of exterior rigid insulation and liquid flashing on 10 homes that were being re-sided. This added R-5 and also served as a weather-resistive barrier and air barrier.

Though the drop in air infiltration wasn’t huge, according to Liaukus. “Anecdotally, people felt the comfort difference and thought the house was quieter,” she says.

To validate its work, the team conducted blower door tests and used infrared cameras to analyze thermal bridging. As the cameras showed, the opaque portions of the envelope were better insulated buttoned up, but the windows were still losing significant heat. 

Window replacement is expensive, and it rarely pencils out for the homeowner when looking at it strictly from the perspective of energy savings. So with its latest Building America project, the NJIT team is developing an alternative to window replacement that still accomplishes significant energy savings. 

Called “Renew-Wall,” the strategy involves adding two inches of exterior rigid insulation with an integrated air barrier to reduce air infiltration, and installing exterior secondary windows with a thermal barrier surround.

The project began with months of iterative design. Last fall, the team built two wall mock-ups on the NJIT campus. They include novel products from two companies. Alpen Windows agreed to modify an interior secondary window product so that it can be installed on the outside, and BRINC Building Products , whose high-density foam-and-resin ThermalBuck eliminates thermal bridging around the edges of windows, agreed to modify its product so it can be used on existing homes.

“Those companies are innovative and nimble enough to take on a new idea and try it out,” says Liaukus. “They’ve been excellent partners.”

Constructing the mock-ups allowed the team to work out details such as how long screws need to be and the sequence of steps. The goal is a replicable process that isn’t too fussy and can be easily adapted by siding contractors and their crews in the field. Now that the mock-up has been validated, the team will test out the system on at least five actual homes in 2024.

The Renew-Wall assembly could achieve heating and cooling energy savings of 30 percent or more, and it could be an important step toward electrification.

Retrofitting Roof Cavities

In 2017, another Building America team led by Building Envelope Materials set out to answer a challenging question: How do you retrofit enclosed roof cavities without tearing down the ceiling? 

Doug Lamm, CEO of Building Envelope Materials , has been working on this problem since the Great Recession of 2008, when he obtained a grant to develop a minimally invasive process for adding additional insulation to wall cavities that were already filled with fiberglass batts. The technique, called Pinole Insulation, involves injecting polyurethane spray foam insulation through tiny, quarter-inch holes through the drywall. 

“We had developed Pinole Insulation for walls and had done big buildings already,” says Lamm. “I said, ‘How about if we develop a similar process for enclosed roof cavities?’” 

The majority of existing homes have some type of enclosed roof cavity. This may include cathedral ceilings, dormers, or flat roofs. In the Northeast, where historic “triple decker” multifamily homes with flat roofs are common, the opportunity is especially ripe.

After developing systems in the lab, Building Envelope Materials tested them on pilot projects, including one where it treated half of a cathedral ceiling with existing R-19 fiberglass batts. After injecting closed-cell polyurethane foam insulation into the cavities through tiny holes in the ceiling, builders used an infrared camera to validate the quality of the installation. 

Snow piled up on the side of the roof that had been retrofitted with injection foam but not on the side insulated only with R-19 batts. “The R-19 batts allowed so much warm air to escape from the interior that it completely melted the snow,” says Lamm.

Pinole Insulation can be used in walls and ceilings. The most critical part is calibrating the “fill rate” so that cavities are filled completely—but not overfilled. “A lot of people have tried to do what we do and failed,” says Lamm. “Blowing out walls is the number one reason.” 

To avoid this, Building Envelope Materials uses a proprietary smartphone app to calculate an exact shot time for every cavity. Because the foam heats up slightly as it is injected, installers can use an infrared camera to verify that cavities are filled completely. “We monitor every cavity we insulate and guarantee a complete fill and good quality foam that won’t shrink or off-gas,” says Lamm.

Lamm’s company has treated hundreds of buildings, mostly in Massachusetts and other Northeastern states. On average, the process increases insulation values in older, fiberglass-filled walls from R-10 to R-24, with a 22 percent to 26 percent reduction in thermal energy use. The material comes out “like shaving cream” and doesn’t emit formaldehyde or produce hazardous particulates.

“We’ve now injected 35,000 to 40,000 cavities,” says Lamm, who adds that they are exploring using the technique as an alternative to invasive “gut rehabs.” “We would not have been so successful had it not been for Building America’s help.” 

Less-Invasive Air Sealing

As important as insulation is to a home’s comfort and energy performance, it’s only half the solution. The U.S. Environmental Protection Agency (EPA) estimates that homeowners can save an average of 15 percent on heating and cooling costs (or about 11 percent on total energy costs) by adding insulation plus air sealing—filling the cracks and holes in the building enclosure where air can escape. 

Unfortunately, manually sealing these cracks is time consuming and often doesn’t yield the promised energy savings.

Building America has long supported the development of aerosol-based technology that can be used to effectively air seal homes. The research has led to successful commercialization through a company called Aeroseal . 

This solution involves pressurizing an interior space and releasing an aerosol fog inside. As air escapes the building, sealant particles are carried to tiny cracks and holes in the envelope. The sealant particles make contact and stick to the edges of these leaks, eventually sealing them. 

The technique was initially used in new homes, but Building America has been supporting efforts to expand its application to existing homes and to adapt the process so it’s less invasive.

From March 2020 to January 2022, the Center for Energy and Environment (CEE) partnered with Aeroseal and the University of California, Davis’ Western Cooling Efficiency Center to test the technology on 34 existing residences in Minnesota and California. Furnishings were removed, and the residences were air sealed while they were vacant. 

Aerosol sealing reduced leakage by an average of 47 percent, but the project also revealed a challenge particular to existing residences: how to protect horizontal finishes from aerosol particles while ensuring the aerosol fog reaches leaks. This was especially tricky where baseboards meet carpet or wood flooring—a common path for air leaks. 

“It’s not just protecting the surface and allowing airflow,” says Dave Bohac, senior director of research at CEE. “When you pull off the floor protection, you don’t want to pull off all the aerosol sealant with it.”

This got the team thinking: Was there a better way to aerosol seal existing residences like these? Curtis Harrington, senior engineer at UC Davis, tested an alternative approach at three townhouses. 

Instead of pressurizing the house and releasing the aerosol fog inside the occupied space, they depressurized the home and released the aerosol in the attic, sealing it from the outside in. 

“For these three townhouses, we saw total leakage reductions of 54 to 55 percent overall, just by doing the attic,” says Bohac. “That led to, ‘Wow, that’s a viable method; let’s pursue that.’”  

In 2023, CEE won funding from the DOE’s BENEFIT program to develop an advanced aerosol sealing technology that can be used in attics, crawlspaces, unconditioned basements, and attached garages without having to disturb occupants or cover surfaces in living spaces. 

Aeroseal modified the sealing equipment for the project, and the team is currently testing its methods in 40 townhouses and apartments in Minnesota, California, and two other climate zones. 

The same team is working on a new testing method that will allow it to more accurately measure house-to-attic leakage so they can tell what proportion of leaks the aerosol sealing has addressed. 

“If we’re doing an attic, for example, we want to be able to measure both the overall leakage reduction and the reduction specific to that space,” explains Bohac. “With a lot of these residences, especially if we can do pre-sealing of larger leaks, we think we can see as much as a 90 percent reduction.”

More Solutions to Come

In 2023, Building America named nine “Retrofit Solutions Teams.” These multidisciplinary expert teams, selected from around the country, will address hard-to-solve technical challenges of decarbonizing the U.S. housing stock. Teams will work closely with community-based organizations to identify technical integration issues and appropriate solutions for improving their existing housing stock. 

The mock-ups consist of two inches of rigid insulation, liquid flashing, a thermally broken window surround, high-performance exterior storm windows, and new siding. Courtesy U.S. Department of Energy

Infrared images reveal the difference between the side of the ceiling insulated with fiberglass batts and the side fortified with polyurethane foam. Courtesy U.S. Department of Energy

Releasing the aerosol spray in the attic offers a minimally invasive and effective approach to air sealing a home’s ceiling. Credit: Center for Energy and Environment

No leaks allowed. Aerosol sealant can be used to air seal common leaks in attics, including around ducts, fan housings, ceiling light boxes and wire penetrations. CREDIT: Curtis Harrington/UC Davis