Photo by Tim Griffith, courtesy of ZGF Architects

Airtightness in Buildings: Don’t Let it Slip Through the Cracks!

At a ribbon-cutting ceremony for RMI’s new Innovation Center today, RMI is being presented with Passive House US+ Certification, partly due to the airtightness of the building.

Many of us have experienced how uncomfortable it can be to sit next to a drafty window or door. Not only does this air leakage impact comfort, it can also cause a home or building to use a lot more energy. Achieving cutting-edge energy goals, such as net-zero energy, requires a very high level of building efficiency in design and operation—airtightness is one critical component.

RMI’s new Innovation Center in Basalt, CO, was recently tested for airtightness and achieved one of the lowest air leakage values recorded for any building in the U.S.— 0.36 air changes per hour (ACH) at one pound per square foot pressure (ACH@50 Pa). This means that the total area of cracks, gaps, and holes in the entire building envelope (walls, roof, etc.) has an effective leakage area equivalent to 61 square inches, about the size of a basketball. The building, meanwhile, is a 15,610 ft2 two-story office building, with a total envelope area of over 30,000 ft2. For comparison, a typical older office building has an equivalent hole 24 times larger, or 12 ft2.

Achieving this level of airtightness is a critical step towards achieving the net-zero energy, comfort, and durability goals we set for the Innovation Center.


Airtightness is important for both energy efficiency and comfort. Cold air leaking into the building through gaps and cracks increases the building heat load. Some studies have suggested air leakage could account for approximately 40 percent of heating energy for older offices in cold climates. In homes, infiltration typically accounts for 25 percent or more of heating and cooling energy consumption. Across the entire residential sector this equates to over 600 average-sized coal-fired power plants!

In addition to saving energy, reducing infiltration also helps drive down a building’s peak heating and cooling demand. Since infiltration is driven by temperature difference and wind speed it can be a significant portion of the peak heating and cooling demand. Achieving a high level of airtightness enables smaller heating and cooling equipment to be used, reducing capital cost. For our Innovation Center, aggressive reduction in building loads, including designing to the Passive House airtightness target, allowed us to save 258 square feet of space in our mechanical room, and $9 per square foot worth of heating, cooling and ventilation equipment.

Air leakage, on the more severe end of the spectrum, results in drafts, which can have a significant impact on thermal comfort. In addition to discomfort, this also leads to higher energy consumption as people often adjust the thermostat up in the winter (or down in the summer) to attempt to mitigate.


The part of air leakage less commonly understood is the relevance airtightness has to durability. There is a common misconception that airtightness is detrimental to durability. This stems from the fact that for many older homes, the primary source of ventilation was through uncontrolled air leakage. Of course, ventilation is very important—to avoid excessive humidity, remove pollutants, and ensure an adequate supply of oxygen. However, most existing homes are significantly over-ventilated through infiltration, and there are smarter and safer ways to ventilate a home or building.

Air leakage through the envelope can result in condensation inside of the envelope, which in turn can cause rot. Warm moist air leaking out of the building can condense onto cold framing elements in the wall. While often in very leaky homes the frequent airflow can dry out the condensation before it becomes a problem, and drying happens quickly in thin poorly insulated walls, it causes a significant energy penalty.

The smarter approach, which is now being implemented in some progressive building codes and standards and most high-performance buildings, is to build a very tight envelope, and then provide ventilation in a controlled, reliable way. One great way to do this is through a heat recovery ventilator—a fan-powered ventilation unit with a heat exchanger. These systems are often set up so that fresh air is ducted to each room. The heat exchangers can recover up to 90 percent of the heat from the exhaust air, meaning you can ventilate your home or building safely with minimal energy penalty.


The most aggressive airtightness standard is that of the Passive House standard— 0.6 ACH at 1 pound per square foot pressure (ACH@50 Pa). This target has been met by over 25,000 certified Passive House buildings in Europe, as well as the 152 buildings certified in the U.S. through Passive House Institute U.S., and is a target we used for the Innovation Center.

Setting this clear and firm requirement up front allowed the team to design to the target, and also incorporate a plan for inspecting and testing during the construction process, so corrective actions could be taken while still relatively easy to address. Our team carried out a preliminary airtightness test (blower door test) after the building was fully enclosed, but before installing drywall and interior finishes. In our case no major corrective action was needed, but in many cases additional holes must be found and sealed in order to meet the airtightness target.

Our final result, after all finishes were installed, is that our building exceeded this requirement by a factor of two! This was done through good design and attention to detail during construction, and didn’t require new technologies or significant additional investments. The team used an exterior liquid applied air barrier—an approach that has become more common for commercial buildings. Careful attention was paid to taping and sealing all connection points and transitions and all sub consultants were aware and educated about the importance of high-quality craftsmanship. Our building features quite a few operable windows, to allow for natural ventilation and passive cooling. We installed air locks at both primary entrances, specified windows with low air-leakage ratings, and ensured windows have latching mechanisms for a tight seal.

Airtightness isn’t just for new construction, air-sealing measures such as caulking and weather stripping can be very effective, increasing thermal comfort and yielding energy cost savings paybacks within a year.


Although building owners and design teams can achieve high levels of airtightness through project requirements and contracts, as we have done with our Innovation Center, mandatory airtightness targets are needed to address the majority of new buildings which are not owner-built projects. A number of European countries have building codes that require minimum levels of airtightness, including the UK and France. The International Energy Conservation Code (2012), which forms the basis of many state codes, has introduced mandatory measured airtightness levels for residential construction (3 ACH@50 Pa), and ASHRAE codes for commercial buildings have specific design requirements for air barriers.

These are steps in the right direction on both fronts, but in order to fully capture the potential energy savings, we need more-stringent targets and actual airtightness testing for both residential and commercial buildings. The U.S. Army Corps of Engineers has an air tightness requirement of 0.25 CFM/ft2 of envelope area @ 75 Pa for all its new buildings (roughly equal to 1.3 ACH@50 Pa for a typical office building), and requires testing to show demonstration. For comparison, the Passive House U.S. (PHIUS+) standard requires 0.08 CFM/ft2 @ 75 Pa, and our innovation center achieved 0.05 CFM/ft2—so there is considerable opportunity for the Army’s standard to increase in stringency. However, this type of portfolio-wide building standard shows exciting progress in this area and provides an example for others to follow.

Mandating airtightness in buildings, in the range of the passive house requirements, along with effective ventilation systems such as heat recovery ventilation, will help protect building owners from potential envelope issues and higher energy bills.

Working to accelerate adoption of such airtightness targets into codes is one important lever for broader change in the building industry. For building owners and developers working on new construction projects, setting such targets in your owners project requirements is a great way to ensure this happens for your building.

Our Basalt team has been in our new Innovation Center since mid-December, and has been enjoying the incredible comfort of this office, as well as the gratification of knowing how little energy the building is using—both features that owe credit to the superior level of airtightness of this building.