Code & Application Ambiguity

Interestingly, there is no air barrier requirement for commercial buildings in most states and codes. Although European countries have air barrier requirements in their building codes, Massachusetts was the first state to require an air barrier by code, but not until after 2000. And only a half dozen states have followed suit since then. LEED does not address air barriers either. Yet air barriers can save up to 40 percent of the energy a building uses.

House wrap is commonly used for residential construction and is effective if installed properly. Continuity of the airtight barrier is key — nailing an air-impermeable barrier to the structure is problematic, since any punctures/holes makes the barrier less than airtight. Consequently, fluid or spray applied air-impermeable barriers are best or a “stick-on” application with overlapping seams can work well too.

The location of the air barrier varies by climate, but is typically applied on the interior of a house. Conversely, air barriers are usually applied between the structure and the cladding on the outside of a commercial property.

Before giving any air barrier assembly its seal of approval, the Air Barrier Association of America (ABAA) is requiring manufacturers to prove their technologies can pass the ASTM E 2357 Air Leakage of Air Barrier Assemblies test. The purpose of this assessment is to determine whether air-barrier products, when used with other typical wall components, collectively function as an air-barrier assembly.

The test is performed on an 8-by-8-foot wall mock-up that includes typical wall penetrations — a window, galvanized duct, PVC pipe, post-applied brick tie-ins, electrical junction boxes, roof and concrete-foundation interfaces. The air barrier assembly is applied to the wall, complete with flashing and sealing materials applied around all penetrations and at air barrier joints in specified locations.

The wall specimen is then mounted in a sealed test chamber with an air supply that allows application and measurement of both positive and negative air-pressure differentials across the wall structure. The assembly then undergoes a “conditioning” process where the assembly is subjected to both positive and negative loads to confirm that the various materials will actually work together to provide an airtight seal. According to the E 2357, the air leakage of an approved assembly must not to exceed 0.2L/(s•m²) @ 75Pa. (0.04 cfm/ft² @ 1.57 psf). Based on the results, the air barrier assembly is assigned an air leakage rating.

The benefits to air/weather barriers are many, including:

1. Reducing the footprint of a building through reduced energy use.
2. Using recycled or bio-based materials in its manufacture.
3. Low or no VOCs.
4. Engineered to last the lifetime of the structure, minimizing resources used.

6. Concrete

The last of the six building envelope elements to discuss is concrete. On the surface, concrete seems pretty straight forward, but on closer examination, chemistry has probably done more to manipulate and improve the sustainable aspects of concrete than any other element discussed.

Admixtures are additives that are engineered to enhance specific performance attributes and they fall into these categories:

• Water Reducers — usually reduce the required water content for a concrete mixture by about 5 to 10 percent.
• Plasticizers — also known as superplasticizers or high-range water reducers (HRWR) reduce water content by 12 to 30 percent and increase flow rates.
• Accelerators — increase the rate of early strength development; reduce the time required for proper curing and protection, and speed up the start of finishing operations.
• Retarders — slow the setting rate of concrete and are used to counteract the accelerating effect of hot weather on concrete setting.
• Corrosion-inhibiting — used to slow corrosion of reinforcing steel in concrete.

Specific admixture formulations vary depending on the characteristics required. The performance priorities for concrete used in the construction of a major bridge are different from those for concrete used to lay the foundation of a house. Climactic conditions, setting times, the type of equipment used and the availability of local Portland cement or local sand all have an impact on the admixtures needed to produce the performance characteristics desired. Different admixtures can be used for ready-mix, pre-cast, masonry, paving and underground applications.

One development in concrete admixtures that achieves new levels of performance, economics and sustainability is an environmentally friendly, cost-effective concrete with optimized proportions in which supplementary cementitious materials, non-cementitious fillers, or both, are used with special high-range water-reducing admixtures and/or workability-retaining admixtures to meet or exceed performance targets.

Relative to a baseline reference mix, it attains desired setting characteristics, strength, durability, and if needed, a higher slump at a reduced cost to the producer. A simple equation sums up the recipe and its results:

Another innovation gaining ground because of its sustainability benefits are pervious pavement technologies — including pervious concrete and pervious asphalt — which play a significant role by providing uniform distribution of runoff into vegetated areas to keep the water from directly entering the storm-drain network, reducing runoff volume and promoting distributed infiltration. Pervious concrete helps to recharge groundwater, maintain aquifer levels, provide nourishment for trees and plants, reduce untreated runoff to storm sewers and eliminate hydrocarbon pollution from asphalt pavements and sealers.

Pervious concrete is a mix of Portland cement, coarse aggregate, water and admixtures, as well as recycled materials. Some also use a urethane-based adhesive/coating. Because there is little or no sand in the mix, the pore structure contains many voids that allow water and air to pass through. Chemical admixtures are used to enhance these formulations to improve ease of installation, including increased working time with improved concrete flow. The admixtures also increase compressive strength.

Summary

The role of chemistry in advancing high-performance building envelopes is encouraging. It is good to know that other professionals — chemists and building product manufacturers, in particular — are deeply committed to finding new and more sustainable ways to refine and reinvent every aspect of architecture and construction, just as design professionals are. The fundamental building blocks of the built environment discussed here are, at once, plain and sexy — the same yet entirely different, basis stuff and also totally exotic.

As a fitting example of what these building envelope elements can contribute to more sustainable homes, in this case, can be observed at a demonstration project in Canada, completed in 2007 (http://www.cmhc.ca/en/corp/nero/nere/2007/2007-11-09-1400.cfm). Alouette Homes lead one of 12 teams that designed and built a home in Eastman, Quebec, Canada — as participants in the Canadian Mortgage and Housing Corporation (CHMC) EQuilibrium housing initiative. Their demonstration house consumes only 10 percent of the energy used in a standard house with the same surface area. That’s a 90 percent savings! Meaningful change is very possible.

The CMHC’s EQuilibrium initiative demonstrates a new approach to housing in Canada, and represents a fundamental change in the way Canadians will think about their homes in the future. It strives to balance their housing needs with those of the environment. It brings together — under one roof — the principles of occupant health and comfort, energy efficiency, renewable energy production, resource and water conservation, and reduced environmental impact and pollutant emissions. EQuilibrium also refers to energy use and generation — striving for a net-zero house, a sustainable home.

By Roger C. Brady, AIA, LEED AP, with contributions from Mary MacLeod Jones and Stephanie Inglis, on behalf of BASF Construction North America.