Party Walls

As the latest versions (2012 and 2015) of the International Energy Conservation Codes (IECC) push for more efficient homes, we are getting more questions from architects on how to achieve the air tightness requirements of 3 ACH50. There is no one correct answer, but it can be often achieved through taping of exterior structural or insulated sheathing, air sealing of wall cavities prior to insulating, and/or the use of insulation that is restrictive of air movement. The most common approach that we are asked about is the use of open cell spray polyurethane foam (ocSPF), as it is air impermeable (required thickness is dependent on the specific product, so check requirements in the ICC Evaluation Services Report), reasonably priced, and theoretically, doesn’t require any changes to standard builder practices. While it is true that ocSPF will provide air sealing cost-effectively, we typically do not recommend it in our cold climate region without additional measures due to risk potential over time. To effectively build a home with ocSPF, thoughtful detailing and a high level of execution is required to ensure that it remains effective 5, 10, 15…25 years from now.

While this wall assembly was not insulated with ocSPF, poor window flashing details are a common issue that we see and is one of the reasons we are cautious with this insulation approach.

• ocSPF is vapor permeable, so there is a greater potential for condensation in the building enclosure than if closed cell spray polyurethane foam (ccSPF) is used. A hybrid approach of ccSPF and an alterative insulation (ocSPF, cellulose, fiberglass, etc.) is often used to keep costs down.
• ocSPF can absorb 40% of water by volume. Therefore, if bulk water from leaks does make it into the building enclosure, the ocSPF will retain the water until saturated. Pinpointing the source of the leak may be difficult as the water can migrate within the foam.

Our main concern is that the performance of the product requires several trades to meet a high level of quality to ensure success and hope that the homeowners don’t unwittingly cause problems down the road through lack of maintenance. Here’s what we suggest… 

• The builder must ensure that the drainage plane is continuous on the exterior of the building enclosure to keep bulk water out of the home. Not just at the time of construction, but over time. This requires proper cladding, weather resistant barriers, and flashings.
• The insulator must make sure that ocSPF is applied so as to create a continuous air barrier within the building enclosure. With any spray polyurethane foam, a quality installation (right mix, temperature, and surface prep) is necessary to minimize potential for shrinkage or other defects during installation.
• The builder must ensure that an appropriate vapor retarder is located within the building enclosure (typically on the interior in our cold climate, but a smart vapor retarder might make sense to address summer time conditions.
• The HVAC contractor must ensure that the mechanical systems control interior humidity conditions to minimize the potential for condensation within the building enclosure. This requires right-sized air conditioners to provide latent cooling in the summer. A whole-house ventilation system that is either balanced or results in a slightly negative indoor pressure will likely be necessary. In addition, stand alone dehumidification for Fall through Spring may be needed. Humidifiers should not be used.
• The homeowners must maintain all of these assemblies and systems over time so that they continue to operate as designed. The first homeowner might do this, but how often is the subsequent homeowners ever aware of the operational and maintenance requirements of a home?

As the IECC continues to push energy efficiency, the risk of designing and building homes continues to grow. Sticking to standard building practices and just adjusting material selections will not be sufficient. Architects and builders will need to ensure that there is continuity and longevity of all required control layers: water (bulk and moisture), air, and thermal.