"The Passive House approach can meet the demand for both affordability and energy-use reduction"
From an article in the Green Fire Times...
The first generation of certified Passive Houses has been completed in Santa Fe. Optimizing the economic advantages of the Passive House approach, these projects establish the cost competitiveness of Passive House construction when compared with typical regional construction projects.
The American Southwest has a long history of environmentally based architecture. However, the typical approaches to sustainability, such as earthen or strawbale construction, are extremely labor intensive and therefore can be quite expensive. The associated passive-solar designs follow an energy model of high thermal losses balanced by high solar gain—a model requiring sustained solar exposure that is not often available.
The following case studies demonstrate that the Passive House approach can meet the demand for both affordability and energy-use reduction.
Passive House Project One: Balance House
The first Passive House—Balance House—was completed in April 2011. With a construction area of 3,313 sq. ft. measured from the exterior thermal envelope, Balance House includes two units: a 2,590 sq. ft. residence and a 723 sq. ft. office building. The final construction cost was $451,406 for 3,313 sq. ft. This results in an amortized construction cost of $135 per sq. ft.
Passive House Project Two: VolksHouse
The second Passive House project—VolksHouse—was designed to introduce a Passive House product to the Santa Fe housing market and, as such, was designed to meet standard market expectations for size, program and cost.
Additionally, VolksHouse is intended to serve as a model for Passive House construction techniques that are allied with standard U.S. design and construction sequencing and practices. A successful Passive House model in the U.S. must adapt standard construction practices, since, unlike our European counterparts, project architects in the U.S. often have little or no role in the construction process of smaller scale affordable or production-home projects. This break in continuity can lead to the failure of a project’s final performance, especially in terms of airtightness and thermal-bridge elimination. These elements, central to the Passive House approach, are currently undervalued in the U.S. design and construction industries.
VolksHouse was completed in February 2012. Its basic design follows a typical, detached single-family residence pattern with three bedrooms, two baths and a two-car garage. The residence is 1,700 sq. ft. measured from the thermal-envelope exterior, with an additional 560 sq. ft. of unconditioned space for storage and vehicle parking.
The final VolksHouse construction cost was $259,799 for 1,700 sq. ft., resulting in an amortized construction cost of $153 per sq. ft.
Process, Lessons & Modifcations
VolksHouse was designed concurrently with the construction of Balance House, and we modified the thermal envelope and system designs in response to lessons learned through the Balance House construction process.
The primary modification was to the VolksHouse thermal envelope. By simplifying critical details and systems—foundation forming, air-tightness detailing and materials, thermal envelope, and the mechanical system—we increased the project’s air-tightness, decreased thermal bridging and improved the performance consistency of the completed building.
VolksHouse foundation detail showing the simplified version that reduced overall project costs.
Balance House foundation detail showing the different pieces that required difficult on-site installation.
The perimeter-insulation design for Balance House incorporated several small sections of rigid EPS insulation based on Passive House thermal-performance requirements. This sectioned-EPS approach minimized material usage; but it proved difficult to implement with consistency and was labor-intensive. For the VolksHouse, we simplified the foundation system and thereby eliminated much of the forming and rigid-insulation installation labor.
Airtightness, Detailing & Material
The airtightness layer on the Balance House used plywood that was fully taped from the exterior. The Larsen Truss detailing around the windows did not, however, allow window installation to align with the airtightness layer, a condition that required subsequent labor-intensive air-tightness detailing and resulted in higher air-infiltration levels. Additionally, the material inconsistencies of the plywood led to air leaks that required patching. The leaks were confirmed during positive pressurization testing using smoke to locate air leaks.
Although Balance House passed the Passive House requirements for airtightness (0.47 ACH @ 50 Pa.), we felt that the process was not replicable with quantifiable results. The airtightness detailing for VolksHouse was changed to address these issues. VolksHouse windows and doors were aligned directly with the air barrier without any exceptions, and the project employed a zip-panel as an alternative airtightness material. The zip-panel included a simple overlay component and provided a very consistent surface for air-sealing-tape application and easy inspection. Labor for air sealing was consequently reduced, and the blower-door test resulted in a 0.25 ACH @ 50 Pa.
Thermal Envelope Design
Balance House utilized all-cellulose insulation for the walls and roof—a material we selected for its relatively environmentally friendly characteristics. However, during installation, it became apparent that the regional quality control and installation standards for cellulose did not meet the standards required for Passive House durability and gap elimination. A significant number of inspections, reviews and contract revisions were ultimately required to achieve our desired results. As it was our intention that these projects create a repeatable model for Passive House construction, we revised the thermal envelope for VolksHouse to a continuous layer of rigid EPS insulation.
The solar-thermal system for Balance House was successful, but the project’s greater scale informed the cost benefits of the system design since its cost is amortized across a large conditioned floor area. VolksHouse is roughly half the size of Balance House, and installing a similar, yet smaller, system would have been financially inefficient. Also, since we wanted VolksHouse to meet market norms, we reanalyzed the mechanical system in terms of the level of interaction that environmental controls would require of the owner.
Balance House primarily uses manual shading to control over-heating, but theren were concerns that manual controls would hinder market penetration and create an inappropriate association between Passive House construction and owner inconvenience. We therefore decided that a cooling system would bolster the overall success of a repeatable project. VolksHouse uses a ducted mini-split heat pump and relies on the ventilation ducting to distribute heating and cooling loads.
Code Built House Cost Comparison
In order to compare the construction cost of VolksHouse with that of a similar home built to regional building codes (Code Built House), we cost-modeled VolksHouse using RS-Means estimating software. This software uses industrystandard and regionally specific construction costs based on assembly type. The assembly types we used reflect Santa Fe’s current local building code, which requires compliance with IECC (International Energy Conservation Code) and U.S. Energy Star Certification with a HERS (Home Energy Rating System) score of 70 or below.
For accuracy in comparing the project costs, we matched certain line items, such as builder profit and overhead. Our modeling indicated that the construction cost for a typical residence of identical size and configuration to VolksHouse would be $154 per sq. ft.—an estimate corroborated by the Santa Fe Area Home Builders Association (SAHBA). This number was then used as a baseline for comparison between code-built construction and the Passive House projects.
Comparisons indicate that both Passive House projects were built for less than the model Code Built House. The modifications to the thermal envelope assembly of VolksHouse provided significant advantages over Balance House in thermal-bridge elimination and airtightness, which result in reduced energy use. However, the costs were slightly higher. In repeated projects, the environmental impacts of EPS use should be considered, as well.
Simplifying details increased overall performance consistency in all cases. This will be a critical benefit as Passive House construction is introduced to the U.S., given the industry separation between professional architectural responsibilities and construction management. The U.S. Passive House industry will need to address this gap in responsibilities in order to successfully move Passive House construction into the mainstream.
The completion of Balance House and VolksHouse demonstrate that Passive House projects can meet typical U.S. single-family home construction costs. This is especially notable in the Santa Fe area, where construction costs are high relative to other areas in the country. Simple and replicable systems that align with standard US.. construction sequencing and practices are key to keeping costs in check—and to the eventual mainstream market acceptance of the Passive House approach.
The environmental advantages, lifestyle benefits and cost competitiveness of these projects raise the question of why the Passive House approach is not employed more often in the U.S. We suggest that the answer is primarily a matter of unfamiliarity, and that the solution will require effective contractor and consumer education. Importantly, Passive House education in the U.S. must address mainstream expectations of building performance, which are chronically low.
Jonah Stanford is a Certified Passive House Consultant and a principal partner at MoSA – Mojarrab Stanford Architects.
505.577.4295, firstname.lastname@example.org, www.mo-s-a.com