Passive house (German: Passivhaus) is a voluntary standard for energy efficiency in a building, which reduces the building's ecological footprint. This article presents the basic concepts of passive house design and how to use cove.tool to rapidly analyze each strategy (insulation, infiltration, heat recovery, high-performance windows, and solar gain).

Before getting to the principles of the passive house standard, let's revisit what makes an energy model. Building Energy Modeling (BEM) is the practice of using computer-based simulation software to perform a detailed analysis of a building’s energy use and energy-using systems. Users input geometry, climate data, constructions, schedules, and energy-using equipment to calculate the heat flow into and out of a building (thermal balance equation). Results are generated for annual performance which includes the space cooling and heating loads, daylighting impacts, equipment energy use, resource consumption, energy costs, and other performance-related parameters.

A project which masterfully balances these thermodynamics equations creates ultra-low energy usage and a year-round quantifiable level of comfort. Passive House is a standard that recognizes such accomplishments in design. In the next section, we will explore the standard and strategies involved in certifying your project as a passive house design.

The PHIUS+ Passive Building Standard is a set of rigorous design principles that provide climate-specific goals for aggressive energy and carbon reduction as well as overlap with cost-effectiveness. Passive House Institute US, Inc. (PHIUS) is a non-profit that maintains the PHIUS+ climate-specific passive building standard, and also certifies and quality assures passive buildings. PHIUS+ standard can be used on any type of building, including single-family, multifamily, and large scale commercial buildings. A passive house is designed and built in accordance with these 5 building science principles:

Buildings that meet the PHIUS+ standard typically use 40%-60% less energy for space conditioning than conventional buildings. These projects provide superior indoor air quality, resilience during power outages, and an extremely quiet, comfortable indoor environment. Because of these rigorous standards, a passive house is also the best path for Net Zero or Net Positive designs.

For each of the design principles guiding passive house design, cove.tool allows users to customize inputs and assess the impact on the overall energy consumption and cost premiums.

The first step to thinking of a passive design is to understand the local climate. PHIUS+ has distinct recommendations for different climate zones in North America (US & Canada), including strict guidelines for window performance. Cove.tool can readily generate a climate report for a location that not only breaks down the radiation received but also explains this in terms of beneficial locations for windows, in different seasons.

For all projects, energy consumption is a cumulative result of the various inputs. By default, these are set to be code minimum in the project's location but users are encouraged to customize them to achieve higher efficiency and meet stricter targets like those set by PHIUS. In each of the principal categories, the inputs that can be modified in cove.tool are listed below:

This is a measure of the resistance to inward and outward air leakage from a building's envelope. Excess air leakage can impact the operational energy and also lead to decay of the building fabric due to dampness. Cove.tool's infiltration input references EN 15242 Annex B Table B.1 for a pressure difference of 4 Pa. This is closer to actual infiltration that occurs at natural pressure differences, denoted as flow rate per envelope area (cfm/sf or m3/h per m2). This ranges from less than 0.1 for a super-tight house to 2 or higher for a sieve of a house. Having sufficient fresh air in a building is critical to control humidity and keep occupants healthy.

PHIUS+ 2018 requires a whole building airtightness test at either a 50 Pa or 75 Pa pressure difference, commonly known as a blower door test (3.2 Airtightness Criterion, page 20). There are maximum limits set for buildings depending on the number of stories (more or less than five) and combustibility of construction (i.e. whether wood-based or paper-based insulation is used). These values are measured in cubic feet per minute (CFM) / square foot of gross envelope area. As such they yield either a CFM50 or CFM75 value, which can, in turn, be converted CFM4 for inputting into cove.tool via this method. If you need to convert to ACH or from ACH, the following formula can be used.

ACH = CFM x 60 / (Area x Height)

This conversion provides the ACH50 or ACH75 value that is simply the air changes per hour of the structure at 50/75 pascals of negative pressure. A higher number means more infiltration (loose) and a lower number means less infiltration (tight).

Based on the building science inputs and geometry, cove.tool will analyze hundreds of possible alternatives and generate optimized solutions for that project. This unique and powerful feature allows users to make cost-efficient design decisions, which aligns perfectly with the passive house design goals as well. Users can further narrow down the results by prioritizing specific aspects such as energy savings, cost premium ($), and payback years.

Designing a project up to passive house standards is a complex, multi-layered process. It stresses the interactions between different aspects of the building like its location, geometry, envelope design, and seasonal space conditioning strategies. By running a holistic analysis, cove.tool makes this process a lot quicker and more intuitive. Parametric studies combined with a keen understanding of passive design principles can push all projects further along in the direction of high performance.