Passive House Design Principles

Understanding Passive House requires understanding the physical mechanisms by which buildings gain and lose heat. There are three:

Conduction: Heat moves through solid materials from warm to cold. Metal conducts heat readily; insulation resists it. The resistance to conduction is measured in R-value (US) or RSI (metric). Doubling the insulation halves the conductive heat flow. The relationship is linear — more insulation always means less heat loss.

Convection: Heat moves in air currents. In a building, warm air near the ceiling is at higher pressure than cold exterior air and finds any gap to exit, drawing cold air in behind it. This infiltration/exfiltration is responsible for a substantial fraction of heat loss in conventional buildings. Stopping convective loss requires stopping air movement through the envelope.

Radiation: All objects emit infrared radiation proportional to their temperature. A cold window surface in a cold climate draws radiant heat from your body even if you are not touching it — this is why you feel cold near a single-pane window even if the air temperature in the room is comfortable. High-performance windows with warm interior surfaces reduce this radiant loss and improve thermal comfort.

Solar gain: Heat entering through windows from direct and diffuse sunlight. South-facing windows (in the northern hemisphere) provide solar gain that partially offsets heating demand in winter. The same windows must be shaded in summer to avoid overheating. Passive design manages solar gain as an asset, not a problem — but it requires careful calculation.

Internal heat gains: Occupants, appliances, and lighting generate heat. A person at rest emits approximately 80 watts of heat. A family of four generates roughly 250-350 watts continuously — comparable to a modest space heater. Appliances add more. In a well-insulated building, these internal gains contribute meaningfully to maintaining temperature.

The Passive House approach attacks all three loss mechanisms simultaneously and captures gain sources. The result is a thermal balance that requires almost no external energy to maintain.

Wolfgang Feist at the Darmstadt Institute of Housing and Environment, working with Bo Adamson of Lund University in Sweden, developed the Passive House concept in 1988. The first certified Passive House was built in Darmstadt-Kranichstein in 1991 — four townhouses that have been continuously monitored since construction.

The Kranichstein houses were not an experiment. They were a demonstration that the concept worked in practice, not just theory. They have met the energy targets for over 30 years with minimal maintenance intervention. The heating system in the original buildings is a single 1.5kW electric element — smaller than a hair dryer — that serves as the backup for the entire heating season.

Feist founded the Passive House Institute (PHI) in 1996, which developed PHPP and the formal certification process. The institute has trained certifiers globally and established regional offices. In North America, the Passive House Institute US (PHIUS) developed a North American variant (Passive House Plus / PHIUS+) calibrated to the diverse climate zones of the continent.

The European adoption has been substantial. Germany, Austria, and Switzerland have tens of thousands of certified buildings. Ireland made Passive House the mandatory standard for all new construction. Several Scandinavian cities have mandated Passive House for public housing. The market maturation in Europe has driven construction costs down significantly — German Passive House builders now price their work within 3-8% of conventional construction.

The insulation levels required by Passive House feel extreme by North American standards. Typical targets:

Climate zone comparisons:

Mild climate (e.g., San Francisco, Seattle): - Walls: R-30 to R-40 - Roof: R-50 to R-60 - Foundation: R-20 to R-30

Cold climate (e.g., Minneapolis, Toronto): - Walls: R-40 to R-60 - Roof: R-60 to R-80 - Foundation: R-30 to R-40

Extreme cold (e.g., Fairbanks, Scandinavia): - Walls: R-60 to R-80+ - Roof: R-80 to R-100+ - Foundation: R-40+

These values are achieved through wall system design, not simply adding more batts. Common high-R wall assemblies:

Double stud wall: Two rows of 2x4 or 2x6 studs separated by a space, filled entirely with mineral wool or dense-pack cellulose. The gap between the stud rows eliminates thermal bridges at the structural studs. A 12-inch double stud wall with dense-pack cellulose achieves approximately R-42. This is a simple, contractor-familiar assembly using inexpensive materials.

Advanced framing with exterior insulation: Standard 2x6 wall with advanced framing (reducing stud count to minimize thermal bridging), filled with mineral wool batts for R-21, plus 4-6 inches of continuous exterior rigid mineral wool or EPS foam board for an additional R-20 to R-24. Total: R-41 to R-45. The continuous exterior insulation keeps the structural layer warm, reducing condensation risk.

Structural Insulated Panels (SIPs): Factory-made panels with OSB skins and EPS foam core. Fast to erect, inherently airtight at panel joints (with proper sealing), but limited to standard sizes and challenging to modify in the field.

Insulated Concrete Forms (ICFs): Hollow EPS foam blocks filled with concrete. R-22 to R-26 in the foam layers, with additional thermal mass from the concrete. Inherently airtight, excellent thermal mass, extremely durable. Popular in regions with wide temperature swings where thermal mass provides comfort benefit.

A thermal bridge exists wherever a thermally conductive material passes through the insulation layer. The insulation level of the wall between the bridges is irrelevant if bridges are conducting heat freely.

Common thermal bridges in conventional construction:

Structural studs: A standard 2x6 stud wall has a stud every 16 inches. Wood has R-value of approximately 1.25 per inch — a 5.5-inch stud has R-7. Mineral wool batt between the studs has R-23. The effective center-of-cavity R-value is R-23; the clear-wall R-value accounting for studs at 16-inch spacing is approximately R-15. One-third of the insulation value is lost through thermal bridging at studs alone.

Concrete slabs and foundations: In slab-on-grade construction, the concrete slab at the edge directly contacts the exterior, and every inch of that contact conducts heat. This is managed by running insulation under and around the entire slab perimeter, keeping no concrete exposed to exterior conditions.

Window and door frames: Window frames that contact the exterior air pocket and interior framing simultaneously are thermal bridges. Passive House window installation typically wraps the frame with insulation or uses purpose-designed clips to hold the window away from the framing, with insulation in the gap.

Cantilevered balconies: A concrete balcony extending from a concrete floor slab is one of the most severe thermal bridges in modern construction. The entire slab edge conducts heat outward. Solutions include structural thermal break elements (Schöck Isokorb is the standard product) that interrupt the conductive path at the balcony connection point.

Psi values: The heat transfer rate through thermal bridges is quantified using psi (Ψ) values — heat loss per unit length per degree of temperature difference. PHPP calculates thermal bridge contributions from the geometry of the building (perimeter of foundation, area of window reveals, etc.) and adds them to the transmission heat loss from the envelope proper.

Standard North American windows are double-pane, typically with an argon fill and a low-emissivity (low-e) coating on one surface. These achieve approximately U-0.30 (R-3.3) in the center of glass.

Passive House windows require: - Triple pane glazing: Three layers of glass with two gas-filled cavities - Gas fill: Argon (80% efficiency) or krypton (better performance, higher cost) - Multiple low-e coatings: Applied to interior glass surfaces to reflect infrared - Warm-edge spacers: Non-metal spacers at the glass edges that reduce the edge thermal bridge - Insulated frames: Frames with thermal breaks and minimal conductance

Results: U-0.12 to U-0.18 center of glass (R-5.5 to R-8.3 at center); U-0.14 to U-0.20 whole-window including frame.

Cost: Triple-pane Passive House windows cost 40-80% more than standard double-pane windows. This is a real cost premium on a large window area. The payback comes through reduced heating system cost (smaller system or no system at all), reduced heating energy cost, and significantly improved thermal comfort.

Solar heat gain coefficient (SHGC): How much solar radiation passes through the window. High SHGC (0.5+) is desirable for south-facing windows in cold climates to maximize passive solar gain. Low SHGC (0.3 or below) is desirable for east and west-facing windows to reduce summer overheating.

Orientation strategy: In Passive House design, windows are concentrated on the south facade (northern hemisphere) to maximize winter solar gain and minimize east/west windows that gain heat in summer when solar angles are high. North-facing windows are minimized to reduce heat loss (north windows never receive direct sun and only lose heat). This is not absolute — daylighting, views, and cross-ventilation matter — but orientation is a design parameter, not an afterthought.

Most people understand that insulation reduces heat loss. Fewer understand that air leakage often causes more heat loss than poor insulation.

In a conventional house, the volume of air that leaks out (and is replaced by cold outside air) in a day can equal 10-15 times the interior volume of the house. Each air change brings in cold air that must be heated. Stopping infiltration is the most cost-effective energy retrofit in most older buildings — it requires no materials beyond tape and caulk applied correctly.

Blower door testing: A blower door is a calibrated fan installed in the front door opening that depressurizes the building to 50 pascals below ambient pressure. At that pressure difference, the flow rate required to maintain pressure (in cubic feet per minute, CFM50) divided by the building volume gives the air changes per hour at 50 pascals (ACH50).

Typical results: - Average US house: 7-15 ACH50 - Code minimum (2021 IECC): 3 ACH50 in most climate zones - Passive House standard: 0.6 ACH50

Getting from 3.0 to 0.6 ACH50 is not linear — it requires rethinking construction sequencing and detailing. The airtightness layer must be: - Continuous (no gaps) - Accessible for inspection - Durable (interior membrane will last for the life of the building; exterior membranes must handle UV and weather) - Clearly defined in construction documents

Airtightness materials: - Fluid-applied membranes (sprayed or rolled): Can be applied to irregular substrates, cover gaps and penetrations. Products: Prosoco R-Guard, ZIP System liquid flash. - Variable permeability membranes: Allow vapor to pass in summer (when interior is drier than exterior) but resist vapor passage in winter (when interior is warmer and moister). Product: INTELLO Plus (Pro Clima) is the Passive House community standard. - Tapes: All penetrations and laps in membranes must be taped. Standard tapes fail over time; purpose-designed air barrier tapes (Siga, Pro Clima, 3M 8067) are UV and temperature resistant. - ZIP System sheathing: OSB with a built-in air and water resistive barrier, taped at all joints. A common assembly in North American Passive House.

Where airtightness fails: - Penetrations: Every pipe, wire, duct, and anchor bolt that passes through the air barrier must be sealed. A standard house has dozens to hundreds of penetrations. - Transitions: Where the air barrier transitions from wall to roof, or from wall to foundation, requires careful detailing. These transitions are often complex geometry and require specific products. - Windows and doors: The window-to-rough-opening interface is a common failure point. Passive House window installation requires expanding foam, sill tape, and in many assemblies, additional air-sealing tape on the interior jamb.

The Passive House standard requires that all fresh air supply come through an HRV or ERV. This is not optional — a sealed building without mechanical ventilation would accumulate CO2, moisture, and pollutants to unhealthy levels within hours of occupation.

How HRV works: Two air streams — stale exhaust air from bathrooms and kitchen, and fresh supply air from outside — pass through a counter-flow heat exchanger without mixing. The exhaust stream gives up 80-95% of its heat to the incoming fresh air before exiting. The result: fresh air arrives indoors at near-indoor temperature. Only the remaining 5-20% of heat is lost with the exhaust stream.

HRV vs. ERV: - HRV transfers heat only - ERV transfers heat and moisture (through a permeable membrane that allows water vapor to pass between airstreams)

In dry cold climates, indoor air becomes extremely dry in winter as cold outdoor air is brought in. An ERV returns some of the moisture from exhaust air to the supply air, maintaining comfortable humidity without humidifiers. In hot humid climates, an ERV transfers moisture from the hot humid incoming air to the cooler drier exhaust, reducing the cooling and dehumidification load.

Sizing and distribution: The HRV must supply adequate fresh air per occupant (ASHRAE 62.2 specifies about 7.5 CFM per person plus 1 CFM per 100 SF of floor area) and distribute that air throughout the occupied space. Distribution can be through a simple, compact duct system — Passive Houses have no large HVAC ductwork because the space conditioning loads are so small.

Passive House heating systems: When a building's heat loss is low enough, the heating system can be extremely simple. Common solutions: - Electric resistance coil in the HRV supply duct: 1.5-3kW is often sufficient for an entire house - Minisplit heat pump: A single small unit (6,000-9,000 BTU) that would be wildly undersized in a conventional house handles a Passive House in most climates with ease - In warm climates, no dedicated heating system — cooking, appliances, and occupants maintain comfortable temperature year-round

PHPP (Passive House Planning Package) is the energy modeling tool that verifies whether a design meets the Passive House standard. It is a complex spreadsheet (Excel-based) that calculates: - Monthly heat demand from transmission losses, ventilation losses, solar gains, and internal gains - Annual cooling demand - Total primary energy demand from all sources - Airtightness verification

Certification thresholds (PHI Classic standard): - Specific space heat demand ≤ 15 kWh/(m²a) — approximately 4.75 BTU/ft²/year - Specific space cooling demand ≤ 15 kWh/(m²a) - Primary energy demand ≤ 120 kWh/(m²a) for all uses combined - Airtightness: n50 ≤ 0.6 ACH50 (verified by blower door test)

PHPP is not free (available from PHI for €200-400 depending on license), but Passive House designers use it as the core design tool, running early design iterations to test massing, window placement, and insulation levels before committing to construction documents.

PHIUS+ standard: The North American Passive House standard uses climate-specific targets rather than universal ones, recognizing that a building in Phoenix and a building in Minneapolis have fundamentally different optimal designs. PHIUS+ targets are tighter in some climates and looser in others compared to PHI.

The Passive House premium varies significantly by market, design, and contractor experience.

Europe (mature market): 3-8% premium over code-compliant construction for simple buildings. Experience has driven costs down dramatically from the 15-25% premiums of the early 2000s. German mass-market developers build Passive House as standard for apartment buildings without significant premium.

North America (developing market): Typically 5-15% premium, higher in markets where contractor experience is low. A custom home that might otherwise cost $300/SF costs $315-345/SF Passive House. The premium is largely in design fees (PHPP modeling, detailed construction documents), learning curve markup from contractors, and higher-performance window cost.

Operating cost reduction: A house that uses 90% less energy for space conditioning saves real money. At typical US energy prices, a conventional 2,000 SF house might spend $1,500-3,000 per year on heating and cooling. A Passive House equivalent might spend $150-300. Over a 30-year mortgage, this is $40,000-80,000 in saved operating costs (undiscounted).

Comfort premium: The economic calculation often misses the comfort value. A Passive House with no temperature gradients, no cold walls, no drafts, constant fresh air, and excellent sound attenuation from dense construction is noticeably more comfortable than a conventional house. Resale premiums exist in markets where Passive House is understood.

Starting with Passive House principles without full certification: Full certification requires professional verification and is appropriate for new construction. Applying Passive House principles to renovations or to uncertified new construction still yields substantial energy reduction. The principles — more insulation, better airtightness, better windows, HRV — can be applied in combination without targeting the specific certification thresholds.

The Passive House standard exists because building codes set minimum performance floors, not optimal performance targets. A code-compliant building is legal. A Passive House building is designed. The difference matters for energy independence, comfort, and durability — for the life of the building.