Passive House Engineering: Airtightness, Thermal Bridge-Free Detailing, and Why Ventilation Becomes Mandatory

Passive House is often described as a very well insulated building. That undersells the difficulty. Insulation thickness is the easy part; you can always add more. What makes the standard demanding is that it sets a performance target verified by measurement, and hitting it requires the envelope to behave as a continuous system rather than as a set of well-specified components.

The core targets are specific: annual heating demand at or below 15 kWh per square metre of treated floor area, or a peak heating load at or below 10 W per square metre; total primary energy within a defined cap; and airtightness at or below 0.6 air changes per hour at 50 pascals pressure difference, verified by blower door test on the completed building.

That last one is the requirement that most projects find hardest, and it is the one that drives most of the detailing. This post covers what airtightness actually requires, why thermal bridges become the dominant heat loss once insulation is good, how the ventilation requirement follows from the airtightness, and where the design process differs from conventional practice.

1. Airtightness at 0.6 ACH50 Is a Detailing Problem

A conventional new building in the United States might test between 3 and 7 air changes per hour at 50 pascals. Reaching 0.6 is roughly an order of magnitude tighter, and it cannot be achieved by being careful with the same details. It requires a different approach to the envelope.

The continuous line principle

The method that works is deceptively simple to state: there must be a single continuous air barrier that can be traced around the entire thermal envelope, on a section drawn through any part of the building, without lifting the pen.

Applying it exposes the places where conventional construction is discontinuous. Where a floor slab passes through the wall to a balcony. Where an interior partition ties into the exterior wall. Where the wall meets the roof and the material changes from sheathing membrane to roof membrane. Where a beam penetrates the envelope. Each of these is a junction between different materials installed by different trades at different times, and each requires a specific detail showing how the air barrier is made continuous across it.

The practical technique is to draw the air barrier as a highlighted line on every section and detail, and to require that every drawing showing the envelope shows that line. Junctions where the line stops are the ones that will leak.

Penetrations

Every service crossing the envelope is a hole in the air barrier. Ventilation ducts, plumbing vents, electrical service, gas lines, and any structural element passing through all require sealing, and the seal must accommodate movement and remain functional over the building's life.

The approach that works best is to minimise the number of penetrations by consolidating services, then to seal the remainder with purpose-made airtight grommets and tapes rather than with expanding foam, which is not an air barrier material and degrades. Where a penetration cannot be sealed reliably, a service cavity inboard of the air barrier lets the wiring and plumbing be run without crossing the line at all, which eliminates the problem rather than solving it.

0.6 ACH50

The Passive House airtightness limit, verified by blower door test. Roughly ten times tighter than typical new construction, and achievable only through a deliberately continuous air barrier rather than careful conventional detailing.

Testing during construction, not after

A blower door test on a finished building tells you whether you passed. It does not tell you where the leaks are, and by then the air barrier is buried behind finishes. The practice that works is to test as soon as the air barrier is complete and before it is covered, with a smoke pencil or thermal camera to locate leakage while it can still be fixed for the cost of a roll of tape. A second test at completion verifies the final result.

2. Thermal Bridges Dominate Once Insulation Is Good

In a poorly insulated wall, heat flows through the whole assembly and a thermal bridge is a modest addition. In a wall at R-40, most of the assembly barely conducts, and any path that bypasses the insulation carries a disproportionate share of the total loss.

Passive House targets thermal bridge-free construction, defined by a linear thermal transmittance at or below 0.01 W per metre-kelvin at junctions. Reaching that means eliminating bridges by design rather than accounting for them in calculation.

Where the bridges are

Balconies and canopies.  A cantilevered concrete slab passing through the insulation line is one of the worst bridges available in a building. The solutions are to support balconies independently on their own structure outside the envelope, or to use a structural thermal break, a proprietary connector combining stainless steel reinforcement with insulating material that maintains structural continuity while cutting the conduction path.

Foundation to wall junction.  The wall insulation has to connect to the sub-slab or perimeter insulation without a gap. In conventional construction the foundation wall is often uninsulated at the top where the framing bears on it, creating a continuous bridge around the entire perimeter. Solutions include insulated foundation forms, load-bearing rigid insulation blocks below the sill, and continuous exterior perimeter insulation carried down past the slab edge.

Window installation position.  A window installed at the inner face of a thick wall creates a bridge through the reveal. Installing it within the insulation layer, so that insulation laps the frame on all four sides, substantially reduces the junction loss. This is why Passive House windows are commonly mounted on brackets projecting into the insulation zone rather than fixed to the structural opening.

Structural penetrations.  Steel members crossing the envelope, canopy supports, sunshade brackets, and railing posts all conduct. Where they cannot be eliminated, thermal isolation pads and minimising cross-sectional area at the crossing reduce the effect.

Modelling

Junction losses are calculated by two-dimensional or three-dimensional finite element heat flow analysis, which produces the linear transmittance value used in the energy model. This is a specialist activity that has to happen during design development rather than at the end, because the results frequently indicate that a junction needs redesigning.

3. Ventilation Becomes a Requirement, Not an Option

A leaky building ventilates itself by accident. Air moves through the envelope continuously, driven by wind and stack effect, providing uncontrolled and inefficient but real air exchange. Seal the envelope to 0.6 ACH50 and that stops.

At that airtightness, mechanical ventilation is not an efficiency measure but a habitability requirement. Without it, carbon dioxide accumulates, moisture from occupants and cooking raises humidity toward condensation risk, and indoor pollutants from finishes and furnishings have no path out.

Heat recovery

Supplying outdoor air continuously in a cold climate would impose a heating load that defeats the envelope work. Heat recovery ventilation solves it by passing outgoing and incoming air streams through a heat exchanger where they exchange thermal energy without mixing. Passive House requires at least 75 percent sensible heat recovery efficiency, and good units exceed 85 percent.

Energy recovery ventilators additionally transfer moisture, which is valuable in humid climates for reducing latent cooling load and in very cold dry climates for retaining indoor humidity. The choice between heat recovery and energy recovery follows from climate rather than preference.

Design considerations that get missed

The ventilation system has to be quiet, because it runs continuously and occupants who find it noisy will turn it off. Duct velocities are kept low, which means ducts are larger than a conventional intermittent system would need, and that has to be coordinated with structure and ceiling space early.

Filtration matters because the system is the building's only air path. Filter access has to be convenient enough that filters actually get changed, which sounds trivial and is a common cause of degraded performance in operation.

And in cold climates the exchanger can frost when exhaust moisture condenses and freezes in the cold side. Defrost strategy, whether by preheating incoming air, by periodic recirculation, or by bypass, is a design decision with an energy cost that belongs in the model.

Why the Modelling Tool Is Part of the Method

Passive House certification uses PHPP, a spreadsheet-based energy model validated against measured performance in certified buildings. It differs from typical code compliance software in that it is a design tool used iteratively from early concept, not a compliance check run at the end. Window specification, shading geometry, insulation levels, and ventilation strategy are all tested in the model as design decisions are made. Projects that treat it as a final verification step rather than a design instrument tend to discover late that the target is not being met, at which point the available fixes are expensive.

4. Windows and Solar Gain

Windows are the weakest part of any high-performance envelope. A wall at R-40 next to a window at R-5 means the window loses eight times as much heat per unit area, so window area, specification, and orientation carry disproportionate weight in the energy balance.

Passive House climate zones set minimum window performance, typically triple glazing with two low-emissivity coatings, argon or krypton fill, an insulated spacer at the edge, and a thermally broken frame. Whole-window U-value targets are usually at or below 0.8 W per square metre-kelvin in cold climates.

Solar heat gain coefficient is the parameter requiring the most judgement, because it works in opposite directions by season. High SHGC on south-facing glazing delivers useful winter heat; the same glazing overheats the building in summer. The resolution is geometric rather than material: fixed external shading sized so that the high summer sun is blocked while the low winter sun passes beneath. Because the sun's altitude at a given date is fixed by latitude, the overhang depth that achieves this can be calculated exactly.

External shading is far more effective than internal blinds, because it stops solar radiation before it enters. Once radiation has passed the glass, the heat is inside the building regardless of what is hung behind the window.

5. What Changes in the Design Process

The most consequential difference is when decisions get made. In conventional practice, envelope performance is often resolved during design development and mechanical systems sized afterward from the resulting loads. Passive House inverts this: the envelope determines whether the target is achievable at all, so envelope decisions belong in concept design, and the heating system that results is small enough that its selection is a comparatively minor decision.

The second difference is that airtightness is built by trades who have to understand it. A subcontractor who cuts a hole through the air barrier to run a cable and patches it with foam has undone work that will show up in the blower door result and be expensive to trace. Site induction covering the air barrier, a visible marked-up drawing on site, and a named person responsible for envelope continuity are standard practice on projects that succeed.

The third is that certification requires evidence: measured airtightness, product data for every envelope component, and the completed model. Assembling that documentation is a project management task that has to be resourced rather than assumed.

Conclusion

Passive House is demanding because it sets a measured performance target rather than a prescriptive specification. Airtightness at 0.6 ACH50 requires a continuous air barrier planned as a system and tested while it is still accessible. Thermal bridges have to be eliminated by design because at high insulation levels they dominate the remaining loss. And the resulting envelope makes mechanical ventilation with heat recovery a habitability requirement rather than an option.

None of the individual techniques are exotic. What the standard demands is that they be applied consistently across every junction, penetration, and interface, verified by measurement rather than assumed. That consistency is the difficult part, and it is achieved through detailing discipline in design and comprehension on site rather than through any particular product.