Airtightness

How is airtightness different from insulation?

Airtightness and insulation address two separate building-envelope properties often confused by clients and even some practitioners. Insulation (measured in U-value in W/m²K) resists conductive heat transfer through the material itself, slowing the rate at which heat escapes. Airtightness prevents uncontrolled air movement through cracks, gaps, unsealed joints, and penetrations for pipes and ducts. A building can be well-insulated with poor airtightness, losing energy through cold-air infiltration when heating, or well-sealed but poorly insulated, with heat flowing steadily through thin walls. Both properties are essential for energy efficiency. Many clients conflate them because both affect heating costs, but they require separate design intent and construction discipline.

What does the n50 rating mean and what are typical targets?

The n50 value measures air changes per hour (ACH) at a 50 pascal pressure difference during a blower-door test. A value of n50 = 3/h means the entire interior air volume leaks and is replaced three times per hour under that pressure condition. The 50 Pa threshold is chosen because it simulates realistic wind-driven and stack-driven infiltration on a moderately windy day, making n50 a practical real-world metric rather than a purely theoretical laboratory measurement.

In Slovakia, STN 73 0540-2 (energy performance of buildings) sets energy-performance classes with different airtightness targets. For passive houses, the Passive House Institute standard requires n50 ≤ 0.6/h, one of the most stringent airtightness requirements in the world. Nearly zero-energy buildings (buildings that consume minimal energy on an annual basis) typically target n50 ≤ 1.0–2.0/h. Standard new construction without explicit airtightness measures typically achieves n50 = 4–8/h, sometimes higher in poorly detailed work.

Standard or classification	Typical n50 target	Effort required
Passive House Institute	≤ 0.6/h	Meticulous construction, continuous membrane, testing
Nearly zero-energy building	≤ 1.0–2.0/h	Careful design, sealed joints, trained crew
Energy class A or B (Slovakia)	≤ 3.0/h	Good residential practice, basic sealing discipline
Standard construction	4–10/h	No specific airtightness measures applied

Where does the airtight layer sit in Slovak construction?

The airtight layer's location varies by building assembly but always sits on one side (usually the warm interior side) of the thermal insulation as a continuous, unbroken barrier. In typical Slovak exterior walls with mineral-wool insulation and external render finish, the airtight layer often sits on the inner surface of the masonry structure (brick or concrete block), covered by a vapor-retarder membrane, followed by the mineral wool. In timber-frame construction, the airtight layer is typically the interior faces of the sheathing boards (covered by plasterboard on the interior), with seals at all joints and penetrations.

For pitched roofs, the airtight layer sits on the ceiling structure (interior side), with insulation layered above it and a vapor-open roof membrane on the exterior. In flat roofs with insulation above the structural deck, the airtight layer is usually a polyurethane or polystyrene membrane applied directly on top of the structural element, sealed carefully at all edges and around penetrations (pipes, ducts, skylights).

A critical detail: the airtight layer must be airtight but allow vapor diffusion (breathability) toward the interior. Cheap vapor-tight plastic sheeting that blocks both air and moisture vapor traps condensation, creating the conditions for mold and decay. This is why careful material specification matters, not just membrane installation.

Who enforces airtightness during construction?

The primary responsibility falls on the stavbyvedúci (construction supervisor) and the main contractor executing the envelope work. Every trade (masons, carpenters, electricians, plumbers, HVAC installers) must respect the airtight layer and minimize penetrations. The architect and energy consultant specify the location, materials, and details. The site manager or supervisor enforces the protocol daily and performs intermediate checks using partial blower-door tests or smoke tests before interior finish.

Common failures: electricians running cables through membranes without sealing holes, plumbers drilling without prior approval, carpenters cutting openings without caulking. Prevention requires a written site protocol specifying "no penetration without prior approval," clear marking of the airtight layer, and mid-stage blower-door testing at the roof-sheathing stage to catch leaks before hidden damage occurs.

Role	Responsibility
Architect and energy consultant	Specify airtight-layer location, materials, detailing, and testing protocol
Main contractor	Execute without puncture; ensure subcontractor compliance and cooperation
Site manager or supervisor	Enforce protocol daily; conduct mid-stage blower-door or smoke tests
Electrical, plumbing, HVAC trades	Seal all penetrations immediately with approved tape or sealant

How does airtightness relate to mechanical ventilation?

Airtight houses must breathe through controlled mechanical ventilation, not uncontrolled envelope leakage. This is not a design trade-off but a necessity. The old practice of natural draft through chimneys and unavoidable cracks provided fresh air; modern airtight dwellings eliminate that uncontrolled leakage and introduce controlled fresh air via heat-recovery ventilation (HRV). The HRV system extracts stale air from bathrooms and kitchens, passes it through a heat exchanger to warm incoming fresh air, then supplies the fresh air to living spaces. The result: fresh air, no drafts, no heating loss. An airtight house without ventilation becomes stuffy, humid, and uncomfortable. Airtightness plus mechanical ventilation is the system; neither works well alone. This is a key client-education point: the ventilation requirement is a feature (fresh air without drafts or heat loss), not a deficit.

What moisture damage can result from airtightness failures in timber roofs?

A leak in a timber roof's airtight layer is particularly dangerous because timber is hygroscopic and vulnerable to fungal decay and insect damage when moist. If the airtight membrane is punctured and warm interior air escapes, it condenses on cold surfaces inside the roof cavity, causing timber members and insulation to absorb moisture. Over weeks and months, fungi (wood-rot species like Serpula lacrymans) and wood-boring insects thrive in damp conditions, weakening the structural integrity. By the time the problem is visible (sagging roof, mold staining on underside, musty odor), significant wood loss may have occurred, requiring expensive removal and replacement of affected members.

Prevention is far cheaper. A properly sealed airtight membrane in a timber roof, combined with a vapor-open exterior membrane, allows the assembly to dry quickly to the exterior. Regular blower-door testing during construction catches leaks before hidden damage accumulates.