Why Is It Important To Include Airtightness With Insulation? – Can I Just Ask? – Ep.11

In sustainable building, insulation often gets the spotlight. It’s what keeps homes warm in winter, cool in summer, and reduces energy bills. But without airtightness, insulation can be wasted. Gaps and leaks in the building fabric allow warm air (and with it, moisture) to escape, making insulation far less effective and risking moisture damage. In this article we explore how airtightness complements natural and other insulation strategies, why you need both for a high performance build, and what best practice looks like in detailing and specification.

If you’d rather hear Andy and Chris talk through the ideas in this article, you can watch Episode 11 of Can I Just Ask? below. 👇

Why “Insulate Only” Is Not Enough

Insulation works by slowing conductive heat loss: heat flows through materials from warmer to colder zones. But buildings lose heat in more than one way. When internal warm air escapes through gaps, that’s convective heat loss. Even the best insulation can’t stop that. In fact:

• Without airtightness, air leakage may bypass insulation altogether, rendering it much less effective
• If warm moist air leaks into cooler parts of the structure (especially behind internal insulation), condensation and moisture problems can arise. Over time that damages materials, reduces thermal performance, and can cause mould

So insulation and airtightness must be designed together.

What Is Airtightness?

To avoid confusion, here are some key terms:

Fabric airtightness: The limit or removal of uncontrolled air movement (leaks) through the building envelope (walls, roof, floor), excluding intentional ventilation. Gaps around windows, junctions, services etc. are the usual culprits.

Ventilation: Deliberate, controlled exchange of air (natural or mechanical). Airtightness is not opposed to ventilation—it just prevents uncontrolled leakage.

Moisture vapour control / vapour control layer (VCL / AVCL): A layer included in the build to control movement of water vapour from warm humid interiors toward cold surfaces, reducing risk of condensation.

How Airtightness Supports Insulation, and Vice Versa

1. Reducing convective heat loss
   Even with thick insulation, uncontrolled air movement (leakage) can carry away heat, reducing comfort and increasing demand on heating systems. Airtightness limits this.
2. Preventing moisture problems
   When warm, humid air moves into colder building components (e.g. insulation layers, wall cavities), it can condense. This is especially problematic with internal insulation, where the inside part is warm and the outside part is cold. Including vapour control layers and airtight detailing helps keep humidity movement under control.
3. Improving indoor air quality (IAQ)
   If the building envelope is airtight (i.e. leaks reduced), it becomes easier to ventilate in a controlled way, preventing drafts, cold spots, and ensuring fresh air supply without excessive heat loss.
4. Durability and performance
   Moisture, temperature cycles, condensation damage—these degrade building materials, reduce insulation effectiveness, and over time increase maintenance. Airtightness (with moisture management) protects the fabric of the building.

Key Details and Best Practices

• Place air barrier layers correctly (often on the warm side of the insulation) and ensure continuity across junctions (walls to roofs, walls to floors, around windows, services etc.)
• Use vapour controlled airtightness layers in internal insulation systems. These control vapour movement as well as air leakage
• Seal all penetrations: services, pipes, cable entries, window frames, doors etc. Use tapes, membranes, grommets or specialised junction details
• Testing is important: air leakage tests (e.g. at 50 Pa) to verify that airtightness targets are met. Making sure these tests happen before finishes or sealing up so any defects can be accessed and fixed
• Coordination and detailing: design, specification, contractor training, sequencing on site—all contribute heavily. A well specified and well managed airtightness strategy is essential

Common Misconceptions

Airtight means no ventilation: Not true. Airtight means controlled ventilation. You still need fresh air (natural or mechanical), but you don’t want unplanned leakage that wastes energy and causes cold drafts.

Breathability vs airtightness are opposed: They aren’t necessarily. Many high performance and natural builds use breathable or vapour open materials (wood fibre, hemp, straw etc.), while still achieving airtight construction. The trick is balancing vapour diffusion, controlled air leakage, and ensuring moisture can escape in a managed way.

Conclusion and Takeaways

• Insulation and airtightness are complementary. Insulation reduces conductive heat loss; airtightness limits convective loss and moisture ingress
• Without airtightness, insulation often underperforms. Worse, moisture problems can arise that degrade both the building fabric and indoor comfort
• For high performance building (natural or otherwise), airtightness must be detailed, specified, and tested properly. Vapour control layers are often part of that
• A building that’s both well insulated and airtight is easier to ventilate in a controlled way, ensures comfort, protects materials, and reduces energy use (and carbon)