Greener Living

A beginner's guide to airtightness and home retrofit

Air can escape through hundreds of small gaps in your home, carrying heat and comfort with it.

When homeowners think about improving energy efficiency, they often focus on insulation, new windows, solar panels or heat pumps. But there's another factor that can have a surprisingly big impact on how your home performs: airtightness.

Done well, airtightness improvements can help reduce draughts, improve comfort and support the performance of other retrofit measures. The key is getting the balance right, because a healthy home also needs good ventilation.

Our previous guide explains what airtightness means, why it matters during retrofit, and what products can be used. This guide covers a quick recap on airtightness, as well as how airtightness is tested and the practical steps homeowners can take to achieve a warmer, more energy-efficient home.

What is airtightness?

Airtightness describes how much air escapes unintentionally through a home's structure, including the walls, roof, floor, windows, doors and the junctions between them.

These unwanted air leaks are often hidden in:

Gaps around windows and doors

Floorboards and skirting boards

Loft hatches

Pipe and cable penetrations

Chimneys and fireplaces

Roof and wall junctions

Unlike opening a window or using an extractor fan, air leakage is uncontrolled. It varies depending on wind, weather and temperature, often causing heat loss where you least expect it.

Think of it like a winter coat

Insulation works like the warm filling inside a coat.

Airtightness is the windproof outer layer.

Even a thick, well-insulated coat won't keep you warm if the zip is open. In the same way, insulation can be less effective if warm air is constantly escaping through gaps in the building fabric.

Top tip: Airtight doesn't mean sealed shut. A well-designed home still needs fresh air. The goal is to stop uncontrolled draughts while ensuring ventilation happens in a planned way.

Why airtightness matters during retrofit

Improving airtightness is often a key part of a whole-house retrofit approach.

Benefits can include:

Greater comfort
Reducing unwanted draughts helps create more even temperatures throughout your home and can eliminate cold spots near windows, floors and walls
Reduced heat loss
Warm air that escapes has already been paid for through your heating system. Controlling air leakage can help retain heat and improve overall energy performance.
Better performance from other upgrades
Insulation, heat pumps and ventilation systems can often work more effectively when uncontrolled heat loss is reduced.
Improved moisture management
Warm, moisture-laden air can travel into colder parts of a building through hidden gaps. Good airtightness, combined with appropriate ventilation, can help reduce condensation risks.
Confidence in quality
Testing provides measurable evidence of how a property is performing and can help identify areas that need improvement before work is completed.

How is airtightness measured?

The most common method is an air pressure test, sometimes known as a blower door test.

During the test, a temporary fan is fitted into an external doorway. The equipment creates controlled pressure differences between the inside and outside of the home and measures how much air leakage occurs.

What happens during a test?

Prepare – the property is measured and prepared for testing.
Pressurise – a calibrated fan creates controlled pressure differences.
Measure – specialist equipment measures airflow and leakage rates.
Diagnose – smoke testing, thermal imaging or other techniques can help identify where air is escaping.

The process is non-destructive and is typically completed within a few hours.

When should airtightness be tested?

Many homeowners assume testing only happens after work is complete.

In reality, testing during a retrofit can be even more valuable.

Stage	Why it matters
Before work begins	Establishes a baseline and identifies major leakage routes
During the project	Allows issues to be found and fixed while they remain accessible
At completion	Confirms the final result and records performance improvements

Testing before finishes are installed can help avoid costly remedial work later.

Lessons from Ferndale Rise

At our Ferndale Rise home retrofit project, airtightness testing was carried out at multiple stages throughout the renovation.

The project's final airtightness result was 0.54 air changes per hour (ACH) at 50 Pascals, demonstrating the value of testing and quality control throughout the retrofit process. More importantly, staged testing allowed issues to be identified and addressed before final finishes concealed the airtight layer.

Where do homes commonly leak air?

Most air leakage doesn't come from one obvious hole. Instead, it's usually the combined effect of many small gaps.

Common problem areas include:

Loft hatches and ceiling penetrations

Window and door frame junctions

Suspended timber floors

Kitchens and bathrooms

Pipes, ducts and service penetrations

Chimneys and fireplaces

Roof junctions and eaves

Extensions and previous alterations

Hidden cavities within walls and floors

Did you know? The place where you feel a draught isn't always where outside air is entering. Air can travel through hidden cavities before emerging elsewhere in the home.

The golden rule: build tight, ventilate right

One of the biggest misconceptions about retrofit is that a leaky home is a healthy home.

In reality, air leakage is unpredictable. A house may be over-ventilated on windy days and under-ventilated on still days. Air may also enter through lofts, cavities and other undesirable routes.

As airtightness improves, effective ventilation becomes even more important.

Depending on the property, this might include:

Background ventilators

Extractor fans

Continuous mechanical extract systems

Mechanical ventilation with heat recovery (MVHR)

What homeowners shouldn't do

Avoid:

Blocking trickle vents or air bricks

Sealing combustion air supplies to boilers or stoves

Relying solely on opening windows for ventilation

Installing powerful extract fans without considering replacement air

Treating MVHR as a simple product purchase rather than a designed system

A practical homeowner checklist

If you're planning retrofit works, consider these steps:

Take a whole-house approach
Think about insulation, windows, heating and ventilation together rather than as separate projects.
Define the airtight layer
Ask your designer or contractor to identify how the airtight layer will be maintained throughout the property.
Set a clear target
Decide whether your goal is improved comfort, lower energy bills, regulatory compliance or a specific performance standard.
Assign responsibility
Ensure someone is responsible for sealing penetrations and maintaining continuity between trades.
Use suitable materials
Membranes, tapes and sealants should be appropriate for the building and expected conditions.
Record progress
Photograph key airtightness details before they are covered up.
Test before completion
Where possible, carry out an interim test so issues can be identified and resolved early.
Commission ventilation systems
Any ventilation system should be tested and explained to occupants.
Review future changes
Extensions, window replacements and loft conversions can all affect airtightness and ventilation performance.

Questions to ask your retrofit team

Before work begins, consider asking:

Where is the continuous airtight layer shown on the drawings?

How will existing and new parts of the property connect?

Who is responsible for sealing service penetrations?

What ventilation strategy is proposed?

How will ventilation performance be commissioned and verified?

Can airtightness be tested during the project, not just at the end?

What evidence and documentation will be provided on completion?

Start your warmer home journey

A successful retrofit doesn't rely on accidental gaps to provide fresh air.

Instead, it combines improved airtightness with appropriate ventilation to create a home that is warmer, more comfortable and more energy efficient. Whether you're planning a small renovation or a whole-house retrofit, considering airtightness from the start can help ensure every improvement performs as intended.

This guide was produced with ATSPACE Ltd, the air pressure testing and diagnostic specialist involved in The Cambridge Building Society's Ferndale Rise retrofit project. This article is intended as general guidance and should not replace project-specific professional advice.

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