Cozying up: Why airtightness is important for a low energy home

Airtightness is important for a low energy home 

When planning a renovation or a new build, you will likely have a conversation with your architect or builder around insulation. 

While insulation is undoubtedly critical for retaining heat, it is only part of the equation when keeping your home warm. Another vital component is airtightness. 

Think of your home’s insulation is like the synthetic fibres or fleece lining of a good quality coat, this keeps you warm. Then airtightness is like the outer shell of a high-quality winter jacket.

Without the outer shell, cold drafts can blow right through the fibres and heat can escape through the gaps from the zip (if the coat’s open) and the neck and cuffs of a coat. 

Achieving better airtightness means sealing those tiny gaps so that your insulation can do its job more efficiently and using ventilation that’s designed and not accidental. More on ventilation is explained later in the blog post.  

What is airtightness?

Airtightness is the measure of how much air leaks in and out of a building through unintentional gaps. In standard construction, a home is rarely a sealed envelope. 

There are tiny cracks and holes everywhere, particularly where the floor meets the wall, around windows and doors, and wherever pipes or cables penetrate the external walls.

Current building regulations in England still allow for a surprising amount of air leakage. It has been likened to having a hole the size of a 20 pence piece in every single square metre of your building envelope. 

If you add all those small holes together, you would end up with a gaping window open to the winter air all year round. That is hardly keeping warm air in and cold air out. 

When a home is not airtight, you face a constant cycle of energy loss where your heating system works hard to warm the air, only for it to escape immediately through these thousands of tiny gaps.

Measuring performance with Air Changes Per Hour

The airtightness of a home is measured by a metric called Air Changes Per Hour (ACH). This is determined by an ‘air pressure test’ that simulates a breeze to see how many times the air inside the house escapes from the home in an hour. 

While this test is more intense than normal daily conditions, it is the best way to see how well performing your home’s outer shell is.

The energy savings from a tighter seal are significant: a home with a rating of 5 ACH will use roughly 15% less energy to heat than one with a rating of 10 ACH. 

While older homes often score 13 or higher, aiming for a lower number ensures your home needs less heating.

The role of ventilation in an air tight home 

If you’re increasing the airtightness of a home then the role of ventilation becomes more important and you need to make it change from accidental to controlled. 

To pick up on the coat analogy from earlier. The outer shell of our coat is increasing it’s airtightness. However, there will usually be ventilation designed into the coat with zips at the front, under the arms or around the neck to allow for heat to escape and for fresh air in.

A proper ventilation strategy ensures that stale, humidity-laden air is consistently removed from source areas, while fresh air is supplied to living spaces. 

This provides a healthier indoor environment and eliminates the risk of condensation, giving you fresh air exactly where and when you need it. 

No matter how leaky your home, you should still consider ventilation as an important component of your comfort and health. 

In an ideal world you’re changing the air in your home once every 3 hours. This keeps air pollution and humidity at comfortable and healthy levels. You don’t want to rely on dusty cracks to provide you and your family with your fresh air.

How does an airtight home breathe?

A common concern is that sealing a house too tightly will make it stuffy or cause damp issues because the house can’t breathe.

However, it’s important to distinguish between uncontrolled air leakage and the breathability of materials. 

Air leakage is simply draughts carrying heat out and bringing cold, damp air into the building. 

Breathability, on the other hand, can refer to the use of vapour permeable materials that allow moisture to escape naturally without letting the heat out. Or materials that can absorb moisture from the air when it’s humid and release that moisture when the air dries out. 

The aim is to create a home that is airtight to prevent draughts but uses breathable materials to manage moisture and prevent condensation.

How airtightness can protect your home 

Having a more airtight home is also important to maintaining the building’s physical health. 

In a house that isn’t properly sealed, warm, moisture-laden air internally from activities like cooking or showering is forced through tiny cracks and gaps in the construction. 

As this humid air travels through your walls or roof and hits colder surfaces, it turns back into liquid water, known as interstitial condensation.

Because this dampness is trapped deep inside the structure, it often goes unnoticed but can create mould or damage the structure of your home.

By creating a solid airtight barrier, you block these hidden pathways, keeping  your home dry, sturdy, and free from the mould that can impact your indoor air quality.

Building for the future

Ultimately, airtightness is the difference between a house that looks efficient on paper and one that actually feels warm to live in. 

While it requires attention to detail and a shift in how we think about construction, the benefits are considerable.

By sealing the small gaps in your building fabric and pairing this with a considered  ventilation strategy, you prevent constantly reheating cold air and you get a home that’s draught-free, healthy and cheaper to run.