Let's learn about... Insulation (Part 1)

One of the most important things to understand for healthy home-making

Part of my Cleanest Greenest Home Series: demystifying eco upgrades, healthier materials, and smarter ways to live well inside the homes we already have.

Photo by Niek Doup on Unsplash

Insulation. Sounds really boring, right? But let me tell you — in combination with air-tightness and moisture control — it is probably the single most important thing to understand about your home. Why? Because insulation is the infrastructure of comfort. Get it right, and everything else in your home works better. Get it wrong, and you’ll forever be compensating with candles, radiators, jumpers, or dehumidifiers.

So let’s start with what it actually means to have a well-insulated home.

• When a home is well insulated, the surfaces inside the space warm up — walls, ceilings, and even the furniture against them. This matters more than you might imagine because we humans respond not just to air temperature, but to radiant temperature — the warmth coming off the surfaces around us. Evenly warm walls deliver more radiant comfort, which feels intrinsically soothing to the nervous system. It’s why a 19°C room with warm walls feels cosy, while a 21°C room with cold walls can still feel miserable and draughty. As such, with good insulation you can lower your thermostat without losing comfort, saving roughly 5% on energy costs for every degree reduced. Comfort up, bills down. Win-win.

• Effective protection against mould: insulation is one of the most powerful mould-prevention tools we have. Humidity from daily life (showers, cooking, drying laundry, breathing) and moisture from outside air will only condense on cold surfaces. Cold wall = water droplets = the perfect breeding ground for mould. Insulation warms the interior surface temperature of your walls, meaning condensation cannot form. Without damp patches, mould is categorically unable to grow. Given mould is linked to respiratory irritation, asthma flare-ups, fatigue, and impaired immunity, this is no small benefit. It’s a major health win.

• Less unwanted air movement means fewer draughts, and cleaner air: In uninsulated rooms, a subtle but important process is constantly happening: warm air hits a cold wall. It cools, becomes heavier, and sinks. It then slides along the floor rising up again once warmed in the middle of the room. This convection loop creates micro-draughts, uneven temperatures, and the constant movement of dust — all of which undermine comfort. In a well-insulated home, this loop is dramatically reduced or eliminated. Air becomes stiller and more stable. The room feels quieter, calmer, more restful. Less dust is disturbed, which is especially good for allergy sufferers. Comfort isn’t just about heat, it’s about stability, and insulation creates that stability.

Side note: if you see a lot of dust settling in your home, this might be the cause, not just the dog! I’ve noticed it specifically in one room in my new home that has two external walls that are always cold because they’re North-facing and not adequately insulated. So fascinated was I by this, I researched this further, see below…

Insulation vs Air-tightness

Insulation and air tightness are not the same thing — but they are absolutely dependent on each other, and people often confuse them, or believe air-tightness means a vacuum sealed home. It doesn’t. Understanding the distinction is crucial.

• Insulation slows heat transfer. It resists the movement of heat through your walls, roof, and floors. Think of it as a thick winter coat.

• Air-tightness stops uncontrolled air movement. It seals the gaps, cracks, and holes that allow warm air to leak out and cold air to whistle in. Think of this as like zipping up the coat.

You need both. A winter coat is useless if it’s unzipped. And a zipped-up jacket is pointless if it’s paper-thin. Your home works in exactly the same way. But here’s the bit that’s most often misunderstood: Insulation only performs to its stated value if air isn’t moving through it.

In other words, if air slips behind insulation boards, through gaps in old plaster, around window frames, under skirting boards, or through tiny cracks in the walls, it behaves like wind cutting through a jumper. The insulation is technically “there” but it’s not able to do its job. Walls stay colder than they should. Heating bills stay depressingly high. This is where air tightness comes in.

Let’s get this super clear because it’s really important!

So air-tightness means the building envelope has no unintended gaps where air can leak out or through. In practical terms, this means: caulking gaps (even tiny cracks) around window or door frames. Taping the joints between insulation boards using specialised air-tightness tape. Using air-tight membranes behind plasterboard. Sealing holes in old plaster. Filling joints around sockets, skirting, or pipes (a classic place for draughts as the holes are usually cut larger than the pipes to make installation easy). Right through to adding draught excluders to windows, letterboxes and keyholes, and fitting brush strips along the bottom of your front and back doors.

Think of it as both creating a consistent, unbroken skin behind your insulation and in all the obvious places in front of it, as a line that warm air cannot cross. Obviously, if you were doing a really deep retrofit involving the removal of old plaster or plaster boarding, this is easier to do. But as listed above, there’s lots that can be done retrospectively without requiring a messy makeover.

But make no mistake, even small leaks create cold corners and uncomfortable draughts. Patches where mould can form. Rooms that never feel evenly warm. This is why you might hear people say, “But I added insulation and my house still feels cold!” It’s not the insulation that’s failed, it’s the air movement around it.

Key, your airtightness layer must never be in the form of a plastic, vapour-blocking layer that might trap moisture (like wearing a plastic raincoat on a hot day — everything gets wet underneath)! You want your home to be airtight to air, but still vapour permeable to moisture.

Quick explainer: What is vapour permeability? Vapour permeability refers to a material’s ability to let water vapour pass through it, rather than trapping it, specifically inside the walls of your home. Think of water in the air (humidity) as an invisible mist. In a healthy building, this mist needs to be able to move, to escape, to dry out, and to pass through materials naturally. Vapour-permeable materials allow this to happen slowly and safely.

To underline though, an airtight home is not a sealed, suffocating box. The goal is to stop draughts, not drying.

But hang on… you’re always telling me to open my windows?

I know — it sounds contradictory. Seal everything up but also ventilate? Ventilation and air-tightness are not opposites. Again, you absolutely need both. One without the other is a problem. A draughty home is uncomfortable and wasteful. A sealed home without ventilation becomes humid and unhealthy. Here’s the crucial distinction: Air-tightness stops uncontrolled air movement. Ventilation introduces controlled air movement.

• Air-tightness protects your home’s warmth, comfort, and structure.

• Ventilation protects your health and indoor air quality.

The first is about eliminating random leaks through cracks, gaps, and cold corners. The second is about giving your home the fresh air it actually needs — opening windows when cooking or showering, cross-ventilating when weather allows, or using mechanical systems that exchange stale indoor air for fresh outdoor air without losing heat. I’ve listed three levels of ways to ventilate (and control moisture) below.

The final piece that completes the comfort trio: moisture control

Once a home is warm and better sealed, the way it handles humidity becomes critically important. After all, moisture is produced constantly just by living. A family of four produces roughly 6–10 litres of water vapour a day through breathing, showers, kettles, cooking, drying laundry and even plants. The issue isn’t the moisture itself; it’s what happens when it lingers. If that moisture has nowhere purposeful to go, we’re back to condensation, damp patches, and ultimately mould.

This is why good insulation and air-tightness (using vapour permeable materials) must always be paired with controlled ventilation (as discussed above): to give excess humidity a clear exit route, but without losing the hard-won heat you’ve created. In a well-designed home, warm air stays in, and moisture knows exactly how to get out.

So how do you take control of moisture in a practical way? There are three levels — and most homes need a combination of them.

1. Everyday, zero-cost habits (the stuff everyone can do)

These are the simplest and often most effective first steps — especially in older or poorly sealed homes. These habits alone can dramatically reduce condensation and mould risk — and they cost nothing.

• Open bathroom windows after showering. A five-minute purge of humid air prevents it from travelling through the house and settling in colder corners.

• Use lids on saucepans and switch on extractor fans when cooking. Most kitchens have extractors; few people use them properly. Run them during cooking and for 10 minutes afterwards.

• Cross-ventilate when weather allows. Open two windows on opposite sides of your home to create a quick, powerful, five-minute flush of stale, humid air. I do this every morning upstairs and down without fail, unless it’s pouring with rain.

• Don’t dry laundry directly on radiators. It dumps litres of water into the air. Use a drying rack in a well-ventilated room, or place it near an open window. Or in a room in with a portable dehumidifier. I have one a low energy one from Meaco.

• Keep furniture slightly off external walls. Even 5–7cm allows airflow and prevents micro-climates where cold air can condense unnoticed. This is particularly important behind large, hard to move, pieces of furniture that might be placed against external walls (like beds and wardrobes).

2. Assisted ventilation (for when windows aren’t enough)

If your home is well insulated and reasonably airtight, opening windows might be all you need. But for many households, especially in cooler or wetter seasons, everyday habits might not fully solve the problem. These middle ground options are affordable, accessible, and helpful in bridging the gap between “open the windows” and more complex systems.

• Humidity-sensing extractor fans. In kitchens and bathrooms, these automatically switch on when moisture rises. No thinking required.

• Trickle vents. Often overlooked, these small slots above windows allow slow, continuous air exchange without draughts. In new builds, these are now regulation. A competent window fitter or joiner can retrofit trickle vents into many existing frames, usually by cutting a slim slot into the top of the window sash or frame and fitting an internal/external cover. It’s a fairly simple job if the window material allows it (timber and uPVC are usually fine; aluminium can be trickier).

• Positive Input Ventilation (PIV). A simple box in the loft pushes fresh filtered air into the home (usually through a discreet ceiling vent in the hallway) at a gentle rate. This creates slight positive pressure — meaning the air inside your home is very gently nudged outward through the natural gaps and vents that already exist (extractor fans, trickle vents, chimney flues, and the building fabric itself). It dilutes moisture and stale air and is far cheaper than full MVHR. To note, PIV is an option in most UK homes only because a typical loft isn’t airtight — it’s deliberately ventilated through the eaves, which means the air drawn in is continually refreshed.

• Dehumidifiers. A lifesaver in rental homes or properties with unavoidable moisture loads. Look for ones with humidity sensors and quiet night modes.

3. Mechanical Ventilation with Heat Recovery (MVHR)

The gold standard — but not essential for every home, MVHR systems continuously remove stale, humid air and replace it with fresh air while capturing the heat from the outgoing air — meaning you ventilate without losing warmth.

They are brilliant for new builds, deep retrofits, homes that are already highly insulated and airtight, people with allergies or asthma, or anyone wanting consistently fresh, low-humidity indoor air.

However, they require careful and expert installation that can be highly intrusive if you’ve already decorated. But if you don’t have a good foundation of insulation and airtightness for all the reasons already discussed, they won’t help you. For example, retrofitting MVHR into a Victorian terrace, without a deep insulation and airtightness upgrade at the same time, is rarely cost-effective and can cause more issues than it solves. And they’re not cheap. At all.

For most homes, option 1 and 2 are your go-tos.

Quick explainer: Why it’s never a good idea to fill wall cavities with foam insulation. It’s tempting: a contractor drills a few holes, injects expanding foam, and suddenly your home is “insulated”. Quick, cheap, done. But wall cavities exist for a reason. Cavities act as a drainage and ventilation gap, allowing any rain that penetrates the outer brickwork to run down and escape through weep holes. Fill that space with foam and any moisture that gets in has nowhere to go, often becoming trapped against the inner wall. Many foams also behave like plastic, so indoor humidity now cannot pass through and dry out naturally. The result is a perfect storm of cold external air meeting trapped internal moisture — leading to condensation, damp patches, mould, and long-term damage to plaster and timber. Worse, if the wall is already taking on water due to cracked brickwork or failed render, foam simply hides the problem while allowing it to deteriorate unseen.

And one last, crucial point…

Even with perfect air-tightness, and ventilation, insulation can only ever be as healthy-making and effective as the material it’s made from. This is the part that’s often glossed over. This matters because we are not just trying to make our homes warmer, more comfortable and ultimately cheaper to run — we’re trying to make them cleaner, greener, and genuinely healthier too. You are what you breathe.

It’s easy to assume that “insulation is insulation”, that it’s all performing the same job in the same way. But the type of insulation you choose determines how it works, what chemicals are introduced into your home and how breathable the building fabric remains. Many off-the-shelf insulation boards rely on petrochemicals, plastic foams, and added flame retardants. These materials tend to be non-breathable (trapping moisture); vapour resistant (preventing walls from naturally releasing humidity), contain a hidden chemical load (which off-gases into your indoor air), and they’re often rigid and unforgiving, creating moisture traps where they meet older, uneven walls. Not to forget, incompatible with traditional building materials, leading to long-term damp issues.

This isn’t scaremongering — it’s building physics. If you wrap a home (particularly an older property) in non-breathable, non-vapour permeable materials (whether insulation or plaster and paint), you fundamentally interrupt the moisture movement it was designed to depend on. Moisture becomes trapped. Walls can’t dry out. Hidden damp accumulates. Mould follows.

Yet over in the cleanest greenest healthiest camp are materials like wood fibre, cork, hemp, cellulose, and other natural, vapour-open products that work with a building, not against it. I’ll dive deeper into these products in another post as I’m still researching what to specify in my own home!

Conclusion

When insulation, air-tightness, and moisture control work together, the result is a a home that feels naturally comfortable, and supports your health automatically, in the background, every single day. Not overheating. Not too dry. Always fresh. This is the goal we want to aim for. A clean cosy home.

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