“Just insulate your home” is advice that’s easy to give but difficult to follow. When you’re looking at extortionate energy bills it can seem like all insulation does is make clear advice less clear. Which type of insulation should you do? How do you know where to start when you can’t afford to do everything at once? And why are the savings estimates always so wildly different?
I fell down this particular insulation rabbit hole in my Victorian terrace. With build quality implying that whoever built it thought warmth was a holiday bonus not a fundamental human requirement, I’ve learnt about insulation at work and via Google until my eyes bleed. Professionally, personally and, lets be honest, through bank account after bank account draining mistakes.
My learned experience told me that there was a right way and a wrong way to insulate your house. Do it in the wrong order and you could spend thousands making your home cosier to stand in but with minimal effect on your heating bills. Get the order right though and you could cut your heating bills in half.
There’s a science to insulation and heat loss which governs what to do first. UK homes lose heat in predictable ways, but some lose heat more through certain routes than others. And some insulation upgrades are more expensive than others. Knowing where your home loses heat, and how much it loses through each route, is the first step to working out whether you should prioritise your loft, your walls, or skip both altogether and start with windows.
The infographic below shows how much heat the average uninsulated UK home loses through different routes, and how it changes as you tackle different upgrades.
The building physics of home heat loss
Heat doesn’t travel through houses randomly. It’ll always lose heat through the same routes, in the same proportions. And if you understand how it behaves you can make the right decisions about insulation. Let’s look at each route heat escapes your home through individually.
Conduction: Heat moves through solid materials at set rates, which is known as their u-value or thermal conductivity. An old single glazed window will lose heat around 10 times faster than a typical insulated wall, which is why you notice cold windows so much. The same is true for the walls in your home. Uninsulated solid walls can lose heat up to five times faster than an insulated cavity wall. (Energy Saving Trust)
Warm air rises: Okay so this isn’t heat loss exclusively. But it’s the physics behind why so many homes struggle with loft insulation. Poor loft insulation creates a thermal chimney which sucks the warmest air out of your home. Up and out. (Energy Saving Trust) That’s why loft insulation tends to be both the easiest to install and see an immediate impact from even if the rest of your house is badly insulated.
Thermal Bridging: Have you ever touched an exterior wall in your house and felt cold? That’s thermal bridging; areas of your home that are much better at conducting heat than others. Metal ties in cavity walls are a common cause, as are lintels above windows in solid wall homes.
Proportion of total heat loss: When you insulate your home correctly you reduce heat loss through all avenues. As you’ll see in the infographic above this doesn’t reduce the priority of other insulation upgrades but it does change how your home loses heat. An uninsulated house will lose around a third of its heat through its walls. But insulate your loft correctly and suddenly your walls become by far the biggest issue.
Why loft insulation should be the first thing you do
Loft insulation always trumps other types when it comes to insulation priority. Across all metrics; impact on your energy bills, cost and ease of installation. There’s even regs mandating loft insulation since the 1960s.
Why does loft insulation work so well?
Simply, heat moves around your home, eventually pooling in your loft. Insufficient loft insulation means that heat is quickly lost through your roof and into the atmosphere. The majority of homes in the UK have inadequate loft insulation. (National Insulation Association)
Most homes should now have loft insulation upgraded to a depth of 270mm. Much higher than even 15 years ago. Upgrading to modern standards will reduce heat loss through your roof by approximately 75%. (Energy Saving Trust) That saves you up to £285/year for a detached home. Lesser amounts for smaller properties but it’s still the biggest insulation money can buy.
Installing loft insulation is also something the average homeowner can do themselves. Meaning you’ll avoid expensive installer margins if you’re willing to get dusty climbing around your loft for a day.
How much will loft insulation cost?
If you go professional expect to pay around £1,000 (Which). DIY and its £200-400 depending on the size of your house and how much insulation you need to purchase. You’ll save that back in around 3-7 years (Carbon Trust) So its financially one of the simplest decisions you can make.
GotKids? Install loft insulation anyway. Your energy bills will still drop by two-thirds of what they could otherwise achieve. But don’t be tempted to skimp on the depth and save a few quid. Loft insulation needs to be installed correctly to work. Three common pitfalls can noticeably reduce its effectiveness.
You compress it. You won’t misunderstand how much you need to fill your loft to reach the recommended depth.
You don’t fill gaps around pipes etc. These create thermal bypasses that can account for 5-10% of total heat loss through your roof.
Your forget about your loft hatch. It needs the same level of insulation as the rest of your loft to avoid another thermal bypass.
Cavity Wall Insulation
There isn’t a lot of Physics to explain why cavity wall insulation should be your next priority. Simply, once you’ve insulated your loft correctly your walls will be your property’s next biggest weakness. Cavity wall insulation saves up to £395/year (Energy Saving Trust). Making it the second biggest insulation money can buy.
Installing cavity wall insulation is something you have to employ someone to do. Using specialist equipment they drill holes in your external walls and blow/spray insulation into the cavity.
It costs around £1,500-3,000 depending on the size of your home and which material you choose. Not something most people can do themselves.
But what type should you choose?
| Cavity width |
| Narrow (<50mm) – Choose mineral wool or polystyrene beads. |
| Medium (50-74mm) – Both of the above still apply plus foam can be used. |
| Wide (>74mm) – All of the above can be used but wider cavities may have to be modified (augured) to accommodate higher performance materials. |
| Property condition |
| If your home has had moisture issues in the past, or an investigation reveals problems with the cavity, skip cavity wall insulation. Or consider partial fill methods which leave an air gap between the insulation and inner wall to prevent moisture bridging. |
Solid Wall Insulation
Externally thermally bridging through your walls is limited by installing solid wall insulation. There are two types; internal insulation, and external insulation. External works better and costs more.
There’s no magic Physics behind solid wall insulation. Once you’ve insulated your loft and have cavity walls your solid walls are by far the biggest loss area. External solid wall insulation saves up to £660/year in detached houses. (Energy Saving Trust) Making it single biggest insulation upgrade by saving potential. But it costs £10,000-15,000 (Which). Yikes.
How does external solid wall insulation work?
Well the physics behind it is lovely.
You fix rigid insulation boards to your external walls and render over them. Eliminating thermal bridging through walls by adding thermal mass.
And internally?
Internal wall insulation fixes insulation materials to the inside of external walls. Cheaper than external but reduces the floor area of your home. It also doesn’t protect your walls exterior which can be important if you live in a busy street where you bounce sound off your neighbours walls.
Installer stupidity to avoid
Regardless of what type of insulation you choose there are several common mistakes installers make. That will drastically reduce the effectiveness of what they’ve just installed.
Mistake #1: Truncated loft insulation.
It sounds obvious but loft insulation needs to meet in the middle of your roof. If your installer tries to overlap the insulation by more than a few feet to avoid this they aren’t doing their job properly.
Mistake #2: Skipping ventilation when upgrading loft insulation.
Yes it’s tempting to ignore loft hatch and pipework leaks when you’re installing loft insulation. But doing so will leave significant thermal bypasses that negate the effects of your new loft insulation.
Mistake #3: Installing cavity wall insulation without checking it can be done.
Cavities with existing moisture problems, structural defects, or inappropriate filling materials may require removal of insulation if issues arise. Having your installer check for common problems will save you a huge headache. Expect them to do a borescope crawl of your cavities as part of the survey.
Insulation job done
Installing insulation is never going to be cheap. And if you have an older property with solid walls the priority insulation jobs will likely cost you well over £10,000. But done correctly you’ll cut your energy bills and improve your home’s comfort at the same time.
At Chris Credit Me blog I’m trying to offer an alternative to the traditional blog format. Rather than cover this topic in several smaller posts I want to go deep and give you everything you need to know in one place. If you want to make an informed decision about what to do and in what order sign up to my mailing list below to get updates and notifications when this guide is ready.
