Your home insulation Guide
We’re a cosseted generation that demands diva like temperature regulation in our homes. In my household there are shivers and grumbles from the wife and kids if the temperature dips below 19C.
Only 40 years ago, people woke up to ice on the inside of their windows and endured temperatures that were, on average, 4 degrees colder that today. We quite rightly now expect our homes to be heated and cooled to a comfortable temperature.
A well insulated house is warmer in winter and cooler in summer while using less energy. Insulation also creates a quieter home, cutting down on noise from cars, planes or noisy neighbours. It’s what all home owners, businesses and the government want, but there has been a very slow progress in retrofitting old homes with insulation. Highly insulated and airtight houses use next to no energy so this would save thousands in energy bills and more importantly, drastically cut down on carbon emissions. So what are you waiting for?
There are lots of different types on insulation from different brands and they all have different applications so it’s not always easy for us to work out what to use and where. Cost is also a big consideration so this guide will hopefully shine some light on how best to go and insulated your home. Some can be installed by yourself, and some will definitely need a professional to do it.
Heat Transfer Explained
To begin, it’s worth understanding heat transfer and how it is lost through the fabric of your home. Heat is transferred via conduction convection or radiation.
Conduction is where heat is transferred from particle to particle that is in direct contact with one another. For example, when you touch a hot surface, the heat is conducted to your skin. Reducing conduction involves trapping air in pockets or bubbles, and because air is a poor conductor of heat, this will form an insulating barrier.
Convection is the transfer of heat through a liquid or gas, typically air flows in this instance, these would be the cold draughts coming through the gaps in your house. Reducing convection heat loss requires a physical barrier to the movement of air.
Radiation is where heat is transferred through infra red which all objects emit if it is above zero degrees kelvin (-273C). For example, you can feel the heat radiated off a brick wall that has been in the sun all day.
To reduce heat lost through radiation you just need a reflective surface. Think of the foil blankets given at the end of marathons to reduce heat lost from rapidly cooling finishers.
Measuring Insulation Effectiveness
Insulation will reduce one, two, or all of these heat transfers. How we measure the insulation is where I can be the nerd and pour over technical specifications. There are other ways to measure the effectiveness of insulation but these are the most common that you will come across most fo the the time. You really only need to consider the following;
U-Value - Expressed in W/m²K
Measures the amount of heat that passes through a given material. The better the insulation, the lower the U-value.
R-Value - metres squared Kelvin per Watt (M2K/W)
Refers to a given materials resistance to heat passing through it. The opposite of U-Value, the higher the resistance the better the insulation.
Choosing Insulation
When it comes to choosing home insulation, there are several factors to consider, such as the climate, your budget, and the specific areas you want to insulate.
It’s no surprise that the colder it is, the more insulation you will need to keep your home at a comfortable temperature.
Generally speaking the cheaper insulation materials have higher U values and will need to be installed to a thicker depth than more expensive, higher performing materials to insulate to the same level. Given enough depth, even simple materials like straw can have impressive insulation properties as many straw bale house owners can attest to.
As you will likely be looking to insulate a particular area, I will cover the areas of the house and the different insulation most commonly used.
Roof insulation
Normally the 1st port of call in improving your home insulation because it offers the best improvements for relatively little time and money.
Blanket Insulation
Fibreglass Insulation: Fibreglass is one of the most widely used insulation materials. It consists of tiny glass fibres and comes in batts or rolls. Benefits include affordability, fire resistance, and it’s available in various R-values. However, if you’ve ever laid it, you’ll know just how nasty it can be, causing skin and respiratory irritation. Personally If I was to buy loft insulation again, I would avoid fibreglass.
Mineral Wool Insulation: Slightly more expensive than fibreglass but with better sound insulation, ease of installation, moisture and fire resistance. They both have similar insulation performance though.
Sheep's Wool Insulation: A natural, more environmentally friendly option that doesn’t irritate your skin or lungs. Treated to prevent little beasties from eating and making nests in it so you don’t have to worry about an infestation.
The only catch is that it’s a lot more expensive than fibreglass and the majority of people tend to opt for the cheapest insulation option.
Recycled polyester: Similar to the filling in your coat, this is a great alternative if you want to lay it yourself but have an issue with fibreglass. Again, it’s much more expensive than fibreglass (although less than wool).
Rigid Foam Insulation
Rigid foam boards, typically made of polystyrene, polyisocyanurate, or polyurethane, offer high insulating value and moisture resistance. The brands that are most common are Kingspan and Cellotex, come with he aluminium foil backing to reflect infra red radiation and are commonly used in walls, roofs, and foundations. Rigid foam is lightweight, easy to cut, and can be layered for increased insulation. However, it can be more expensive than other options and requires careful installation to avoid thermal bridging.
Reflective Insulation
Reflective insulation uses materials like foil-faced kraft paper to reflect heat radiation. It's often installed in attics, roofs, or as a radiant barrier in hot climates. Reflective insulation works best when there is an air gap between the insulation and the heat source. It's less effective in colder climates where heat loss occurs through conduction and convection.
Loose Fill
Cellulose Insulation: Cellulose is made from recycled paper products and treated with fire retardants. It's an eco-friendly choice that offers good thermal and sound insulation and can be very cheap. However cellulose is dusty and messy and is only suitable for loft space that isn’t used or boarded over. It may settle over time, so periodic inspection and reapplication may be required.
Cork Granules: Also a lightweight, natural and sustainable option but is rarely used to to much higher cost.
Spray Foam Insulation
Spray foam insulation expands upon application, sealing gaps and cracks effectively. There are two types available
Open cell - Porous and allows vapour and moisture through, soft to touch but not as effective as closed cell.
Closed cell - Rigid and hard insulation that is a more effective insulator and as a vapour barrier.
Both types provides excellent insulation and will reducing air leakage. Spray foam is more expensive than other options but offers much better energy efficiency. Professional installation is recommended due to the use of specialised equipment.
There is a big catch though; currently in the UK, many mortgages providers won’t lend on a house that has spray foam insulation added to the roof. Forming a vapour barrier will reduce loft ventilation and can cause a build up of moisture that will condense on cold surfaces. There have been some horror stories where roof timbers have rotted because of water ingress and excess condensation.
The Home owners alliance have advised not to use spray foam until the issues are resolved. The royal institute of chartered surveyors also state that a loft with spray foam applied may affect the valuation of a property.
Installed correctly though and spray foam can offer excellent insulation but given the potential drawbacks along with the high cost, it is a hard one to recommend.
Cost per M2
These costs are benchmarked on the best selling brands of each insulation type. There are cheaper and more expensive versions of each type of insulation but below is for a rough guide to their pricing. All methods are for a DIY installation except the spray foam where professionals are needed and this is reflected in the price per M2. They are correct as of July 2023 but will change over time.
Material | Depth (mm) | Cost Per M2 | R-Value |
Fibreglass | 300 | £8 | 6.8 |
Sheeps Wool | 300 | £40 | 8.3 |
Polyester | 300 | £27 | 7.5 |
Rigid Foam | 150 | £23 | 6.8 |
Reflective Foil | 45 | £9 | 3.0 |
Loose Fill (Cellulose) | 300 | £30 | 7.7 |
Spray Foam (Open cell) | 200 | £45 | 5.7 |
Spray Foam (Closed cell) | 165 | £55 | 6.6 |
Spray foam will have varying thickness depending on application. Thermal conducitvity also varies but expect an average of 0.025 W/mK for closed cell and 0.035 W/mK for open cell.
Wall Insulation
A pretty broad category that covers internal, external and cavity wall.
Cavity Wall Insulation
Cavity wall insulation should be highest on the priority list. It’s very cost effective, can be done by professionals quickly and with little to no disruption to your house. If your house was built before the 1920s then it will likely not have a cavity. In which case you will need to internally or externally insulate. With many period properties internal insulation is the only option.
They drill holes in the mortar of the wall and blow either mineral wool or polystyrene beads, both have very similar performance.
Many cavities have lots of debris in them that will need clearing out which greatly increases the cost of installation (unless you’re doing it yourself). Cavities also need to be dry otherwise the insulation can transmit the moisture to the inner walls and cause damp problems. Property inspection with a bore hole camera and installation is key to preventing issues like this.
When building new houses, rigid PIR foam sheets or mineral wool sheets are used which is simple to install with an open structure. PIR board has better insulating performance 0.2 W/m2k versus mineral wool at 0.28 w/m2k.
External Insulation
The most expensive but also the most effective insulation is to clad your house with PIR insulation board then it is typically rendered. Brick slips or wooden cladding can also be applied if you want or need that kind of look. The thicker the better but most people opt for 100mm. Solid brick walls have a U-value of around 2.1-3W/m² and adding 50mm of expanded polystyrene (EPS) will take the U-value down to 0.5W/m²k, which is a substantial improvement, but 100mm takes it down to just 0.27W/m2k. Even thicker insulation offers decreasing improvements and can practically be awkward fitting it around windows and doors.
Internal Insulation
For those who can’t insulate externally, internal is the way to go. Cost wise it will likely be much cheaper than external insulation (sometimes half) but more than cavity wall insulation. However this is the most disruptive process that also involves taking off radiators, skirting board, cornices and architraves. Plumbing and electrics may have to be adjusted to the new walls although you should only have to insulated the walls on the outside of the building.
You can of course opt for wallrock thermal lining paper that would avoid all the disruption to that of wallpapering. 3-4mm of insulation has a noticeable impact and can reduce the heat loss of the room by 15%. As with all insulation, It will also reduce condensation and smooth out rough walls.
The big savings are with thicker insulation though and can vary from 50mm to 100mm. Depending on the thickness of board you go for, it will also decrease the usable space of the room but this often isn’t greatly noticeable. If you have the space and are already having the disruption and repositioning skirting, plumbing and electrics then definitely go for thicker insulation.
Insulation board can be stuck and screwed straight to the existing plaster. Plasterboard goes on top then skimmed with plaster. (The cheapest option). There is also insulated plasterboard available that is easier and quicker to install which is basically insulation bonded to plasterboard.
A much messier option is to take the walls back to brick then insulate. This is the best option If the plaster is in poor condition, there is damp that needs treating, you really want to maximise the space in the room, or a combination of all three.
You also have the option to batton out the wall and insulate the gaps with PIR board or mineral wool batts. This option will however take up more space as mineral wool isn’t as effective an insulator.
Floor Insulation
Much depends on the construction of your house on the type of insulation you need. Clearly we’re only needing to insulate the ground floor.
Suspended Wooden Flooring
If you have carpets you can simply buy a thicker, higher tog underlay up to 12mm thick which is the most cost effective way to improve your insulation. Thicker carpets and rugs can also be worth spending the extra money as well as adding comfort.
For wooden flooring you can buy thin XPS (hard) insulation that goes underneath that will reduce heat loss. These boards are simple to lay and usually only 5 or 6mm thick.
You can of course lay thicker insulation which is normally the same PIR insulation mentioned before but this would have an impact on the floor height, skirting, plumbing and possibly electrics.
Note that if you have underfloor heating, insulation should already have been laid underneath the heating element and screed.
A more invasive and costly way to insulate would be to take up the floorboards and insulating in-between the floorboards with either mineral wool batts or solid PIR foam held in place with netting or batons. This is only really advisable if you are having other work done at the same time.
Solid/Concrete flooring
The most simple to insulate as the only option is to lay hard XPS insulation board over the top before laying your flooring of choice. Your concrete flooring may already be insulated underneath so if in any doubt, drill a hole (careful not to hit any pipes or wires) and see if any insulation comes up in the debris.
The more insulation the better, right?
You can keep adding more insulation to your house but the improvements will have less of an impact. The homeowners in the 1960s who first put 100mm of loft insulation down saw a much bigger reduction in heat loss than the people now increasing it from 200 -300mm.
There is a sweet spot where the right amount of insulation is the best balance between cost and reductions in heat loss. The graph below shows this very well where you see the bulk of the improvements up to an R-Value of 20 whereupon big increases will likely have a less noticeable impact.
New Tech
New Technology has been and is being developed that is transforming the building and insulation industry. Here are some of the best ideas
Aerogel Insulation
Aerogel is a highly porous and lightweight material that consists of 90-99% air. It has excellent insulating properties and a very low thermal conductivity. Aerogel insulation is thin and flexible, making it suitable for various applications, including walls, roofs, and windows. It offers high R-values with minimal thickness, providing effective insulation in limited space.
I had a quote from Thermablok and It’s definitely a premium option at around £70 per m2 whereas 100mm PIR board is around £16 per m2. On the plus side, It can be used selectively around metal beams very effectively and it may also negate the need to dig down in a basement as a much thinner layer can be added before flooring is installed. It may also be worth it, if it is impossible or too expensive to move all the fixtures and fittings needed for the thick PIR insulation.
If these products were more cost effective, it would revolutionise the home insulation market and the performance of all the homes that installed it.
Vacuum Insulation Panels (VIPs)
VIPs are panels that consist of a core material enclosed in a gas-tight envelope, from which air is evacuated to create a vacuum. The absence of air reduces heat transfer through conduction and convection. Vacuum insulation panels provide exceptional thermal performance and are ideal for space-constrained areas. They are commonly used in refrigeration, cold storage, and high-performance building applications.
Phase Change Materials (PCMs)
PCMs are substances that can absorb and release large amounts of heat energy during phase transitions (such as melting and solidifying). When used as insulation, PCMs absorb heat during the day and release it at night, helping to stabilise indoor temperatures. They are particularly useful in regulating temperature fluctuations in buildings with high diurnal variations.
Green Insulation
With a growing focus on sustainability, there is an increasing interest in environmentally friendly insulation materials. These include recycled materials such as recycled denim, cellulose made from recycled paper, and insulation derived from agricultural waste or renewable resources like sheep's wool or hemp. These green insulation options offer reduced environmental impact while providing effective thermal insulation.
Insulated Vinyl Cladding
Vinyl cladding is often used instead of wood for its longevity and maintenance free properties, but it is also being integrated with insulation so that you can clad and insulate your home in one go which would save costs.
Air tightness or Insulation?
It should be pointed out that there is a slight obsession with ultra high R-Values. In the houses I’ve looked at, it is more often air leakage or draughts that have been a bigger factor in heat loss. Energy Star (the US version of Energy saving trust) estimates that houses lose 20-40% of heat through air leaks although figures from the UK vary wildly from 15 - 50%. My advice is that before spending £10,000 on exterior insulation, spend £200 on draught proofing your doors, windows, loft hatch, unused chimneys, floorboards, pipework holes, skirting and any other cracks you can find.
Problems with damp that occur when insulation is installed isn’t a damp or a condensation problem, it’s an air leakage problem. Warm, moist air leaking into cavity wall, or loft space will condense on colder surfaces and cause damp. Small gaps left in the insulation after installation can cause big problems.
Installation and maintenance?
Another issue I’ve seen in almost every house I’ve seen is incorrectly installed insulation. Along with poor maintenance, these have a far bigger impact on the effectiveness of your insulation than chasing a marginal improvement through high tech materials.
The most common issues in the loft are where insulation has been moved by the owner or a past tradesman but not replaced. Insulation that has been compressed with junk, has slipped or been saturated with damp will have reduced effectiveness. Pipes will be poorly lagged, or not at all. Viewed with a thermal camera, gaps in the wall insulation will be visible. This means that insulation often performs to standards below that which is claimed.
Unfortunately much of the fabric of your home that keeps your house warm in winter and cool in summer, is hidden from view. Builders know this and so will often cut corners and have few incentives to make sure your home is as well insulated as it should be.
Getting a heat loss survey will identify the parts of your house that are losing the greatest amount of heat so you can look to reduce it. Using established professional installers who make sure there is continuous and even insulation is as important as the type of material used.
Final Thoughts
Fortunately more and more people are wanting to make their homes more efficient with insulation and there there is a growing number of installers setting up to satisfy that demand. Growth of 19% in the sector per annum shows that everyone is taking it seriously and acting to make the changes.
For the public, you need to know the best, most effective areas of your home to put your money into. Spray foam specialists will always advocate that their methods are offer the best value for money, double glazing salesmen will say their windows offer the biggest savings on your energy bills, you get the idea.
Good energy surveyors or installers who offer holistic approach to insulating your house will be able to point you in the right direction that will give you the best outcome in terms of reducing your energy loss for the money you spend on home improvements. Getting an impartial view on insulation will give you more confidence to do more to improve your home without the stress of getting it wrong. This is what we all need to do if we are to save this planet of ours.
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