Lochside House, by Haysom Ward Miller Architects. Photo by Richard Fraser.
Insulating your house is more than just about keeping it warm; it's also about saving energy and carbon which reduces your utility bills.
Effective insulation is a worthwhile investment, often paying for itself many times over. It also enables the efficient use of low-carbon heating systems like heat pumps, further enhancing your home's sustainability.
If you're planning a home renovation, you might be wondering about insulation costs and whether to focus on individual areas or the entire building?
In this guide, we provide expert guidance on how to insulate your home effectively and where to allocate your resources for the best results.
Quick links:
Important considerations before you insulate
Which types of insulation are best for your home?
How to best insulate suspended timber floors
Insulating existing concrete floors
The best way to insulate cavity walls
How to properly insulate a flat roof
Take a holistic approach to insulating your home
Building Regulations for insulating existing dwellings
How much insulation do I need?
Moving towards sustainable heating
Before delving into the specifics of how to insulate, it’s essential to consider the wider implications of retrofitting insulation to ensure the work performs as desired and achieves the best possible results. Below are the key items you'll need to address.
Vapour Control
Installing insulation can lead to the buildup of mould and damp within the building’s fabric – known as interstitial condensation – unless proper vapour control measures are in place. Vapour control and airtightness are often best achieved through the use of a vapour control layer (VCL). In some situations, choosing a 'breathable' or vapour-open construction buildup can be beneficial.
Airtightness
Installing insulation without proper draught-proofing measures will significantly reduce energy efficiency and thermal comfort. Airtightness is also vital for vapour control as it prevents uncontrolled airflow that can carry moisture into the structure. Without good airtightness, warm, moist air can infiltrate the building, leading to interstitial condensation. Installing an air and vapour control layer (AVCL) will reduce draughts and manage moisture levels, helping you create a warm and dry environment.
Ventilation
Proper building ventilation is crucial to maintaining a healthy indoor environment and will help keep vapour levels under control following the insulation works. Homes can be ventilated by passive means, such as natural ventilation strategies like cross-ventilation through windows and vents, or mechanical systems like extract fans and mechanical ventilation heat recovery (MVHR), which provide continuous airflow and improve indoor air quality through a ducted air system. Further, many existing timber structures rely on ventilation to keep them free of damp and rot. It’s therefore important that the new insulation maintains existing ventilation routes – for example, in suspended timber floors and pitched roofs.
Costs
Many insulation measures can be done economically, such as loft insulation. However, others can be a lot more disruptive and costly. For example, insulating solid walls internally or externally can be expensive due to the complexity of the work and the need to relocate services or refurbish finishes. When planning your insulation project, consider the return on investment in terms of energy savings, comfort, and property value.
Floor Area Loss
Adding insulation to the inside of the building can result in the loss of floor area; a key factor in smaller homes where space is at a premium. If insulating externally isn't an option, consider high-performance insulation which can achieve high thermal performance at reduced thicknesses. Performance and cost benefits decrease once a decent thickness is achieved, so focus on real-term benefits rather than maximising thicknesses to achieve optimal U-values. More on this later on in the article.
Heritage
Evaluate whether architectural details such as floorboards, joinery, and cornicing will be disturbed by the insulation works. Consider materials and installation methods that will protect as much of the heritage value as possible. If you live in a listed property, heritage protections may rule out many types of insulation work.
Building Services
Our homes contain building services – water, heating pipes, electrical cables, etc. – concealed within them. These may need to be relocated to accommodate the insulation. This can add to the cost and complexity of the project, so it's important to account for these factors when planning your insulation works.
READ: Vapour Control Layers Explained: Prevent Damp and Mould When Insulating Your Home
When insulating an older building such as a Victorian house, consider how the work may affect original details such as cornicing, skirtings, architraves and floorboards. Photo by Inigo.
When selecting insulation, it’s important to consider factors beyond just thermal efficiency. Factors such as vapour permeability, thermal mass, suitability, cost, and sustainability are just as important.
Wood fibre
Wood fibre is an excellent all-rounder and a great choice for insulating older homes. It is highly sustainable, offers good thermal resistance and acoustic properties, and provides thermal mass, which helps regulate room temperatures. Additionally, wood fibre is hygroscopic and vapour-open, meaning it can safely absorb and release moisture, helping to manage condensation. Available as rigid boards and flexible batts, wood fibre insulation is suitable for a range of applications and is also cost-effective.
Natural insulation
Natural insulation materials, such as sheep’s wool and hemp, are very sustainable and possess many of the properties of wood fibre, albeit with slightly lower thermal performance. Sheep’s wool is particularly good for insulating timber structures in old buildings.
Mineral wool
Mineral wool insulation is widely used in the UK due to its good thermal and acoustic performance at an economical price. It possesses thermal properties comparable to wood fibre and is permeable. However, being hydrophobic, it cannot absorb moisture, which limits its effectiveness in managing condensation in older buildings.
Plastic foam insulation
Multi-purpose plastic foam insulation, such as PIR and XPS, is vapour-closed (non-permeable) and best suited to buildings constructed with impervious materials, typically those built from the 1960s onward.
Due to its rigidity, it’s less effective at insulating between existing timber structures, which are irregular and difficult to insulate without leaving gaps. These gaps can be filled with expandable foam, but over time, it tends to deteriorate, compromising the structure. Flexible insulation is recommended for such situations.
Plastic foam is not ideal for insulating buildings constructed using traditional methods, as it can trap moisture, leading to issues like damp and mould.
High performance insulation
High-performance insulation materials, such as phenolic and aerogel boards, are good options for areas with restricted space.
Aerogel boards, being vapour-open, are a potential option for insulating solid brick walls where overall thicknesses need to be kept to a minimum. This is beneficial for reducing the impact on existing heritage details or maximising floor area. Some manufacturers claim 10 millimetre of aerogel can improve the U-value of a solid wall by up to 67%.
High-performance insulation can be twice the cost of other types of insulation, so the financial benefits need to be carefully weighed up.
Approximately 15% of heat loss from an uninsulated house occurs through the floor.
Depending on access, suspended timber floors can be insulated from above or below, each requiring specific steps and considerations.
Insulating from Below
For homes with a cellar or basement, insulating suspended timber floors from below can be done with minimal disruption. Begin by installing a vapour control layer (VCL)/ airtightness membrane. Next, insert flexible insulation between the joists, such as flexible wood fibre. If space and budget allow, add rigid insulation underneath the joists, like tongue-and-groove wood fibre boards. Finish by securing a breather membrane beneath the joists and insulation with battens.
Insulating from Above
Insulating from above is more disruptive since it involves lifting the floorboards to access the joists. After removing the floorboards, start by installing a breathable membrane, which supports the insulation and acts as a damp-proof layer. Then, insert flexible insulation batts and a VCL/ airtightness membrane. Finally, reinstall the original floorboards or use tongue-and-groove chipboard subflooring if an alternative finish is planned.
READ: Insulating Suspended Timber Floors
Insulating existing concrete floors presents significant challenges due to the necessity of raising the floor level, which can impact door and ceiling heights and create additional remedial work.
If raising the floor level is feasible, begin by installing a damp-proof membrane over the existing slab (since most uninsulated concrete floors lack adequate damp-proofing). Then, place timber battens with insulation between them and finish with chipboard or plywood sheeting, prepared for the final floor finish. To minimise floor depth, opt for high-performance materials such as phenolic boards or insulating floor finishes like cork.
READ: Insulating Concrete Floors
Approximately 35% of heat loss from an uninsulated house occurs through the walls.
For buildings constructed with solid wall construction, typically those built before the 1930s, external wall insulation(EWI) offers optimal thermal performance. EWI provides a continuous thermal layer across the wall's surface, enabling the masonry to function as a thermal store that enhances internal temperature regulation and thermal comfort.
Selecting a vapour-open, hygroscopic EWI system utilising materials such as wood fibre or cork also aids in managing moisture that naturally permeates through the wall.
If EWI is not feasible, perhaps due to planning restrictions limiting alterations to the external appearance of your home, internal wall insulation (IWI) becomes the sole viable option.
IWI can be applied directly to the wall's surface or installed between battens, finished with traditional lime plaster, plasterboard, or alternative interior linings like timber wall panelling.
Opting for thin internal wall insulation (TIWI) can minimise impact on valuable heritage details such as cornicing.
READ:
External Wall Insulation: A Guide to Retrofitting Homes
Is Retrofitting Internal Wall Insulation Worth it?
Low Energy House by Architecture for London features existing solid brick walls insulated internally with rigid wood fibre and finished with lime plaster. Photo by Lorenzo Zandri, Christian Brailey.
Improving the thermal efficiency of uninsulated cavity walls can be particularly challenging.
Cavity wall insulation (CWI) is often problematic, as it substantially increases the risk of penetrative damp – where rainwater penetrates through the cavity from outside to inside – and interstitial condensation due to alterations in the structure’s hygrothermal characteristics.
A safer approach involves using vapour-open IWI, such as wood fibre. Alternatively, for enhanced thermal efficiency, vapour-open EWI and cavity wall insulation can be considered (subject to planning restrictions), provided a continuous VCL is installed internally.
EWI shields the wall against penetrative damp, while vapour-open insulation and the VCL manage moisture levels to minimise condensation risks.
A considerable amount of heat loss in an uninsulated house escapes through the roof, accounting for about 25%.
If you have unused loft or attic space, insulating at ceiling level with cost-effective quilt insulation is an efficient way to enhance your home’s energy efficiency.
Leaving the space above the insulation uninsulated creates what’s known as a 'cold roof'.
To maintain continuous ventilation, it’s essential to leave a gap at the eaves and ridge. Before insulating between the ceiling joists, ensure to install a VCL. This prevents the accumulation of humid air within the roof void, thereby maintaining the timber roof structure in a dry condition.
If you’re converting a loft space into living accommodation, you will need to insulate the roof at rafter level. The least disruptive method is to create a ventilated warm pitched roof. Here, the roof tiles and waterproof underlay can remain in place, with insulation inserted between and below the rafters. An air gap is created between the insulation and underlay to ventilate away any moisture.
Alternatively, you can form an unventilated warm pitched roof by removing the air gap, installing full-fill insulation between the rafters, and adding a layer of insulation on top. This approach eliminates thermal bridges through the timber structure, significantly improving thermal efficiency, acoustics, and airtightness. The main drawback, however, is that the existing roof needs to be completely stripped back.
Insulating over the rafters with insulating sarking boards eliminates cold bridges through the timber structure and improves airtightness and acoustics. Photo by Ecological Building Systems.
READ: Insulating Pitched Roofs: An Architect’s Guide
Until recently, many flat roofs were thermally upgraded by inserting insulation between the joists while maintaining a ventilation gap above – known as a ‘cold’ roof buildup. We now know that this form of construction is thermally inefficient and prone to interstitial condensation.
Today, it’s recommended to create a ‘warm’ roof build-up, where a continuous layer of insulation is installed over the joists to eliminate cold bridges. A typical build-up consists of roof waterproofing, rigid foam insulation (such as PIR), a vapour control layer (VCL), structural plywood deck, and then the joists and plasterboard ceiling.
If you are creating a roof terrace, consider an inverted warm roof where the waterproofing is installed underneath the insulation to protect it from the roof finish, such as decking or paving.
To reduce the overall thickness of the roof build-up or use more sustainable insulation materials like wood fibre, consider a hybrid flat roof. Here, flexible insulation is placed between the joists, with rigid insulating sarking boards above.
READ: Insulating Flat Roofs: A Design Guide for New Homes, Extensions & Renovations
Glass is the least thermally efficient material in a building's external envelope, with up to 30% of heat loss occurring through windows and doors. Additionally, windows and doors are often sources of draughts, reducing thermal comfort.
Replacing poor glazing with more energy-efficient double or triple-glazed units can significantly improve energy efficiency and thermal comfort. For period properties with original sash windows, consider replacing just the sliding sashes with brush-sealed, double-glazed units to complement the building's architecture.
Installing slim profile insulated reveal boards around the opening, either externally or internally depending on the presence of external wall insulation (EWI) or internal wall insulation (IWI), will reduce cold bridges and maintain a more continuous thermal layer across the wall's surface.
Once you've chosen your insulation method and materials, aim for total coverage to maximise thermal efficiency. If you're working to a budget, it's much better to insulate the floor, walls, and roof to a decent standard rather than insulating just one element to a high standard.
The roof insulation should connect seamlessly with the insulation in the walls and floor to avoid any gaps. Line door and window openings with insulating reveal boards to minimise localised cold bridges.
If your budget will not stretch to full coverage, initially focus on areas that offer the best returns, such as the roof and glazing. Loft insulation is incredibly cost-effective and offers excellent payback. New glazing, although expensive, will significantly improve your home's energy efficiency, as glass is the least energy-efficient material in a building's external envelope.
Aside from the thermal comfort and ecological benefits that insulating our homes brings, it also needs to make financial sense for most of us. Insulating a building to a high standard can significantly reduce the amount of energy needed to heat our homes. However, if the upfront costs do not offset energy savings in the medium term, there is less incentive to carry out the work. The Energy Saving Trust provides insight into the potential savings that insulation and draughtproofing can bring.
Calculating the true cost of retrofitting insulation can be complex when considering all the work required to complete the job. Costs vary depending on factors such as the installation method, building size, complexity, and type of insulation used. For instance, insulating a suspended timber floor from below is much more economical than insulating from above because there is much less disturbance.
To get a more accurate understanding of how much it will cost to insulate your home, arrange quotes from qualified builders and ask them to itemise all the work needed to complete the installation. This should include the removal and reinstallation of radiators, skirtings, and any necessary making good and redecorations. You will get much better value for money if the insulation works are part of a wider home renovation project, as most of the additional work is likely to already be factored in.
Loft insulation is one of the most cost-effective methods of improving a building's energy efficiency. Photo by RJ Insulation.
There are government grants available for cheaper home insulation for some homeowners.
Homes with an energy performance certificate (EPC) rating of D to G or in Council Tax bands A-D in England or A-E in Scotland or Wales may be eligible for the Great British Insulation Scheme.
If you live in England or Wales you can apply through the gov.uk website.
Alternatively, visit Home Energy Scotland if you live in Scotland or NI Direct for Northern Ireland.
Approved Document L outlines insulation requirements for upgrading existing elements in homes. It acknowledges the challenges of retrofitting insulation into older buildings and provides both minimum (‘Threshold’) and target (‘Improved’) U-values to offer design flexibility and ensure the works are practical.
Table 4.3, extracted below, specifies the U-value requirements for these elements in dwellings:
Additionally, the guidelines also include provisions and considerations to mitigate potential issues such as damp and mould, loss of space, and long payback periods when insulating existing buildings.
These include:
When determining how much insulation to install, there is often a tendency to prioritise energy efficiency and U-values – the measure of heat loss through a building element – over practical considerations and real-world returns.
For instance, consider an uninsulated solid brick wall with a U-value of 2 W/m²K. Adding 40 millimetres of wood fibre insulation can reduce heat loss by 69%, achieving a U-value of about 0.60 W/m²K, which is within the Threshold guidelines. Increasing the thickness to 120 millimetres will achieve the Building Regulations' U-value target of 0.3 W/m²K, reducing heat loss by a further 18%. However, in terms of energy savings for an average UK house, this only amounts to about 100 kWh per year, or £10 at a rate of £0.10/kWh, and the difference in thermal comfort will be negligible.
If space is at a premium, it is sensible to reduce the amount of insulation to avoid compromising the interior space. Refer to the article links in the sections above for guidance on insulation thicknesses and U-values when insulating a floor, wall, or roof.
With the fast approaching deadline for the phasing out new gas boiler installations by 2035, many of us will soon need to explore low-carbon heating systems, such as heat pumps.
A well-insulated, draught-free building is vital for an efficient heat pump installation. Adopting a ‘fabric first’ approach will ensure that your home is ‘heat pump ready’ when the time for installation arrives.
Further, achieving exceptionally high levels of insulation can reduce the demand for heating to the extent that centralised heating is no longer required. Many buildings that meet Passivhaus EnerPHit standards are entirely heating-free. Instead, the heating needed is primarily for hot water, which can be provided by a heat pump or solar thermal panels.
READ: Retrofitting Heat Pumps into Houses: A Guide to the Pros and Cons
Insulating an existing building can be a complex task with many variables to consider, such as construction buildup, orientation, and exposure to the elements. It's crucial to balance thermal efficiency with factors like moisture control, thermal mass, suitability, cost, and sustainability to find the best solution for your home.
To navigate these complexities effectively, seek professional advice from a skilled architect and consult with specialist suppliers. They can provide tailored guidance to help you specify the right construction buildup that meets your specific needs.
About the author
Aron Coates, an architect with over two decades of experience, draws upon a wealth of residential architectural expertise, encompassing projects ranging from the restoration of historic structures to the design of contemporary homes.
