How to Insulate Your Renovation

Upgrading your property’s thermal efficiency as part of a renovation is a great way to enhance the performance of your heating system and save money on bills. Richard Webber takes a closer look at the options
by Richard Webber
26th January 2020

Insulation and fabric performance are integral elements in any renovation project. It’s estimated that around a third of a building’s heat escapes through poorly prepared walls. Even more seeps out through ill-fitting windows, doors and lofts.


Yet despite such stark facts, it’s so easy to focus too much on more aesthetically pleasing aspects of a property update, while underestimating the importance of the performance of the envelope itself.

Fabric first approach

A building’s structure – including walls, floors, ceilings, windows and doors – needs to be insulated as effectively as possible to prevent heat loss and, ultimately, minimise fuel costs.

“Some people are more likely to spend, say, £20,000 on a kitchen rather than those things you can’t see, such as insulation,” says Simon Storer, chief executive of the Insulation Manufacturers Association (IMA), the representative body for the rigid polyisocyanurate (PIR) and polyurethane (PUR) insulation industry.

Passivhaus Standard Insulation

To achieve Passivhaus standard, The Fergusons had to tightly insulate their home. The planners refused to allow an external application on the front of the house in order to maintain its traditional appearance, so the couple, and architect Andrew Yeats, came up with a different solution.

Exterior of white render house

There is now internal edge insulation, slotted into a specially created channel one metre below floor level, on the perimeter of the basement. In addition, vacuum insulated panels were used in the front elevation wall, with external insulation elsewhere.

Open plan extension

Paul was concerned that the fit-out could create moisture problems, so he bought WUFI software that enabled him to calculate whether the walls would become wetter or drier over a period of five years, given the orientation of the house in relation to predicted rainfall.

Fabric performance determines how a building will function in terms of retaining heat and providing comfort.

“It’s important that the shell of the house – the chassis, if you like – runs efficiently,” says Simon.

“In winter, you don’t turn the heating on and open the windows. But, in effect, that’s what you’re doing if you have poor performing fabric.”

Upgrading insulation

There are myriad materials to choose from when it comes to wall insulation. The first decision to be made is whether an internal or external application, or cavity wall insulation is best for the property you’re working on. The walls of your house form the largest part of your insulating decision, so it’s important you make the right choice.

Every building is different, however, and the style of yours will dictate what type of application and product is most practical.

Quick guide to U-values

U-values measure a material’s insulating effectiveness.

Generally, the lower the U-value, the better the thermal insulation.

Usually, insulation manufacturers provide guides to the U-values achieved when their products are utilised in building projects; some even have U-value calculators on their websites.

U-values are calculated by taking the rate of transfer of heat through a structure, such as a wall, and dividing it by the difference in temperature across that structure. Its units of measurement are W/m²K.

A huge advantage of pumping insulation, such as mineral wool, polystyrene beads or polyurethane foam, inside a cavity is that it doesn’t affect the building’s external appearance. Nor does it eat into the internal floor space or lead to extensive decorating.

However, many older properties don’t have any cavity walls. According to the National Insulation Association (NIA), about a third of UK homes have solid walls, so insulating the outside of the property or interior walls is the only option.

One benefit of an external application is that the work can be completed without adversely disrupting your day-to-day living. But opting for exterior insulation isn’t always practical because it alters the appearance of the building significantly. If the property is a listed structure or situated in a conservation area, this may be prohibited.

Energy Efficient  Bungalow

Mandi and Tim Horwood wanted to ensure a good level of energy efficiency in
their renovated bungalow.

Exterior of white render cottage

The vaulted ceilings, as well as exterior walls in the two open-plan zones have had insulation board added internally.

Kitchen extension

This took up minimal space and has the added benefit of hiding wiring and pipes without cutting into the walls

Interior products usually comprise ready-made rolls or boards. It’s often cheaper than external insulation but will impact on your floor space and result in everything on the walls having to be removed, including sockets, picture rails and skirting boards. One advantage, though, is that internal wall insulation can be carried out on a room-by-room basis.

To reduce the amount of floor space lost to insulation boards, consider using the thinnest material possible with the greatest performance level. “The product only needs to be applied to the walls facing the elements outside.

“So if you’re a mid-terrace house, it’s just the front and back walls,” says Simon. “PIR is a good solution for these situations because it’s among the most thermally-efficient available. But there are other options, including sheep’s wool, cork, polystyrene and more. But often the thicker the component, the poorer the performance.”

For floors and lofts, PIR boards are equally useful. “If you’re in an older house, you’re more likely to have joists with a void underneath,” says Simon. “So you can insert  the material between joists underneath the floor boards without affecting air flow as you’re not filling the void.”

Insulation materials

There is a wide choice of insulating materials on the market these days. Christine Lellig, campaign director for Wood for Good, the timber industry’s campaign to promote usage of timber in design and construction, says using wood fibre insulation in a renovation has many benefits.

“Natural fibres like this contain chemically-bound water, which absorbs heat energy and helps prevent overheating in summer. Wood fibre is very dense, which also slows the passage of heat through the fabric. This is especially important for rooms in the roof.”


A draughty 17th century Shropshire farmhouse, Houlston Manor, was transformed into a cosy modern home with the help of external wood fibre insulation.

Man rendering external wall

The unlisted box-framed farmhouse had undergone various alterations over the decades, yet remained chilly. The windows were very draughty and the southern gable end let in wind and rain due to a great deal of failed pointing.

To resolve these issues, the owners wanted to replace the fenestration, insulate the walls and improve the overall appearance.

House in construction phase

When thoughts turned to insulation, the owners chose a system that combined external wood fibre board as a protective thermal layer and lime render as a weather-proofer. Being a solid-walled construction with no damp-proof course, breathable insulation materials were chosen to allow vapour to transfer through walls to help reduce condensation risk.

White render house with garden

Available in various thicknesses, 160mm wood fibre insulation boards were fixed directly onto the brickwork before being covered with two coats of lime render. As a result of the improvements, the owners enjoyed a 59% reduction in energy costs and expect further improvements as the original brickwork dries.

Among the material’s selling points is that it helps regulate humidity and prevent condensation and mould growth.

“Wood fibre has good breathability, allowing the dry transport of moisture,” says Christine. Made from renewable resources, these fibre boards can be reused and are easily recycled at the end of their life, too. Cork is another alternative that’s becoming increasingly popular for use as insulation.

Kieran Streames, director of SPD UK, which supplies a wide range of products made from this natural material, including insulation boards, says “Cork can be used in both flooring and wall applications. The unique honeycomb structure of its cells means that it has a low conductivity to heat, noise and vibrations. It’s also highly energy efficient.”

Another important factor to consider from a safety perspective is that cork is fire retardant and burns without a flame. “This can help to slow the progress of fire in your home and avoid the emission of toxic gases, which other materials can produce when burned,” says Kieran.


For anyone whose renovation project centres on a new addition to the property, you need to follow the Building Regulations when it comes to insulation. That said, it’s worth remembering that when the government last updated the rules regarding performance in buildings back in 2013, extensions were excluded.

Minimum standards, therefore, remain at 2010 levels. So, liaise with your designer or builder to ensure you get the best performance rather than the minimum required under the current rules.

Conversely, attaching a thermally-efficient extension to a poorly insulated home won’t make it cheaper to run overnight. If finances allow, consider improving the efficiency of the whole house.

“If not, all you’re doing is heating the environment,” says IMA’s Simon Storer.

Understanding weak spots

It’s estimated that a typical property loses up to 20% of its heat through air leakage. This often happens through gaps around windows and doors, for example.

Mike Easdon, vice chairman of the National Insulation Association (NIA) says, “Draught-proofing can greatly reduce leakage. It’s not just cold, windy days when valuable heat could be lost. As warm air accumulates inside a property, it’s also escaping through gaps all the time.”

While insulation and eliminating draughts from weak spots – such as windows, doors and roofs – are vital in producing an efficient house, attention must be paid to implementing adequate ventilation, too.


These elements hinder the thermal performance of your home.

The lion’s share of damp walls in a property
is caused by condensation, where warm air hits cold surfaces forming water droplets. High humidity in a room can produce the same effect.

A substantial amount of moisture is created in our everyday lives just through living in the space. Envirovent, manufacturers of ventilation systems and extractor fans, state that “four people living in a three-bedroom property would create 112 pints of moisture a week just from breathing, cooking, showering and boiling a kettle.”

According to Envirovent, primary areas and solutions to consider when attempting to eliminate problems with condensation and damp at home are:

Control humidity in kitchens and bathrooms through the use of good extractor fans.

Ensure there is sufficient ventilation throughout the property. When it’s warm outside, open windows in rooms used regularly to improve airflow.

Heat your property adequately in order to improve the internal temperature of surfaces and reduce the likelihood of condensation.

Check your windows. Over time, sealant around frames may become damaged and allow moisture to seep into your home. An excess in moisture levels can result in condensation.

“Airflow is important. You don’t want to create other problems, such as condensation and damp, which is why the work must be carried out professionally,” says Simon.

Complete renovation guide part 6: Transforming your homes exterior

The Energy Saving Trust reports that for a standard detached home, up to £255 per year can be saved on energy bills by installing cavity wall insulation. Adding loft insulation could increase this sum by a further £225.

Protecting solid walls creates an even bigger total – up to £435 – while draught-proofing windows and doors adds additional savings. So, choosing the right workforce is vital to achieve the maximum benefit from your project.

Main image: Amanda Turner

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