‘Hard to Treat’ Buildings – Our Key Considerations

Buildings that aren’t straight forward to insulate are something we work on a lot! There are millions of so called ‘hard to treat’ buildings across the UK which require thermally upgrading to improve the living conditions within them and also to reduce carbon dioxide emissions from them.

Many of these buildings are solid walled and require insulation either internally or externally and to safely do this a good understanding of building physics is required. The buildings are mostly pre-1940 and are made from vapour permeable materials that transport moisture very well.

Some well intentioned retrofit schemes have been implemented using materials which are not compatible with these older buildings, with the result that the living conditions have worsened for the occupants. Some have caused significant building damage and have had to have the work reversed to avoid structural defects and making occupants ill.

The consequence of these issues is that many retrofit projects do not attempt to work on these types of property as they are seen as being too complicated and too risky.

One size does not fit all

Buildings which have been altered and added to over many years will often incorporate many different construction methods. Ensuring that a scheme is going to be successful involves careful assessment of each element and the different construction methods within it. This allows specifications to be created and assessed in line with the targets for the scheme and the preferences of the owners or occupants.

Schemes can have several different wall, roof and floor specifications. Ensuring that each solution is practical, compatible with the rest of the solutions and will give consistent finishes throughout is vital. This often means that different systems are required to complete the work, making projects supported by a single manufacturer unlikely to happen.

This is where having a portfolio of different, complimentary systems becomes useful so that whatever the scenario, systems can be specified and supported by a single, impartial supplier.

U-value is not necessarily everything

Whilst it is important to focus on getting the heat loss down to a minimum the positioning of the insulation makes a big difference to the behaviour of buildings and the sense of comfort experienced within them. Our sense of comfort is partly derived from infrared radiation emitted by objects around us and so ensuring good levels of this radiant warmth can be key to insulating solid walled buildings.

When applying insulation to the interior of the walls of a building you separate the plaster layer from the masonry and the insulation between obviously reduces heat flow. The surface plaster will warm up to the internal ambient temperature, begin to emit this radiant infrared heat and consequently feel warm.

Additionally, even small quantities of internal insulation prevent surface condensation and mould growth and can make a huge difference to indoor air quality and the health of occupants.

External insulation does not do this and much more insulation is required to achieve the same level of radiant heat, and therefore comfort, as internal insulation. However, if the building is to be continuously heated to maintain it’s internal temperature than external insulation may work more effectively, particularly as it keeps the thermal mass of the masonry walls inside the insulation envelope.


Moisture tends to be the key substance to manage when retrofitting buildings, particularly solid walled buildings as they are usually built with vapour permeable materials that often absorb and release lots of water.

When changing the construction make-up of walls, roofs and floors it is imperative that moisture flow is considered, along with any associated risks from interstitial condensation. Whilst the materials used should not create a barrier to moisture flow it is important to use vapour control membranes or other materials to regulate moisture flow.

Materials such as wood fibre insulation are often specified for these scenarios due to their ability to manage, store and release moisture, properties that are much more important than thermal conductivity alone.

Therefore, a good understanding of material and building physics along with the ability to use various forms of software analysis is important when creating specifications. This highlights any risks and ensures that the long term impact of any intervention is positive.

Thermal bridges

Due to the often complicated layout in many older solid walled buildings there are many junctions which create thermal bridges. One of the main reasons retrofit schemes often fail and cause mould growth internally is that thermal bridges have not been eliminated from the building.

Heat can be seen like a river (go with me on this!). If you dam half of it the other half just flows twice as fast. This means that if you insulate most of a building but leave areas where partition walls or other protrusions and obstructions prevent the insulation being installed, you are actually more likely to get mould growth in these areas than if no insulation had been installed.

When creating insulation strategies all of these junctions must be insulated, even if it is with a thinner layer of insulation, to prevent this issue occurring. This will also ensure that the building performs as it was intended to.

Retrofit strategies

The Passivhaus retrofit performance strategy called Enerphit, along with the AECB’s carbonLite targets are very useful when deciding on how to insulate buildings. As stated above, internal and external insulation create buildings that behave very differently which is why different thermal targets are given for the two different types of insulation.

Both standards give a good guide as to what U-values to target and also to what energy savings can be achieved. There is also a wealth of guidance on detailing that can be used to effectively insulate and retrofit buildings.

Insulating with resilience

Insulating timber structures such as roofs or timber framed walls can be achieved using very lightweight, low thermal conductivity insulation materials such as mineral or glass wools.

However, with the changing climate it is not only winter cold we need to insulate against. Summer heat is becoming more of a challenge and can be equally as deadly as winter cold to occupants in poor health or with limited mobility.

The thermal mass of insulations changes what decrement delay is achieved across a structure. Very lightweight insulation allows heat to pass through a structure quickly and in the case of timber framed buildings or roof/attic constructions, can result in significant overheating. Increasing the thermal mass of the insulation used increases the decrement delay and reduces the risk of overheating.

Additionally, denser insulation gives better sound insulation which reduces stress from external noise and improves the mental health of occupants. This is particularly significant in urban areas or areas with high traffic volumes.

Build with confidence

With each of our products and systems we can guide you through from initial assessment, through design strategy to specification and implementation. Through our initial product assessments we know which materials will work in different areas and scenarios so that you don’t need to search endlessly for the right product fit.

After 14 years of building restoration and 13 years of supplying materials, we have the practical and technical understanding of each element of historic and modern buildings. If you have a project that you’d like us to help you with please contact us.

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  1. Do you cover North Wales?. I bought an old signalman cottage a year and a half ago. And the orinal sons built part is colder than the added on extension which was built in 2007. Thanks

    • Hi. Yes, we cover all of the UK so we can easily ship products to you in North Wales. Please feel free to call us and discuss your project further.

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