Insulating solid walls with ISO-THERM - an innovative damp proofing system

Solid wall properties can suffer from damp, condensation and mould, and are less energy-efficient than properties with cavity walls. This is because they’re difficult to insulate and have no air gap to prevent the transfer of moisture.

ISO-THERM is an innovative solution for damp proofing and insulation strategies in solid wall properties. Thanks to its unintrusive installation and high performance, it’s an effective and convenient choice for treating solid internal walls. Read on to find out more as we explore its properties, applications and how to install it.

What is a solid wall?

A solid wall has no gap, or cavity, in it. It’s a traditional form of construction with one layer of brick or stone. 

In contrast, a cavity wall has two parallel layers (or skins) with a gap between them. The cavity blocks moisture transference and prevents damp, and the cavity can also be filled with insulation. 

Buildings constructed before the 1930s were normally built with solid walls. According to a report on Household Energy Efficiency by the Department for Energy Security & Net Zero, there are 8.5 million homes in Great Britain with solid walls. Of these, only around 10% have solid wall insulation. 

How to tell if a wall is a cavity wall or a solid wall

There are two simple methods you can use to tell if your walls are solid or cavity:

• Brick pattern: If your walls are exposed brick, you can tell by the pattern of the bricks they were laid. If they’re all end-to-end, with the longest side visible, it’s likely to be a cavity wall. If they’re laid in a regular pattern with the long side and short end visible, it’s likely to be a solid wall.

Solid wall pattern

Cavity wall pattern

• Thickness: You can also measure the thickness of the wall to determine if it’s cavity or solid at a door or window. If the wall is more than 26 cm thick, it’s probably a cavity wall. Anything less than this is likely to be a solid wall.

Issues with solid walls

Solid walls can cause a number of issues for homeowners. These are normally around insulation and damp - issues that can seriously affect the quality of life.

• Damp problems: It’s easier for damp and moisture to penetrate solid walls than cavity walls. The air gap in a cavity wall helps prevent damp travelling into the property, but it can move freely through solid walls. Solid walls are also more likely to be cold and suffer from cold spots. As a result, they may be more prone to condensation - leading to damp patches. These damp issues can lead to an increase in harmful black mould.
• Insulation/thermal performance: Solid walls are normally built without any insulation, leading to poor thermal performance. Cold spots on walls are more susceptible to condensation, but the inability to retain heat also means you’ll need more energy to heat your home - increasing your energy bills.

These problems can be solved by retrofitting internal wall insulation and damp proofing strategies. Previously, these have been labour-intensive and difficult to install, but ISO-THERM provides a quick and effective solution.

What is ISO-THERM?

As we’ve discussed, using ISO-THERM is an effective, easy-to-install and cost-effective way to insulate and damp proof solid walls, but what is it?

ISO-THERM is a thin thermal wall insulation (TIWI). Supplied in rolls, it can be easily cut to fit any installation location. Due to this, installation causes minimum disruption. It can be applied directly to plastered walls using ISO-THERM Superbond Adhesive.

It’s made from a specially woven polyester-based textile material. Features include:

• Easy to install with minimum disruption
• +/- 4mm profile
• Provided as 1-metre wide rolls
• Flexible and lightweight
• Can be easily cut
• Vapour permeable but waterproof

Once installed, ISO-THERM will keep walls dry and warm, acting as a damp proofing and insulating layer.

Solid wall damp proofing

Because they lack a cavity that prevents damp from travelling through them, solid walls need a damp proofing strategy to prevent them from being affected by this issue. 

There are a number of different solutions available, which we explore in more detail in our comprehensive ‘Damp Proofing Walls’ guide. But protecting solid walls from penetrating damp with an external waterproofing treatment is a sensible measure.   

At Permagard, we have several solutions available. 

• MicroShield ULTRA - a professional product that provides long-term protection against moisture penetration while retaining breathability.
• PermaSEAL Facade Cream - a high-strength facade cream that protects against penetrating damp.
• PermaSEAL Brick Sealer - a water-resistant, breathable brick and masonry sealer for external walls.

Once you’ve protected your external walls, applying ISO-THERM thermal wall insulation isolates finishes from damp, and acts as a solution for both internal damp proofing and insulation. 

The combination of an exterior water repellent and an internal wall insulation can work very well, with the water repellent letting the wall breathe and dry out whilst stopping further water ingress. This already increases the wall’s thermal efficiency, and introducing internal wall insulation further improves this. 

Careful consideration must be given to eliminating external defects before the water repellent and the TIWI is applied. It’s important to reduce the risk of interstitial condensation as much as possible.  

For more information about internal damp proofing, see our Complete Guide.

Insulating solid walls

Heat escapes more quickly through solid walls than cavity walls. With a cavity wall, insulation is almost always a case of filling the cavity with insulation. With a solid wall, there are a few different methods for insulating them, but they all involve retrofitting.

Insulating a solid wall is definitely worthwhile. It will make your home warmer and more comfortable, saving you money on energy bills and reducing your carbon emissions.

Retrofitted insulation can be applied internally or externally. External wall insulation usually offers greater insulation thickness, but it can be very expensive in comparison and there are many properties that it just won’t suit.

Along with the cost, you will likely require planning permission. It can also be tricky to detail around window reveals, sills and eaves, and any slight cracks or poor fitting can lead to water ingress and trapped moisture.

There are lots of different types of Thin Internal Wall Insulation, and these will normally be cheaper than External Wall Insulation. Cost savings usually come down to how long it takes to install the system. For this reason, ISO-THERM Thin Internal Wall Insulation is one of the most cost-effective and performance-effective options. It’s lightweight, flexible and cuts in easily around light switches and window reveals. The ISO-THERM insulation roll is pressed into a trowel-applied adhesive coat and is hung similarly to fitting wallpaper. 

The insulation can then be skimmed, and, overall, you have a system that requires fewer installation hours than alternative systems such as cork renders and foam laminates. Renders have longer drying times, and some other forms of TIWI are harder to form around fixtures and more complicated room layouts. This all adds up to higher labour costs. For this reason, ISO-THERM represents the most cost-effective solution.

Does solid wall insulation cause damp?

As long as it’s correctly installed, solid wall insulation won’t cause damp. But that’s not to say this never happens. As is found with some retrofit internal wall insulation, if it's impermeable, it traps water which leads to a buildup of moisture and, consequently, damp and mould. 

It’s crucial that any insulation is breathable, allowing moisture to escape so it doesn’t become trapped and cause further issues. ISO-THERM is vapour permeable, allowing moisture to pass through, but it’s also waterproof, preventing lateral damp transference and the formation of efflorescent salt. This, combined with its thermal benefit, also makes it mould-resistant.

For more information about the causes of damp in properties, see our guide.

Installing ISO-THERM to damp proof and insulate solid walls

ISO-THERM is quick and easy to install. It’s lightweight, making it easy to work with. This means it can accommodate skirting boards, sockets, light switches and window reveals, drastically reducing the effort needed to apply as most fixtures won’t need to be removed.

Prepare the wall
The first step is to ensure the wall is clean. If you have a problem with mould, you need to ensure you’ve removed this. Our Mould Remover Kit effectively and permanently removes mould, ensuring it won’t return. 

Once the wall is clean, ISO-THERM performs best when applied to bare plaster. If the surface is porous, prime with Permagard SBR, diluted 1:2 with clean water. You can also key the surface will also help improve adhesion.

Apply the adhesive
ISO-THERM adhesive is ready to apply straight out of the tub. Use a 5 mm notched trowel to form horizontal ribs on the wall that are no more than 3mm deep.

Apply the ISO-THERM
Like applying wallpaper, start from the top of the wall and roll out the ISO-THERM down the wall. Ensure the edges line up with the previous piece and smooth by hand and roll into the adhesive with a wallpaper press roller.

Plaster
ISO-THERM can be directly plastered over when it’s dry. When the plaster skim coat has dried, you can paint it with breathable paint. The total finished depth after a skim coat is around 10mm.

Permagard: ISO-THERM suppliers

We supply everything you need to damp proof and insulate your solid walls using ISO-THERM: 

• ISO-THERM Membrane - 7m2
• ISO-THERM Superbond Adhesive - 15kg
• ISO-THERM Membrane Kit - 1 x ISO-THERM 7m2 Membrane and 1 x 15kg ISO-THERM Superbond Adhesive
• ISO-THERM Bulk Membrane Kit - 10 x ISO-THERM Thin Wall Insulation - 0.95 x 7.5 metre rolls and 10 x 15kg ISO-THERM Adhesive (enough to cover 70m²)

Click here to shop now. Or, if you have any questions or queries about suitability or installation, get in touch by calling 0117 982 3282 or [email protected].

Further reading:

• Treating dampness in walls
• How do I protect my outside wall from damp?
• How to prevent heat loss in a house with Permagard