Thermal Insulation

In our approach to building refurbishment, it is crucial to recognise that we cannot simply upgrade thermal insulation without addressing the balance of moisture movement and air leakage. In doing so, we can ensure that the most optimal upgrade solutions are achieved. Strategies to upgrade the energy performance of existing buildings must therefore be carefully considered, and considered holistically, as actions affecting one aspect of the building may have unforeseen consequences in another. Designers must therefore have a good understanding of the materials being used both in the upgrade process and the original construction.

Thermal Insulation Basics

When considering the insulation of buildings, we usually refer to U-Values. A U-Value quantifies the rate of heat loss through a building element such as walls, roof, windows, and doors. The lower the U-Value is, the slower the heat generated by the heating system will escape the building, and the less energy input is required to maintain a comfortable internal temperature. How can we improve the U-Value of a given element? Using a wall as an example, we add thermal insulation boards or blankets to keep the heat in. A specific thickness of any material has thermal resistance, and to work out a U-Value, we add all the R-Values in the wall together and take the inverse of the result. This inverse calculation means that the more insulation we add, the greater the improvement to the U-Value. Not all materials insulate equally well, though, and to determine that, we use a value called the thermal conductivity or Lambda value. For example, materials like bricks or metal have high thermal conductivity, while insulation like mineral wool or rigid foams has far lower conductivity. The thermal conductivity of a material is usually independent of its thickness, making it ideal to use as a comparison tool. Combining all this information, we take the thicknesses of our materials and divide those by their thermal conductivity to get R-Values. We then add those R-Values together and take an inverse to get the U-Value. Finally, the U-Value is adjusted for any penetrations like structural elements, called repeating thermal bridges, any fixings or air gaps present, and a few other correction factors to give us a final U-Value for the element. The next step is to take all our U-Values, weight them by area, and derive an overall average for the entire building. Finally, this average is adjusted to account for non-repeating thermal bridges like corner junctions, floor zones, etc., resulting in our complete heat loss model. Modern new build homes typically have wall U-Values of around 0.2 W/m2 K, whilst traditional older properties with solid walls will be more like 2.0 W/m2 K, an order of magnitude worse, which will have a significant effect on the heating bills of the property and the quality of life of the occupants. This has substantial impacts where occupants are in fuel poverty or belong to a vulnerable social group such as the elderly. For such groups, poor thermal insulation can lead to health problems, so it’s essential that upgrading such homes is a priority In historic buildings, this priority will need to be balanced against retaining architectural features, maintaining the character of the building and aesthetic qualities, and ensuring the internal spaces remain large enough to be fit for purpose.

Types of Thermal Insulation

There are many types of insulations on the market, with a wide ranges of thermal conductivities However, there are some challenges when considering the right choice for refurbishment projects or historic buildings. Firstly, older buildings typically benefit from highly vapour permeable insulation. The traditional building materials used in these older properties, such as lime-based mortar and plaster, have very different hygrothermal properties from their modern equivalents. Adding insulation of low permeability, such as rigid foam, can result in damaging moisture problems for two primary reasons. Adding any insulation will reduce the temperature of the existing wall masonry, making it more likely that condensation will occur to the inside as there will be less heat penetrating from the living spaces to warm up the masonry. Secondly, some insulation types, such as rigid foams, can prevent the drying out inwards into the building, further compounding the problem. If not adequately accounted for in the design, this moisture accumulation can continue unchecked, increasing issues with dampness and mould over time and, in extreme cases causing the masonry to degrade due to freeze/thaw cycling. Permeable insulation prevents moisture from being trapped in the construction and does not disrupt the established balance of moisture flows within the building fabric to the same degree. However, traditional moisture vapour permeable insulation, such as mineral fibre, tends to be very bulky. This results in another challenge in historical refurbishment: preserving interior space. The relatively high thermal conductivity of mineral fibre, wood fibre or natural wool means achieving modern thermal standards can require up to 200mm of thickness. This thickness is often not practical to incorporate into an existing space. On the other hand, aerogel composites such as Spacetherm® can maintain permeability while dramatically reducing the required depth. This makes Spacetherm ideal for use in rooms with limited floor area, at door and window reveals, and in other areas where retaining existing features would otherwise limit the use of thick and bulky insulation.

What is Aerogel?

Aerogel is a very low-density solid developed in the 1930s but has only recently found mainstream uses. An aerogel is a gel in the liquid that is replaced by gas without shrinking via a complex process called supercritical drying. The silica aerogel used in Spacetherm insulation is essentially puffed up sand, best described as being to sand what a Rice Krispie is to rice. The resultant structure is 97% air, entrapped in nano-sized pores, and while a superb thermal insulator, it is very brittle in its raw state, limiting its practical applications. However, Spacetherm aerogel takes this aerogel and embeds it into a fibrous mat to achieve excellent thermal performance and robust flexibility. The thermal conductivity of 0.015 W/mK makes it among the best thermal insulants in use today, which, combined with its high vapour permeability, makes it ideal for refurbishments. Furthermore, Spacetherm aerogel is also hydrophobic, meaning it actively repels liquid water.

Spacetherm can be used directly in the blanket form under render or plaster, utilising its inherent flexibility to fit around curved or uneven wall surfaces, or it can be laminated to a variety of panels and lining boards for more specific applications. Because the aerogel insulation blanket contains no blowing agents, it does not release harmful gasses into the building over time, nor does its performance degrade, with extensive testing showing no loss of thermal performance over 50 years.

Facade Retention Projects and Wraptherm®

At the more extensive end of refurbishments are façade retention projects, where most of the building is demolished and replaced with a modern new-build structure, retaining only the facade from the existing structure. While this method resolves some of the inherent issues in such projects, simply replacing the problematic elements and features with modern equivalents, designing an effective interface between what is replaced and what is retained can still be challenging. Air leakage and cold bridging are the most significant problems to be overcome in these areas, so by combining a vapourpermeable, self-adhering, air barrier membrane with a layer of high-performance insulation, Wraptherm offers a compelling and straightforward solution to these issues. Wraptherm is installed on the reverse of the retained facade stonework. However, as Wraptherm is vapour-permeable, it does not adversely affect moisture movement through the facade stonework. This helps limit damage that can occur when moisture flow through older stonework is altered by refurbishment works. As well as protecting the existing building fabric, Wraptherm’s hydrophobic properties provide a secondary barrier to water ingress and, by sealing tightly to door and window frames and other penetrations, also limits heat loss associated with air leakage. The unique layer of aerogel insulation in the Wraptherm also effectively limits cold bridging by providing a continuous insulation layer across the back of the facade. Hence, there is a thermal break between the new structure internally and the cold outer stonework, further boosting the energy performance of the upgraded building. Combining these properties into a single material, Wraptherm speeds up and simplifies the installation process and reduces on-site defects and requirements for costly remediation.

Smaller-scale Refurbishment Projects and Spacetherm WL

Smaller-scale refurbishment projects can achieve the same performance benefits using the Spacetherm-WL wall liner system. The product comprises 10 mm aerogel insulation bonded to a durable 3 mm Magnesium Oxide facing board and fixed in place internally using a gap-filling adhesive. The panels can then be jointed, painted, and decorated in the same way as a standard plasterboard wall. It is supplied in a 1200 x 600 mm panel weighing just 4.9 kilograms, meaning that sheets are easy to handle by one person. Once installed, the system provides a significant reduction in heat loss. In addition, a nominal 13 mm increase in wall thickness means that sockets, switches, and TV/network points can be left in situ and features such as cornices and windowsills can be left as is, with little or no modification required.

Thermal upgrades to older properties are often avoided due to the cost and problems associated. Therefore, thin systems such as Spacetherm-WL offer specifiers and homeowners a simple option where a traditional thermal upgrade is impossible. Although this system typically won’t achieve a thermal performance in line with building regulations, even a small upgrade in thermal insulation can raise the internal surface temperature enough to avoid condensation problems. The minimal thickness also allows it to be used in space-constrained areas to ensure continuity of thermal insulation. For example, a solid uninsulated masonry wall risks condensation across the whole surface, both the main part of the wall and the window reveals. If insulation is added to the main wall surface but omitted from the reveals, although the wall surface temperature increases, the surface temperature in the windows reveals decreases, leading to increased condensation risk in these areas. Even adding a minimal amount of insulation to the reveals will elevate this temperature enough to mitigate this condensation risk, highlighting the importance of a continuous envelope of thermal insulation.

Wall and Floor Solutions and Spacetherm Multi

Spacetherm Multi uses a similar magnesium oxide facing to the Spacetherm WL board, but with an increased thickness of 6 mm. This additional thickness allows the same board to be used in both wall and floor applications, making Spacetherm Multi a versatile and straightforward one-board solution to upgrading the thermal performance of all types of an existing structure. Spacetherm Multi is available in both 2400x1200mm and 1200x600mm sizes, with the smaller sized board being ideal for projects where access may be restricted, such as loft conversions. At the same time, the inherent moisture resistance of both the Spacetherm aerogel and the magnesium oxide facings make it a good option for floors where the presence or condition of damp proofing may be unknown.

Traditional Finishes and Spacetherm Wallboard and Directfix

Where a more traditional finish is required, Spacetherm Wallboard offers a plasterboard finish with the same aerogel insulation as the multi boards. This is fixed onto timber straps or studwork in the same manner as traditional insulated plasterboard but offers significantly better performance. For applications where space is even more restricted, Spacetherm Directfix can be used to omit the requirements for the timber strapping and its associated depth. This board incorporates an additional layer of plywood to facilitate the use of shot-fired fixings into masonry substrates. The hydrophobic nature of the Spacetherm aerogel insulation blankets means no additional protection is required on damp walls, making Spacetherm Directfix a one-step solution to thermal upgrades on walls with moisture issues. The Wallboard and Directfix panels are supplied with an inbuilt foil layer for vapour control. However, if the design criteria require higher permeability, this can be omitted on request giving a more breathable wall construction and allowing the free passage of moisture.

Hygrothermal Assessment Methodology

When considering the management of moisture in existing buildings how do we best assess moisture movement and predict the effects of refurbishment accurately? The BS:5250 and EN ISO:13788 standards cover the assessment of moisture movement in buildings, and both have a significant limitation which may lead designers to seek further guidance. Both standards rely on a calculation method known as the Glaser method, developed in 1958. Glaser uses steady state heat conduction and average monthly temperatures and vapour pressures to identify vapour diffusion and model how vapour passes through the building fabric. This gives a reasonable assessment with a comparatively simple process and simplifies data input requirements, however there are limitations.

The main problem of the Glaser method is that it only models moisture vapour flowing from inside to outside and ignores the effects of external moisture sources such as driving rain. The model also does not incorporate the effects of material porosity, moisture absorption and water storage in the building fabric. As stated in EN ISO:13788,”’the method assumes built-in water has dried out and does not consider several critical physical phenomena”. The improved EN:15026 standard addresses these issues with a new calculation method based on dynamic numerical simulation of moisture flows. This type of simulation provides a far more detailed and accurate representation of how moisture flows and accumulates in the fabric of buildings and is the method specified for solid walls with internal insulation in BS5250. This more accurate simulation allows for more significant variations in environmental conditions and material properties and offers a more flexible approach to simulation periods. Designers can look at minute by minute predictions on the building performance, taken many years into the future if required. This additional detail and time can often reveal minor problems that can grow to cause severe issues over the years, issues which a simpler Glaser method calculation would commonly miss. The trade-off to using this more advanced method is more complex simulation software and more complex outputs to determine. At the A. Proctor Group, the team of technical advisors are trained to carry out and report on the findings of these hygrothermal simulations, using the WUFI software developed by the Franhofer Institute.