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Building Refurbishment - Systems for Retrofit & Conservation

Welcome to the replay for our webinar, Building Refurbishment - Systems for Retrofit & Conservation. This was originally broadcast live on 4th December 2020 at 10AM. The presentation lasts for approximately 30 minutes, and is followed by a live Zoom Q & A session with the audience. The webinar covers the following topics:

  • Retrofit Thermal Insulation Strategies
  • Managing Air Leakage
  • Controlling Condensation in Roof Refurbishment
  • Strategies for Upgrading Traditional Buildings
  • Hygrotherm Properties of Building Fabric

Building Refurbishment - Systems for Retrofit & Conservation

Today we're going to take a look at the refurbishment and upgrading of existing building, and some of the common problems that can arise. Meeting the increasing demand for housing has been a challenge for some time, and this has only be exacerbated by the changes in priorities and circumstances brought about by COVID.

Even factors like the boom in DIY during lockdown have boosted the appetite for building refurbishment, with everything from redecorating to home office conversions taking place across the country.

Combining this demand for housing with increased emphasis on sustainability and the recycling of existing buildings becomes an effective and attractive solution. Alongside these practical benefits, upgrading existing structures can play an important role in preserving the aesthetic character and individuality of town and cities.

The Architects Journals recently launched RetroFirst campaign is a good example of the drive to shift policy away from demolish and rebuild to a retrofit first approach,

So, today we'll being by looking at some these factors in more detail, and consider the aims and outcomes of refurbishment projects, alongside the types and scope of refurbishments.

We'll also look at the methods, frameworks and guidance used to assess technical performance of existing structure, and subsequently quantify the effectiveness of upgrade works.

Then moving on, we'll discuss the concept of "inherent technical risks" associate with particular systems and works, before finishing up with a look over some common scenarios and the fabric first solutions that can be applied.

Refurb Aims/Types/Categories

Building refurbishment is a very wide ranging topic which encompasses a great many different types of structures and systems, but to simplify things a little, we can consider most project will fall into one of three principle categories:

Conservation & Preservation
Upgrades and Improvements
Repurposing or Change of Use

Each of the three types of project will require a different balancing of priorities, and will necessitate a different set of trade offs to meet the overall goal. Having a clearly identified and realistic outcome in mind before beginning works i s therefore a very important part of the design process.

Conservation and Preservation projects, especially those on listed or historic structures have a number of limitations that may not be present in other types of refurbishment. Even if retaining the actual period material is not required, the preservation of the essential character and any decorative features certainly is.

This requirement can place limits on the extent to which, for example, energy performance can be increased, or may restrict the approaches that can be taken to reduce air leakage or control condensation.

Balancing the intelligent use of materials with a realistic expectation of what can be achieved, and agreeing a suitable approach with any regulatory bodies is an important prerequisite. Establishing constructive dialog between designers, contractors and regulators early in the project cycle helps ensure costs and schedules are kept under control while delivering quality results.

Projects where the main focus is upgrade and improvement work flip these requirements so the primary concern is meeting a performance based objective. That's not to say preserving feature becomes unimportant, just that the balance shifts more in favour of enhancement than conservation.

Although the layout of the building may still impose limits on what can be achieved, particularly in areas such as window and door openings, there's usually a lot more freedom to accommodate fabric insulation, service runs and other modern requirements.

Some thought must still be given to achieving this in the most cost effective manner however, as even without conservation considerations, making substantial improvements to the fabric performance can incur significant costs if not carefully managed.

Finally the last category is changing the actual purpose of the building itself, which can range from subdividing a large dwelling into flats to conversions of former industrial sites. Depending on the purposes of the building before and after conversion each project will have its own unique challenges, with acoustic performance being a particular concern when converting from non-domestic to domestic use as many non domestic buildings, even relatively modern ones, were never designed to incorporate significant acoustic insulation.

In this case, as with thermal insulation, airtightness and controlling moisture, the critical first step in any project is assessing the performance of the existing building. This achieves two important aims, firstly identifying and prioritising required works such as repairs, and secondly establishing a baseline relative to which subsequent improvements can be judged.

While converting a bedroom to a home office or other basic DIY level projects have a pretty clear functional or aesthetic objectives, larger scale refurbishment projects require a more structured and objective approach.

We'll move on to take a look at what structured systems can be applied later on, but first we'll take a quick look over some of the basic building physics we need to consider when working with the fabric of an existing building.

Heat, Air, and Moisture Movement

The movements and interactions of heat, air and moisture are what influences most of the decisions we make when upgrading building fabric, so it's important to have at least a basic understanding of the mechanisms involved.

We've covered these topics before in our series of webinars, so if you would like to learn about this in more depth, those webinars are available on our YouTube channel, or at our learning hub at www.proctorgroup.com.

Firstly, we'll take a look at heat transfer, and there are three main mechanisms by which this takes place. Conduction, Convection and Radiation.

Conduction takes place when two objects are in direct physical contact, and the heat energy transfers from one to the other. This process will continue until either the contact is broken, or the two objects reach the same temperature.
Convection is when heat is transferred by a fluid medium, such as air or water. This happens because where the fluid is next to the warmer surface, it heats up. The warmer fluid is less dense than surrounding cooler fluid, and so rises up and is replaced with cooler fluid and the process starts all over.

This sets up a circular current moving heat between the two objects, which will occur until this movement is restricted, the fluid is removed, or the temperatures equalise.

Radiation is the final mechanism, and in this case heat is transferred directly by means of infra-red radiation. The first object emits the radiation and the second absorbs it. The rate at which the energy is transferred is determined by the emissivity of the respective materials.

Physics in Context

In a building context, these properties interact in various ways, but for a simple example, let's look at a timber frame wall, with insulation placed in between studwork.

This insulation, let's assume it's a mineral fibre, has a thermal conductivity of 0.035w/mK (watts per metre kelvin), this property is constant irrespective of the thickness of the insulation. This is also knowns as a k-value or lambda value and is a measure of how well heat flows across the insulation.

If we have 150mm of this insulation, we can diving this thickness but the thermal conductivity to give us a thermal resistance, or R-value, of 4.3 m2K/W (metres squared kelvin per watt). This does vary with thickness, and is linear. So if we half the thickness, we half the R-value. The higher the r-value, the better the material resists heat transfer by conduction.

The amount of heat flowing through the entire wall assembly is quantified using the thermal transmittance, or u-value, which we calculated by adding all the R-values together, then dividing 1 by the result. The U-value is measured in W/m2K (watts per metre squared kelvin) and a lower result is better.

Old, solid brick or stone walls will have u-value of 2 or higher, while a modern new build timber frame wall will be below 0.2. So this really is an area where substantial improvements can be made in older properties, albeit not always easily.

Cold Bridging

The simple u-value doesn't tell the whole story however, as not all the layers in our example wall are continuous.

The insulation in a timber frame wall is typically between studwork, which has a higher thermal conductivity and therefore will allow more heat flow.

There are also windows, doors, corner junction and floor zones to consider, all of which will affect the local heat flows. This phenomenon is called cold bridging, and we account for this by adjusting the u-value of our wall to account for additional timber, and applying a correction called a psi-value to account for the corners and floor junctions.

By adding additional continuous layers of insulation, or insulating around structural timbers of steelwork we can reduce these corrections and improved the overall heat loss.

Conductive heat flow is not a linear property however, so the more we want to reduce it, the harder it gets to make further improvements. At modern levels of insulation, it's therefore not always practical to accommodate enough extra insulation to make a meaningful difference, so we must look at reducing other heat transfer mechanisms if we want to improve further.


The other main area where can improve our energy performance is by reducing heat losses through convection. This means ensuring unplanned air movement in the building is minimised.

Gaps, crack and poorly sealed penetrations in the building fabric can all allow a substantial degree of air movement, which not only means cold air entering the building in the form of draughts, but also warm air leaking out.

Residents of old houses will be all too familiar with draughts, and how much difference basic steps like using a draught excluder or taping closed old sash and case windows over the winter months can make, and when applied across the entire building envelope it can add up to a dramatic improvement.

The main benefit to reducing air leakage is that it doesn't necessarily add any thickness to the wall or roof, meaning in older properties space can be maintained and period features like cornicing can be preserved.

There are a few ways to improve the airtightness of older properties which we'll consider later on, but it's important to consider the effect it can have on moisture in the building.


All activities that take place in buildings introduce moisture into the internal environment, and while draughty older building might have been a bit chilly to live in, this air movement allowed an easy route for moisture to escape.

If this moisture cannot escape properly, it can lead to condensation problems where warm, moist air contacts colder surfaces.

Alongside this, traditional building materials like lime mortars and lath and plaster walls have very different hygrothermal properties to their modern equivalents. This means the way they store and transfer both liquid water and water vapour can vary a lot, not least because older material tend to have highly inconsistent properties.

If we seal up all the draughts and upgrade the insulation without a proper understanding of this existing fabric, we can introduce problems with condensation or damp ingress that cause damage to these materials, often in ways that are hard to detect and remedy.

For example, adding a rigid foam internal insulation system with a low vapour permeability to a lime plastered stone wall will make the stonework colder, increasing the risk of condensation between the stone and the new insulation. This reduced permeability also limits the capacity for moisture in the stonework to dry out inwards.

A more permeable insulation can offset this by allowing more drying, but will allow more vapour ingress, making this a more complex problem than it may first appear.

So when we upgrade our buildings, we need to carefully balance the benefits modern materials can bring against the risk of upsetting the balance of heat and moisture interactions present in the existing fabric.

So how do we avoid these types of issues? The first step is assess the existing fabric thoroughly, and there are a number of tools and frameworks available to undertake this.

Refurb Assessments/Frameworks

The best practice guidance for undertaking energy efficiency focussed refurbishment projects is outlined in the PAS 2035 and 2030 documents issued by BSI. The PAS or Publicly Available Specification details the steps, assessments and risks associated with each stage of the process from assessment and design through to completion and handover.

While the PAS2035 approach is aimed more towards commercial or larger scale projects such as local authority upgrade schemes, the steps and recommendations are applicable to every refurbishment project. The document itself also provides a useful framework to highlight the factors that should be considered to delivery any project successfully.

It specifies requirements in the following areas:

Assessment of dwellings for retrofit

Identification and evaluation of improvement options

Design and specification of energy efficiency measures (EEMs)

and the Monitoring and evaluation of retrofit projects

The ultimate aim of the standard is to ensure projects deliver a healthy and fit more purpose environment for the occupants while respecting any preservation considerations.

The principle factors recommended for assessment are:

U-Values of the various elements such as walls, roofs, floors, doors, windows etc.

Psi-Values of junctions and other non-repeating thermal bridges

Condensation risks throughout the structure, and the balance of moisture flows

Heat gains from occupants, appliances and solar

Ventilation rates and indoor air quality

As PAS2035 is focused on energy efficiency, it doesn't include acoustic performance in this list but that's also well worth considering, especially in change of use projects. Undertaking acoustic testing as early as possible in the project will allow acoustic upgrades to be designed and built in more easily than if applied retrospectively.

While the other areas on this list are relatively predictable and can be assessed by calculation with a reasonable degree of accuracy the same cannot be said for acoustics. Making sure the existing performance is properly tested before starting work allows the acoustic impact of all upgrade works to be factored in while changes can still be made easily.

Calculation Methods

So what methods to we use to assess these factors?

The simplest step in the assessment is determining the u-values of the existing envelope, and there is a range of software available to undertake this, such as the BRE u-value calculator that we use here at the Proctor Group. Whatever software is used, it's important that the calculation method itself conforms to the methods given in the relevant standard documents, principally BS EN ISO 6946.

As well as using compliant software, it's important that the calculation if conducted properly and any assumptions made are in line with accepted conventions. The standard methods and conventions for undertaking u-value calculation are given in BRE guidance document 443, the most recent 2019 version of which is undergoing consultation right now.

This document gives guidance on the incorporation of thermal bridging, adjustments for fixings and air gaps and other factors that can influence the final calculated result. While there's a lot of scope for interpreting these factors differently in refurbishment jobs, it's important designers have an understanding of how these work when comparing calculation from different sources.

While most software included material libraries, its important the properties accurately reflect the existing building fabric, as it may not be appropriate to substitute the values given for modern materials.

Condensation Risk

Building on the u-value calculation, the next step is hygrothermal assessments. Moisture control measures in building are detailed in BS5250, the 2020 edition of which is due for release soon.

BS5250 references two standards for condensation risk, EN13788 and EN15026.

EN13788 uses the steady state "glaser method" to determine condensation risks, and our example here uses the ICOND software which is linked to the BRE u-value calculator. This adds an additional layer of values to account for vapour transfer, and as with u-values it's important to ensure the values for permeability are appropriate to the building fabric under consideration.

Glaser uses steady state heat conduction, along with 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 is that this method only models moisture vapour flowing from inside to outside, and completely 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 is stated in EN:13788 "the method assumes built-in water has dried out and does not take account of a number of important physical phenomena"

The improved EN:15026 standard addresses these issues with a new calculation method based around dynamic numerical simulation of moisture flows. This type of simulation provides a far more detailed and accurate representation of the way in which moisture both flows and accumulates in the fabric of buildings.

EN:15026 is the method specified for solid walls with internal insulation in BS5250, making more appropriate in traditional properties where this type of wall is more common.

This more accurate simulation not only allows for greater variations in environmental condition and material properties, but also offers a more flexible approach to simulation time 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 small problems that can grow to cause severe issues over the years, issues which would commonly be missed by a simpler Glaser method calculation.

The trade off to using this more advance method is more complex simulation software and outputs which are more complex to determine. Here at the A. Proctor Group we use the WUFI software package developed by the Fraunhofer Institute to undertake these calculations.

SAP and Energy Assessment

Moving on from this, the energy efficiency of the entire building can be assessed and PAS 2035 recommends two method for undertaking this, the SAP or standard assessment procedure calculations and the Passivhaus planning package or PHPP

Both of these methods incorporate a wide range of data covering all aspects of the building fabric, orientation and systems and will give a detailed and comprehensive overview of the pre and post upgrade performance.

Because these models utilise such a wide cross section of data, they allow the interactions between various measures to be assessed, for example the energy performance trade offs between air leakage rates and insulation levels, and allows variations in levels of insulation in different areas.

This is particularly useful as it allows most aspects of the thermal performance envelope to be traded off against one another. For example by reducing air leakage to permit thinner insulation in areas with ornate cornicing, or restricted space. Larger rooms can also be insulated to a higher level to retain space in other areas.

Although the reduced data or RDSAP calculation method is also available, and indeed was developed for refurbishment scenarios, PAS2035 it cautious about its use as it makes a number of default assumptions that may not accurately reflect the building under consideration. That said it may be appropriate to use for guidance purposes or to draw quick comparisons between a variety of different measures provided these assumptions are understood and allowed for.

Whichever holistic energy assessment is used however, it should be remembered that neither SAP or PHPP account for the effect of upgrade works on moisture, so a separate condensation risk review should also be carried out.

This holistic approach to design is well illustrated by Figure D1 in PAS2035, the "measures interaction matrix". This provides a simple, at a glance overview of likelihood of incompatibilities and problems arising between various upgrade works.

For example, by looking at the matrix, systems which are not appropriated to be used together are highlighted red, such as the combination of decentralised mechanical extract ventilation and heat pumps.

Orange interactions, such as that between electric storage heater replacement and heating controls, require additional consideration to ensure compatibility and may require additional complimentary upgrade works.

This is also true of yellow measures such the combination of upgraded doors/windows and internal or external wall insulation, but to a lesser degree and more related to ensure good detailing and proper installation rather than Significant changes to individual specifications.

Green areas, such as draught proofing and solar PV have little or no interactions or effect on each other and therefore represent a very low risk combination.

"Inherent Technical Risk"

The concept of risk in the specification of retrofit is further expanded upon in PAS2035 with the project itself, and individual measures being assigned degrees of risk. Depending on the nature of the project, three risk based pathways are identified, A being the lowest risk and C being highest. "Risk" in this context can be considered analogous to "complexity".

For example, upgrade work to replace the windows of a single house out with conservation areas and of convectional construction, will pose very little risk of complexity. By comparison fitting internal insulation and mechanical ventilation with heat recovery to a listed, solid walled property will be graded much higher.

Upgrade measures are also graded 1-3, with simple upgrades like draughtproofing and hot water cylinder insulation and complex and disruptive measures like ventilation systems and heat pumps graded 3.

The higher the identified risk pathway, the more pre-assessment measure are recommended, along with increasingly detailed project specifications and higher recommended qualifications for specifiers and installers.

The recommendations in all the areas are extensive, so the PAS2035 document, along with the accompanying PAS2030 which covers installation processes are well worth a read for anyone regularly undertaking retrofit project, especially at scale. The processes and detailed recommendations are also a useful framework for anyone wishing to understand what involved in upgrade works even if the full framework is not being applied to the project.

Common Problems

So let's now move on to look at three scenarios which commonly pose problems in the refurbishment of older properties. Beginning with internal wall insulation.

Internal Wall Insulation And Reveals

Solid walls, be they brickwork or stonework are a common feature of older buildings, and often the stone or brick is an inherent feature of the external appearance. This can extend beyond an individual building to be a fundamental part of the character of an area.

In such areas, even where not explicitly defined as a conservation area or listed building, the application of external insulation systems may not be desirable or practical. Internal wall insulation systems overcome a number of these problems, but typically introduce a few of their own.

The main issued that can arise from internal systems are disruption to services and decorative feature, and condensation risks.

The thickness of the internal insulation system can lead to extensive additional works to relocate switches, sockets and other services to accommodate the thicker walls, or cause problems if period decorative features such as cornicing must be retained.

This insulation systems such as our range of Spacetherm aerogel boards, with thicknesses as low as 13mm for the Spacetherm-WL, can go some way to mitigating this. Because of the exponential nature of u-values, even such a thin board can make a significant improvement thermally if no existing insulation is present, but getting closer to the insulation levels of a modern home will unfortunately necessitate a little disruption even with such high performance systems.

The other issue, condensation risk, comes in two form, firstly the risk of condensation forming at the interface between the insulation system and the original wall beneath. And secondly, risks arising in any areas than cannot be insulated to the same degree as the main part so the wall.

In the first case, problems can arise because adding insulation to the wall makes the underlying building fabric colder than it would otherwise be, increasing the likelihood of the dew point occurring in this area.

Avoid this type of problem can generally be accomplished by care hydrothermal analysis using a dynamic simulations over an appropriate time period. By accounting for the moisture storage capacity of the materials and assessing the potential for inward as well as outward drying, this type of simulation can identify the optimum combination of vapour resistance and thermal insulation level to minimise any negative moisture effects.

The inherently permeable Spacetherm and Spacetherm-A2 insulation systems offer a variety of thicknesses and can be specified with or without an intergated vapour control layer. This allows both the thermal and vapour resistance to be tailored to the specific project requirements, as assessed via WUFI dynamic simulation.

In the case of window and door reveals, it's often difficult or impossible to introduce insulation to these areas without compromising the opening size and obstructing the glazed or opening areas. While the effect of not insulation these areas on the overall heat loss is not huge, the bigger issue is condensation.

If the majority of the wall area is insulated but the reveals are not, the warmer surface of the insulated areas if offset by a decrease in the surface temperature where no insulation is present. So internal wall insulation will actually make an uninsulated reveal colder that it was prior to the upgrade.

This reduced surface temperature can give rise to surface condensation at the reveals, a problem only increased by the relatively lower temperature adjacent to the windows. Reducing this condensation can be achieved by adding even a minimal level of thermal insulation, bringing the surface temperature closer to the overall average for the wall surface. Again, the thin, high performance Spacetherm-WL system can meet this requirement without unduly encroaching on opening area.

Retrofitting Airtightness

As we discussed earlier, reducing the air leakage rate of a dwelling is often a more effective route to improving the buildings energy performance while adding minimal added thickness. How this can be achieved in a retrofit situation is however not always straightforward.

Applying draft strips to doors and windows, or completely replacing doors and windows, will go a long way to reducing air leakage. In other situations such as old floorboard over ventilated solums or when fitting new internal insulation to a more air permeable substrate, it's a little more complex to achieve.

The principle issue in such situations is how to limit air movement while ensuring no moisture is trapped. While lining the walls and floors with a polythene membrane may seem an obvious solution, this can also lead to problems as such membrane may affect the flow of moisture across the element.

As we saw in the dynamic assessment of moisture flow, inward drying and the ability of the building fabric to absorb and release moisture is an important component of maintaining hygrothermal equilibrium. So the challenge is how to introduce an airtight layer without preventing this free flow.

In these situation vapour permeable air barrier such as our Wraptite and Wraptherm provide simple and moisture neutral solution. The membrane is self adhesive so simply bond onto the existing wall surface, providing an effective air barrier without impeding the movement of moisture vapour in either direction through the wall.

The moisture neutral property of this material means that insulation can then be added without causing a condensation risk, regardless of where in the wall or floor the insulation and air barrier are placed.

This allows for the effective sealing not only of surfaces, but also of junctions, for example between a timber floor and a stone wall.

The Wraptherm also adds a layer of high performance, vapour permeable aerogel insulation to the self adhesive membrane, providing a highly efficient thermal breaks as well as an air barrier. Because of the moisture vapour permeability of this insulation, it will help minimise moisture and damp problems not only by permitting drying in any direction, but also by increasing surface temperature and helping reduce any cold spots associated with cold bridging.

Re-roofing Projects

The final scenario we'll consider here is re-roof, a daunting prospect for any building, but particularly difficult in traditional or historic properties which may have a variety of period details and feature as well as less straightforward plan layouts.

In roofs with complex geometry or rooms in the roof, it's not always simple to ensure ventilation can provide adequate airflow throughout the entire structure. A lot of existing roofs rely on air movement through leaky fabric with lots of gap sand crack to ensure moisture vapour can escape, so the lack of condensation problems can be more a case of luck than good design.

Such systems cannot always be depended on to perform similarly post upgrade, particularly if energy performance upgrades substantially alter the hygrothermal characteristics of the building fabric and envelope.

Another particular problem is the condition of the existing ceiling, the performance of which may not be known. Some modern roofing membranes require the addition of a vapour control layer, or that the ceiling be "well sealed" in accordance with BS9250, and as with the provision of ventilation, this cannot be relied upon without a full replacement of the ceiling, which may not be possible or desirable.

Taken together these two factors make the choice of roof underlay crucial in ensuring the roof remains trouble free after the upgrade works are completed. This is especially true if there's a large amount of wet trades involved in the refurbishment, as this additional moisture load must dry out over the initial heating periods.

The critical feature of any underlay used in such work must be the ability to accommodate a wide a range of existing situations as possible, without requiring extensive additional enabling works. Particularly in historic or listed properties, getting the outer roof covering removed, any repair work done and the outer covering reinstated in the shortest time possible is paramount.

Air and vapour permeably underlays such as our Roofshield, do not require any additional ventilation below the underlay, nor do they require any special air or vapour control measure at ceiling level.

Being air permeable, air is able to flow in and out across the entire surface of the roof, ensuring the most efficient transfer of moisture vapour possible even during the initial dry out phase after completion.

This simplicity allows for trouble free specification and installation on any size and shape of roof, with minimal changes to the existing building fabric required. Meaning the roof can be reinstated as quickly as possible.


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