Victorian House Retrofit Case Study

Overview of a Victorian Solid Walled Retrofit

An old, faded Victorian house, in a Sheffield conservation area, had been allowed to fall into a state of disrepair by the previous owners. It was a textbook ‘do-er upper’. The new owners were keen to modernise and enlarge the existing dwelling, so it was fit for life in the 21st century and also made much more accessible.

The project essentially happened in two phases. Phase 1 was a partial retrofit to the existing solid walled house. Phase 2 was a side extension using modern methods of construction but with a traditional appearance. This case study is going to focus on the Phase 1 work which is the partial retrofit part.

The building works commenced in April 2022 and Practical Completion was reached March 2023. The completed project has been in continuous use for nearly two years.

Client Goals

A summary of the brief was to embrace open-plan living with a larger, combined Kitchen / Living Space, that reflects modern family life in the 21st century, improve comfort levels and energy efficiency, and to also extend the house to provide extra facilities.

A key part of this was ensuring the house was much more accessible for older members of their family, who are less mobile, and who often visited for extended periods of time. Introducing lots of natural light was also a high priority as the part of the existing house that was intended to be upgraded was quite dark.

The primary rooms needed were a larger open plan Kitchen / Living Space + Utility Room + Side Porch [boot room] + accessible Shower Room.

Further specific requirements are also listed below:

• Lots of glazing was required to improve natural daylight.
• Side extension needs an external front door + an external back door + an internal side door to provide easy access to the front and back gardens + the new Kitchen / Living Space.
• Main materials to match existing and blend in. For example, natural stone walling + a natural slate covered roof.
• New timber framed external doors and windows.
• Re-use and upcycle existing certain materials where possible.
• Underfloor heating required to the new open-plan Kitchen / Living Space.
• Decorative tiled floors required, porcelain and / or ceramic tiles.
• New accessible hard landscaping including rear garden patio and new front and side paths from the front garden to the back garden.

Design Approach

To create the extra space required a small rear extension was designed but when this was reviewed it clashed with an attached offshoot Store and historic Washroom [from before washing machines!], that was at a higher ground level and had different internal floor levels. It was agreed to therefore omit the rear extension and just focus on remodelling the existing Ground Floor rear rooms into a single open plan space.

This simplified everything and made it far more efficient. A single storey side extension was also designed to provide the Utility Room + Side Porch [boot room] + accessible Shower Room.

As can be seen by viewing the existing Ground Floor Plan the existing rooms are arranged in a traditional format of separate Sitting Room + middle Lobby and Toilet + Kitchen. The requirement was for all of these rooms to be knocked through so a single space was created. This may sound straightforward in theory but structurally it was not straightforward.

The existing feature main cut string staircase is also positioned past the main dividing walls between the Sitting Room and Kitchen and the front rooms. Ensuring the existing Cellar door that is under the existing main stairs could be opened was also critical. This essentially creates a protrusion into the new open plan single room space. The stairs also have a dropped level top landing that creates a dropped floor and ceiling above the middle section of this open plan single room space.

The open plan space was subdivided into 2 distinct zones. Kitchen zone on the right hand side [in the old Kitchen] and Social / Dining Zone on the left hand side.

To rationalise the windows and doors and bring in more natural daylight it was decided to remove the small central window and single external door, and replace these with a new window to match the existing Kitchen window. Oversized super tall timber framed French doors were introduced with full height sidelights and accessible flush threshold. The French doors were about 3m wide x 2.6m tall.

To enable this several steel and concrete beams were required above the old loadbearing walls removed and over the new openings. As the French doors were quite large a small ‘goalpost’ steel frame was required, bolted down to new localised concrete pad foundations and also bolted back to the existing main external wall behind with recessed pockets cut into the stonework and lateral restraint steel plate fins.

This meant the ‘goalpost’ frame was hidden from view and did not create a thermal bridge in the Internal Wall Insulation layer. The new rear window was directly in line with the new steel beams supporting the load bearing walls removed so a super strong beam lintel was required above this window for the new primary steel beams to sit on. As the walls were so thick several concrete lintels were needed in place, sitting side by side, to make up the overall structural support over some of the new openings.

Ensuring the existing building was insulated, ventilated and draft stripped [made airtight] up to the current Building Regulations was super important, while bearing in mind the risk of unintended consequences, such as interstitial condensation. As the existing building had no insulation at all and still contained some of the original single glazed windows we were essentially starting from scratch. The existing ground floors needed to be replaced as well. 

The right-hand Kitchen zone floor was constructed of solid stone flagstones, about 75mm thick, bedded in bitumen, directly laid onto the ground. The left-hand Social / Dining zone ground floor was built as a suspended timber joisted floor that had suffered severe decay in some areas as some of the timber joists were in direct contact with the ground, the air space below was minimal and not adequately ventilated.

It was agreed that all the existing plaster would be removed and replaced with Internal Wall Insulation [IWI] and new plaster on top. The old floors were also to be fully removed and replaced with a single consolidated solid floor with integrated underfloor heating. The Internal Wall Insulation would reduce heat loss through the building fabric and the multi coat plaster build up would reduce heat loss through air leakage by creating a vapour permeable yet airtight building envelope.

As the building is solid walled and a vapour open construction, ensuring the new constructional build ups were also vapour open and breathable was very important. For the external walls we originally opted for wood fibre insulation [this was substituted with cork insulation board, read on to find out why] + insulating lime hemp plaster on top. For the ground floors we chose a limecrete floor slab with underfloor heating buried within. Internal walls and ceilings were also plastered with lime plaster, applied to a rigid woodwool backing board. This meant all wall and ceiling surfaces throughout used identical materials and looked the same.

Several parts of the existing building fabric were reused and ‘upcycled’ such as some of the existing stone walling removed to form the new openings were reused to infill other openings on the same wall + original floor boards taken up and reused + the original raised and fielded door leaves were reused in new internal door openings.

Challenges and Solutions

Most of the project team not understanding how traditional solid walled buildings with vapour open construction deal with moisture.

Ensuring breathable constructions were maintained throughout the entire floor and wall build ups also needed to be done. Working with the clients to select materials from a reduced list of suitable materials was quite challenging as non-porous porcelain tiles and adhesives were originally requested which I advised against. This meant that breathable, natural stone flagstones + porous tile adhesive and porous tile grout were specified and sourced. 

Breathable paints were also sourced and specified for the external walls.

Building Regulations Compliance

Building Regulations Approved Document Part L1 2021, Table 4.3 identifies Limiting U values for existing elements in existing buildings. For walls with Internal Wall Insulation this is a maximum U value of 0.30 W / m²K and for floors this is a maximum U value of 0.25 W / m²K.

The Internal Wall Insulation was originally specified with thicker insulation to achieve a U value of 0.30 W / m²K but when we conducted condensation risk analysis this confirmed it was likely that interstitial condensation would occur as the temperate drop and dew point temperature on the cold side of the insulation was too great.

A decision was reluctantly made to reduce the insulation thickness so there was no risk of interstitial condensation. The revised wall U value was 0.50 W / m²K.

The consequences of interstitial condensation far outweighed the perceived energy savings of thicker insulation. This also meant the originally specified wood fibre insulation was not available in the reduced thickness required. However, alternative breathable cork board insulation was available in the required thickness, and this achieved the same U value [similar thermal conductivity], without any interstitial condensation issues.

The limecrete floor slab insulating layer was able to be thickened up to compensate for the slightly higher wall U value. The floor U value was 0.22 W / m2K which is less than the maximum allowable.

It is worth noting that Building Regulations Approved Document Part L1 2021 also makes provision for this scenario. Paragraph 0.10 states that, ‘The energy efficiency of historic and traditional dwellings should be improved only if doing so will not cause long-term deterioration of the building’s fabric or fittings. In particular, this applies to historic and traditional buildings with a vapour permeable construction that both absorbs moisture and readily allows moisture to evaporate.’

Overzealous Building Control Officers who solely focus on low U values, regardless of the existing building fabric, sometimes misunderstand the potential for unintended consequences. Understanding the risk of interstitial condensation is a very important aspect of retrofit, and overzealous Building Control Officers may need to have this pointed out to them.

Material Specifications

Sourcing a tried and tested vapour permeable Internal Wall Insulation system build up and limecrete floor system suitable for solid walled properties, with all materials easily obtained from a single source supplier, was quite difficult.

After a lot of research, a system by a Welsh company called Ty Mawr was selected. They offered a suite of tried and tested systems for the Internal Wall Insulation and limecrete floor slab. Ty Mawr also offered good customer service and helped produce specification breakdowns of all the components required.

At the time of specification, it was understood that these systems were also tested and approved under an approved LABC scheme. 

The Ty Mawr quote lists were included in the Specification as Appendices so the contractors could place their orders quickly and easily.

Internal Wall Insulation

We investigated different methods to build the Internal Wall Insulation system. Initially, we investigated an independent timber framed partition system with moisture variable vapour control layers [such as the Pro Clima range Intello membranes] of but the overall thickness, and extra materials, of such systems meant that too much floor area was lost. This would have meant the residual space would have been too tight to physically use as the clients preferred.

A thinner build up with higher U value was used, this worked in our favour as it reduced condensation risks.

Multi coat build up took a long time, such as lime plaster levelling coat + adhesive + 3 coat lime plaster on top of the insulation boards.

Ensuring other jobs could be completed in parallel with the Internal Wall Insulation installation ensured the contractor’s time on site was used efficiently.

Limecrete Floor Slab

Limecrete is not suitable for all sites and circumstances, especially water laden sites as there may be just too much moisture for the limecrete to handle. Ensuring the ground below the limecrete slabs was dry enough and not liable to saturation during periods of wet weather, with a high-water table was critical. Site investigation was conducted during the early stages of construction to ensure limecrete was still a viable system. 

Ensuring the existing house foundations were not undermined with the limecrete slab excavations. Existing heritage buildings sometimes have low foundations. To ensure limecrete was viable the contractor was asked to excavate small trial holes at each corner of the space to check there was enough wall depth to accommodate the full limecrete slab build up before excavating fully was also critical.

Ensuring the external ground levels were not higher than the internal floor level and that external rainwater could drain away from the house was important. Saturated walls and water that is allowed to pond near walls can reduce the capability of a limecrete floor slabs ability to buffer water below the limecrete, in the foamed glass aggregate insulative layer.

Small, free draining gravel trenches to the perimeter of the external walls were explored but an alternative solution to ensure surface rainwater was drained away from the external walls and limecrete floor slab was to use an Aco channel perimeter drain instead. A decorative cast iron grating was selected to suit the heritage nature of the building, which is a special specification option available from Aco, instead of the semi-industrial looking channel drain cover grilles.

Essential Repairs and Maintenance

Ensuring the existing building was repaired and essential maintenance carried out before the installation of the Internal Wall Insulation was important. This meant that any potential for damp ingress from leaking drain pipes, gutters and failed lead flashings etc. were remedied in advance.

Old, unsuitable, late 20th century cement mortar was removed from stone mortar joints and replaced with breathable lime mortar at the original joint width. This also reduced the chances of the walls and building fabric getting saturated with moisture. Otherwise, this may have led to eventual damp ingress and damage to the building fabric during frost spells where the water freezes, expands and spalls off the stone walling.

External Walls That Become Internal Walls, When the Extension Was Attached to Them

An old external wall was essentially transformed into an internal wall because the side extension walls enclosed this wall.

This internal wall was still buried in the ground with a foundation below, taking up ground moisture and helping the moisture to evaporate and dry out naturally.

The contractors wanted to plaster this wall with non-breathable gypsum plaster and finish with non-breathable paints etc.

Had to have a robust conversation with the contractors and clients on site to explain this principle to them, so they understood the consequences more fully.

Traditional Construction Skills

Finding craft persons / sub-contractors skilled with breathable vapour open construction techniques, in the available timescales, proved difficult.

Mainly the limecrete floor slab and breathable cork insulation + lime plaster build up.

After making a request, via expert heritage, social media forums, a shortlist of local subcontractors was drawn up, interviews held, and suitability checked.

Eventually a suitable sub-contractor was found with whom to work. They completed a sterling job and even made suggestions for improvements. A great example of using social media in a positive and helpful way.

Structural Steel Installation and Corrosion Protection

As the existing walls are solid stone and prone to soaking up water and moisture any steel beams inserted into and bearing off the external stone walls are likely to get wet or slightly damp, which could accelerate corrosion.

Galvanised steel was specified for all new steelwork inserted into and / or bearing off the external solid walls to provide extra damp and corrosion resistance.

The ‘goalpost’ frame needed to have each lateral restraint fin, site measured so the holding down bolts lined up with the nearest uneven stone course. Each fin was in a slightly different position and was literally, a bespoke, made to measure structural frame.

Outcomes / Lessons Learned

Despite taking a bit longer than expected we consider the project is a resounding success, it has been delivered as the clients expected.

While heat demand in kWh /m² before and after the project was not calculated or measured, we understand from conversations with the clients that the retrofitted rooms and spaces are a lot warmer and feel much more comfortable, all year round. Especially when compared to the remainder of the existing house which has not been retrofitted so comprehensively. The air tightness and reduction of unwanted air leakage has probably been a big factor in the improvements.

Ensure interstitial condensation risk analysis is considered properly and adjust the design accordingly.

Ensure specialist heritage and conservation contractors and sub-contractors are used. Check past work, client references and ensure they are familiar with the job at hand.

Ensure enough time is allowed in the construction programme to allow for the curing process of the lime plastering and limecrete floor slab. Ensure a pessimistic outlook, so extra time is allowed. Contractors sometimes use an overly optimistic outlook.

Don’t assume contractors will read and understand the intent of the design and drawings. 

Hold regular site meetings, review details and why they need to be built a certain way, such as thermal bridging reduction at window and door reveals in very thick walls.

Resources

Related Reading

Related Details

Check out the Detail Library for inspiration, our recent sets offer examples of details for the retrofit of internal wall insulation.

Project Team

Clients > confidential

CIAT Chartered Practice > Ikonografik Design > https://www.ikonografik.co.uk 

Structural Engineer > Collins Hall Green > https://www.collinshallgreen.co.uk 

Contractor > Paramount Builders UK > no website

Further Information

Ikonografik Design website case study

https://www.ikonografik.co.uk/architectural-design-services-projects-sheffield/modern-upgrades-tricky-extension/

Ikonografik Design video of the Internal Wall Insulation design and installation process

https://www.youtube.com/watch?v=BTH4pSY8mQI&t=9s

Ty Mawr Internal Wall Insulation website

https://www.lime.org.uk/applications/retrofit-insulation-systems-for-old-buildings/internal-wall-insulation-system/expanded-cork-insulation-system.html

SPAB Old House Eco Handbook

Image Credits

All photos were taken by Nathan Oliver of Ikonografik Design

All drawings copyright Ikonografik Design Ltd.