Replacement Passivhaus dwelling
Blackthorn House is a newbuild timber frame dwelling built to replace an inefficient original building on the plot. The pre-existing building was awkward in many ways: it sat very close to the site boundary, was in poor condition and was only one room wide! It was energy hungry too – with an EPC rating of F and relying on an oil boiler, and so demolishing the building and starting from scratch was an appealing prospect.
As an unlisted building this was a viable option, but the clients were mindful of the carbon implications of so significant a building project. So with sustainable design, construction and operation at the heart of the brief, plans for Blackthorn House were born – a new timber frame Passivhaus with recycled newspaper insulation, powered by heat pumps, solar PV panels and low-carbon electricity. In June 2020 planning permission was granted and Greengauge was appointed to provide Passivhaus consultancy and design the new mechanical and electrical systems.
Strategy
A key part of the planning application was to replace the existing dwelling with a new one that would fit in better with its surroundings. To achieve this, local firm Affinity Architects created a design with two pitched roof ‘blocks’ connected at an angle to reflect the landscape of agricultural buildings huddled together and to open up public views to the countryside beyond.
The shape of the building was challenging because there was a large area of walls and roof compared to the usable floor area. The ratio of these two areas (the form factor) is directly linked to the amount of energy needed to heat the building, and with a form factor of 3.3, the design was going to need a lot of insulation to enable the Passivhaus heating target to be met.
Project team
Client: Prue & Allan Castle
Contractor: Maidment & Carter
Passivhaus Consultant: Greengauge
M&E Design: Greengauge
Architect: Affinity Architects & Ink Box Architecture
Timber frame: Cullen Timber, James Jones & Son & Severn Valley Timber
Passivhaus certifier: Etude
Key data
” Sustainability issues had clearly been at the centre of the project throughout, with formal Passivhaus accreditation about to be achieved. The judges’ overall conclusion was that this was a house which it would be a delight to live in, and they had no hesitation in giving it an award.”
Salisbury Civic Society
Challenges & solutions
Form factor
The solution to the challenges brought on by the form factor was to use a timber-frame construction, filled with insulation and covered in an extra layer of insulation just for good measure. The contractor constructed the timber frame on-site from I-Joists to which 100mm woodfibre insulation was fixed externally and airtight OSB board fixed internally. The timber cavity was then blown full of recycled newspaper insulation to give a total U-value of 0.1 W/m2K (with a total construction depth of around 600mm including cladding) – more than twice as insulating as the Building Regulations minimum standards.
The same construction was used for the roof (also achieving a U-value of around 0.1 W/m2K). The floor was built on an insulated raft foundation which enabled the wall and floor insulation to be continuous around the perimeter of the building. This thermal-bridge free design, along with an air tightness test result of 0.3 air changes per hour (25 times more airtight than Building Regulations) enabled the home to comfortably achieve the Passivhaus heating target of 15 kWh/m2.
Avoiding ‘over-ventilation’
Fresh air is provided via a Brink mechanical ventilation heat recovery (MVHR) system. The system provides a continuous supply of fresh air to living rooms and bedrooms and is pre-heated with the air extracted from bathrooms and kitchens. MVHR systems maintain stable indoor temperatures and CO2 levels, however they can sometimes lead to overly dry air in winter – not usually a problem in UK homes but something which did need addressing on this project.
When cold outside air is heated up its ‘relative humidity’ (the amount of moisture in the air compared to how much moisture the air can carry) can become very low. For example, 0°C air at 100% RH equates to only 25% RH when heated to 20°C. Relative humidity is important for comfort because low RH (less than around 40%) can lead to feelings of itchy or dry skin and high RH (over 70%) increases the prevalence of mould, mites and bacteria.
Avoiding over-ventilation in Passivhaus homes is therefore an important design consideration. The Building Regulations Part-F sets a minimum dwelling ventilation rate based on the floor area of the dwelling, however for large homes with low occupancy this can often be too high – something which was flagged in the Passivhaus Planning Package (PHPP).
We were aware of this potential issue at Blackthorn House and specified an MVHR unit which could be retrofitted with an enthalpy heat exchanger – a device that recovers both heat and moisture from the extracted air stream. In the end a regular heat exchanger was installed, with the clients opting in the first instance to try manual means to regulate moisture levels, e.g. by turning down the MVHR during fixed periods or taking the opportunity to dry laundry indoors!
MVHR is sometimes considered a ‘commission and forget’ solution, but this project has highlighted for us the importance checking whether a home is at risk of over-ventilation and to ask to what extent we can expect users to actively manage their ventilation systems. Is the home large with variable occupancy? Will the user be able to access and understand the controls systems? Can the systems be retrofitted with additional sensors or components to make life simpler in the future? For social housing in particular, maintainability is key and it’s worth considering that sometimes a more complex system with humidity sensors and automatic turn-down may be easier to maintain – an MVHR that’s difficult to use, expensive or noisy to run will just be turned off and potentially create damaging moisture issues later on down the line.
Energy performance
Prue and Allan moved into their new home in the summer of 2022 and two years on we’re pleased to see that the building is performing as expected.
The measured space heating demand from the Air Source Heat Pump is 11 kWh/m2/year – lower than the predicted 13 kWh/m2/year.
Allan notes that the house is comfortably warm throughout the year and that they hardly notice when the heating is on – satisfying evidence that the low temperature weather-compensated heating system is working well.
Allan’s regular manual meter readings have also made it possible to estimate the home’s total electricity demand, which appears to be around 28 – 33 kWh/m2/year, making the home’s operational energy demand compatible with LETI’s climate emergency target (35 kWh/m2/year).
Currently around 30% of the home’s energy demand is met by the 4.3kWp roof-mounted solar array. Due to mismatch between when the solar energy is generated and when energy in the home is needed, 40% of the generated solar energy is currently exported back to the grid. A home battery storage system could potentially supply up to 50% of the home’s energy demand, or alternatively power an electric car for around 5,000 miles per year – something the clients are currently looking into.
Blackthorn House recently won a local Salisbury Civic Society award and has been commended for its carefully chosen forms, materials and sustainability.
By putting Passivhaus at the centre of the project throughout, Prue and Allan have built a cosy and efficient home which they find a joy to live in.