Howgate Close in Nottinghamshire is a collection of nine low-energy, earth-sheltered rural homes developed by landowner Chris Parsons. Parsons’ intention was locally focused, but with national implications: to address the shortage of affordable rural homes, tackle fuel poverty and introduce rental security, as well as promoting fossil fuel-free lifestyles, community cohesion and biodiversity in the surrounding countryside. His vision has been likened to almshouses for the post-hydrocarbon era.
Central to the approach adopted at Howgate Close is an airtight, super-insulated and thermally massive concrete structure. The homes are built on a 250mm in-situ concrete raft slab, with 300mm of expanded polystyrene (EPS) insulation underneath, giving a U-value of 0.07W/m2K – 38% lower than the notional building in the 2021 Building Regulations. The rendered external walls are solid, with no cavities, and comprise 140mm of high-density blockwork with 240mm of external expanded polystyrene insulation.
These deliver a U-value of 0.13W/m2K (28% lower than regulations). The roofs, which have a U-value of 0.08W/m2K (36% lower), are concrete too, with 220mm of prestressed concrete planks topped with a 100mm in-situ slab and 300mm of extruded polystyrene (XPS) insulation.
The envelope is completed with triple glazing to all windows, with a U-value of 0.78W/m2K (35% lower). The building average U-value is 0.2W/m2K. The contiguous external insulation essentially wraps the entire building in 220-300mm of thermal protection, eliminating thermal bridges from joints, wall ties or other penetrations. Window and door jambs are located externally to further reduce thermal bridging.
On completion, the concrete structures achieved exceptionally low air pressure tests of 0.45-0.77m3/h/m2, an improvement of 87% on the notional building. A condensation risk analysis concluded there was no risk, in any month, of interstitial condensation forming at any interface of the fabric build-up.
The thermally massive structure helps to store heat and regulate internal temperatures as part of a passive solar design strategy. The homes have no conventional heat source, instead drawing in warmth from south-facing windows. They do have electric underfloor heating as a back-up, and in colder months a heat recovery ventilation system uses the heat in the stale exhaust air to pre-warm incoming air. In summer, they are cooled by natural cross-ventilation.
The other key element of the energy strategy is an array of 138 roof-mounted photovoltaic panels, which generate about 50,000kWh of electricity annually, mitigating 16 tonnes of carbon emissions. Residents receive the full financial benefit from this energy generation. The rewilding of the surrounding 10 acres – previously used for intensive arable farming – adds a further 9 tonnes of carbon sequestration, and the scheme is likely to exceed Natural England’s 10% biodiversity net gain target.
One of the most significant aspects of the project has been its commitment to post- occupancy analysis. Sensors were installed in all of the properties to monitor energy use, relative humidity and temperature. Nottingham Trent University, in conjunction with the project’s technical design consultant Jerry Harrall, are engaged in an ongoing programme to analyse the data. After 16 months of occupation, the findings are impressive.
All nine homes are net annual generators of renewable energy, with little or no heating demand. Six of the nine homes remain naturally heated with no resort to the underfloor system. On a recent visit to one of them on a sub-zero day, Harrall found it was a comfortable 21.6˚C inside at 9am. Third-party energy testing places the homes in the top 0.01% of the 12 million registered UK energy performance certificates. The average daily energy bill for a two-bedroom home on Howgate Close is just 10 pence.
