Passivhaus Plus first for Shetland Isles

A Shetland self-build project has achieved Passivhaus Plus certification. It is one of only a handful of Passivhaus Plus schemes in the UK and the first in the Shetland Isles.

The Skeotaing project comprises a 4-bedroom house certified to the Passivhaus Plus standard, alongside a 2-bedroom Passivhaus Classic annex and double garage. The Passivhaus Plus standard was chosen to reduce future energy bills and address the climate emergency. The house is located on a north-facing slope with views to the north and west. The future addition of a wind turbine or more PV panels may change the rating of the annexe.

Key stats

Construction: Timber frame with SIPs panels
TFA: 306.8 m² (main house), 138.7 m²(annex)
Build start date: 2021
Completed: 2023
Certified: Passivhaus Plus (main house), Passivhaus Classic (annex), 2023

Skeotaing Passivhaus Plus. Image credit: Allen Duncan

Construction

The house is built with timber frame, SIPs panels and a rendered concrete block exterior. The original design proposed highly insulated blockwork cavity walls to achieve high thermal capacity & retain heat in the winter. However, there was a shortage of block layers in the area. Timber-frame kits were then explored, already familiar to local builders.

The 235mm studs of a standard timber frame kit needed to be upgraded to a 245mm frame to accommodate the continuous layer of 50 mm of PIR wall insulation fixed to the inside of the kit.

Skeotaing Passivhaus Plus. Image credit Allen Duncan

The project's floor has 240 mm insulation plus a 100 mm thermo block insulation board around the perimeter under the walls and wall plate to minimise thermal bridging. High-performance triple-glazed doors and windows, with a bespoke non-standard spacer bar, have been complemented by Passivhaus-certified quintuple-glazed rooflights.

U-values

Floor: 0.074 W/m²K

Ground floor concrete on PIR insulation boards

Wall: 0.088 W/m²K

Timber frame SIPS panels with rendered concrete blocks Wall build up exterior - interior:

100mm concrete block,
50 mm air gap,
245 timber frame kit with factory injected insulation,
50 mm continuous PIR insulation,
38mm service gap for cables,
15mm plasterboard.

Roof: 0.089 W/m²K

Roof tiles on SIPS panels supported on Glulam ridge beam

Building performance

Skeotaing Passivhaus Plus. Image credit: Allen Duncan

Designed energy performance

Airtightness n₅₀ (≤ 0.6ACH @ 50 Pa)

0.4 @ 50 Pa

Space Heating Demand (≤ 15 kWh/m².a)

13.55 kWh/m².a

Heating Load (≤ 10 W/m²)

8.65 W/m²

Primary Energy Renewable (PER) Demand (≤ 60 kWh/m².a*)

25 kWh/m².a

Primary Energy Renewable Generation

55 kWh/m².a

*+/-15 kWh/m².a allowance if offset by energy generation. See Passivhaus criteria.

Services

Skeotaing Passivhaus Plus. Image credit: Allen Duncan

The services for the Passivhaus Plus project include solar PV, storage batteries, solar thermal, ground source heat pump, heat storage tanks, and two MVHR systems.

43 JA385W solar panels, each with individual solar edge optimiser
Roof-mounted solar thermal glass tubes for hot water
Electrical power storage via three 9.7kWA solar edge batteries
8kW ground source heat pump linked to five boreholes each 47 metres in depth
Two water tanks, one 500 litres for hot water plus one 200 litres for underfloor heating

Challenges

The site's orientation provided the biggest challenge to the project. It was clear that the low winter sun in Shetland would not permit much solar gain on the north side of a hill in Skeotaing. Larger windows were proposed to take advantage of the north/northwest views but had to be reduced in size to meet the PHPP energy balance requirements. South-facing windows to obtain much-needed winter solar gains were included but the design team found it difficult to get the heat balance to work. Extra insulation was added, and window performance was increased, by adding a higher-performance spacer bar and reducing glazing sizes. After 3 days of re-design, the PHPP model finally showed that the design could meet the Passivhaus Plus standard.

Drilling for the ground source heat pump also proved challenging, as the adjacent bog was not deep enough to accommodate the coils. The design was changed to drill into the granite rock below the site. However, the only available drilling rig in the Shetland Islands was used for dynamite at a quarry with a different drilling specification so the scheme was redesigned to work with a smaller pipe diameter and deeper holes.

The Passivhaus Plus standard was adopted to provide comfort, low energy bills and to prove that the Passivhaus Plus, energy positive standard, was possible even on a north facing slope at 60 degrees north in the Shetland Islands.

Allen Duncan, Allen Duncan Architects

Lessons learned

This was the first Passivhaus project for almost the whole design & construction team; goal-setting was vital. An early-stage training afternoon was held in a local sports centre for all site workers, joiners, electricians, plumbers, and the air pressure tester. It proved critical in getting the team to understand the importance of a continuous thermal envelope and airtightness.

It was instilled with all working on site that they were not building a normal project,but were, in fact, building a submarine where one tiny leak could sink the boat!

Allen Duncan, Allen Duncan Architects