Raised in Just 10 Days, This Airtight Prefab Is a Lesson in Efficiency

By Lucy Wang
This stunning forest retreat in England uses prefabricated panels to minimize site impact, shorten construction time, and protect against weather.

London-based architecture and design studio Alma-nac designed House in the Woods, a contemporary home surrounded by pristine forest in England’s South Downs National Park to replace a decrepit, 1950s bungalow. The new 240-square-meter dwelling retains the former building’s simple gabled form and footprint, but offers greater flexibility with its ability to change from a single bedroom home to five bedrooms for large family gatherings. The project’s construction value was approximately $830,000, not including value-added tax.

To speed up construction and to create a weathertight envelope, House in the Wood’s superstructure was prefabricated offsite using structurally-insulated panels. The external walls are clad in thermally-treated timber and hand-cut brick, while natural slate tops the sloped roof. 

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"Nowadays, timber is no longer considered an insulating material in energy-efficient design: it is a cold bridge in wall and roof construction," says Alman-ac partner Tristan Wigfall. "Constructed using SIPs (Structurally Insulated Panels) technology, panelized construction far out-performs timber frame methods both in terms of thermal efficiency and airtightness of the completed structure."

Each panel of rigid insulation, lined on each side with timber skins, stretches the height of the full story and carries the building load. After all the wall/floor junctions are sealed, the resulting building is both incredibly airtight and thermally insulated. This lowers the heating requirement in winter and, thanks to passive ventilation via Velux roof lights, keeps the interior comfortable in summer.

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To complement the highly insulated construction, floors were built with concrete that function as "heat sinks," storing heat during the day and releasing that heat at night. If the homeowners do need heating, they can use the air source heat pump-powered underfloor heating installed in the concrete slabs, localized radiators, or a wood-burning stove. A heat-recovery unit continually introduces fresh air into the airtight home.