Prefabricated timber frame house in Batumi from factory-made panels

A prefabricated timber frame house is built from the same studs, insulation and sheathing as an ordinary timber frame house, except that the walls and floors arrive on the plot as ready-made panels. They are made in a workshop to the design, and on site they are set on the foundations, tied together and closed in with the roof. In Batumi this sequence has an obvious advantage: the layers of the wall most sensitive to water are assembled under the workshop roof, not in the autumn rain.

How building with panels differs from framing on site

In an ordinary timber frame house, carpenters cut the timber to fit and build the house wall by wall, and a small design error can be put right on site. With panels the work is split differently: almost everything is decided in the drawings, and the plot is only for installation.

Panels can be open or closed. In an open panel the workshop builds the frame with the external sheathing, and the insulation and vapour barrier are added on the plot. In a closed panel the whole wall is finished, sometimes with the windows in: installation is faster, but you will only see what is inside on the photos from the workshop. How the wall itself is built, and why in Batumi the vapour barrier is chosen with air conditioning in mind, we explain on the page about timber frame houses built on site.

The design and the panel order

The architect divides the house into panels so that they can be delivered and lifted, and so that the joints don't fall at the corners of windows. Before the order, the design has to show every opening, the water and drainage entries, the cable channels, the fixings for the kitchen and the water heater, and the opening for the flue. Once the workshop has cut the timber, moving a window by half a metre means a new panel and another week of waiting.

We don't have our own production: we order the panels from a manufacturer to our design. We check the manufacturer's working drawings against the architectural design and the structural engineer's calculation before they go into production. We also look separately at what timber the manufacturer uses for the frame and at what moisture content, what insulation goes in and how the panels are fixed to one another.

Foundations for panels: accurate to the millimetre

A frame built from timber on site can be adjusted to fit; a panel can't. If the top of the ground beams or slab varies in height, the panel joints open up, the windows go in out of square and the roof lies in waves. So we bring the top of the foundations dead level with a surveyor's level, check the diagonals of the footprint, and set the anchors for the sole plate using a template taken from the panel layout.

As with any light house, we choose the type of foundation from the ground survey: a slab on level ground, piles with ground beams on a slope. The sole plate goes on a damp-proof course and is raised above the ground, and the floor panels over the crawl space are covered underneath with mesh against rodents.

Delivering and installing panels in Batumi

The size of a panel is set not only by the drawings but also by the road. A long panel has to get along the street, turn at the gate and get onto the plot. The crane truck needs a level standing area next to the house, with no overhead wires above it. In tightly built-up streets and on a steep slope we make the panels smaller, and some of them are put up by hand.

The panels arrive in the order of installation and go straight into place, without being stacked on the ground. A small house is taken up to roof level in a few days, so we schedule installation for a window of dry weather and close up the joints the same day. For mountain plots this is the main argument for panels: how delivery and the building season work in the mountains is covered on the page about mountain houses in Adjara.

Joints: moisture, heat and vapour

In the workshop the wall is built true and dry; the weak points appear where the panels meet. Heat escapes through a joint if a void is left in it, and water gets in if the breather membrane of one panel doesn't overlap the next. So we close every joint in the same order:

• we fill the joint cavity tightly with mineral wool;
• on the outside, we lap the breather membranes of neighbouring panels over each other and tape them;
• on the inside, we continue the vapour barrier across the joint and tape it, at the floor and ceiling too;
• we photograph the corners and the junctions with the floors before the inner lining goes on.

We fit the facade on battens with a ventilated air gap on site, over the taped breather membrane, so that the cladding runs continuously and doesn't follow the joints between panels.

Panels filled with mineral wool behave like an ordinary timber frame in humid conditions, not like a SIP panel with expanded polystyrene. How these walls differ in a humid climate is shown in the comparison of SIP panels and timber frame.

Seismic design and fire safety

An earthquake puts only a small force on a light panel house, but all of it passes through the connections: the house is assembled from flat panels, and what holds it together are the ties between them. Panel to panel, panel to floor, sole plate to foundations: the structural engineer calculates every connection to Georgian codes for the ground on your plot. A manufacturer's standard details may be designed for a region without earthquakes, so we don't accept them without a check. A floor made of panels has to work as a rigid diaphragm, and the floor panels are tied together along the full length of each joint.

Fire is held back by the same layers as in a timber frame: the mineral wool inside the panel and the plasterboard that lines the panels on the inside, fitted on site. The workshop makes the cable channels to our wiring layout, and the cables run in them in protective conduit. Exposed timber in the loft and the rafters gets a fire-retardant treatment.

Where a panel house makes sense

Panels pay off most where the design is repeated. Several identical guest cabins on one plot are built from one set of drawings, and each one goes up faster than the one before. What a rental property needs, from a bathroom in every room to quiet between rooms, is described on the page about guest houses. Panels also suit a country house and a mountain plot with a short dry season.

They are a poorer fit for a house of complex shape that you would like to change as the build goes on, and for a plot where a crane can't get in. If you want an even higher degree of factory completion, with finishes and plumbing, that is a modular house.

How much a prefabricated timber frame house costs

The shell of a prefabricated timber frame house costs from 690 to 840 ₾ per m² of floor area. The price covers the foundations, walls and floors made of ready-made insulated panels, the roof, windows and assembly on site. Building services and finishing are priced as later stages, and the design separately as well. A turnkey house costs from 1,470 ₾ per m².

For comparison, the shell of a timber frame house built from timber on site starts from 750 ₾ per m², and a SIP panel shell from 780 ₾. If you have ordered a panel kit yourself, assembly on ready foundations costs from 125 ₾ per m² of the house floor area, labour only. A screw pile with a head plate for a small house costs from 300 ₾ each.

Costs go up with panels from abroad, which bring delivery and customs clearance, with tight access, where the panels have to be made smaller, and with any changes after the order. They come down with a series of identical houses, a simple rectangular plan and a level plot by the road. We send a preliminary estimate from the plan within 24 hours on business days; the final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.