Timber facade cladding

What is timber facade cladding and how is it built up?

Timber facade cladding is a skin of boards or battens hung on a substructure in front of an insulated wall. It is not a structural layer and it is not an insulating one: it is the weather-facing surface of a ventilated facade, and everything about how it is detailed follows from that single fact.

The build-up is fixed in its logic. The insulation sits against the wall, counter-battens create a clear cavity of roughly 25 to 30 mm, and the boards are fixed to battens over that cavity. The cavity must be open at the bottom and at the top so that air can enter, rise and leave, carrying moisture out of the insulation and off the back of the boards. Timber conducts at roughly 0.13 W/(m·K), so the cladding adds practically nothing to the thermal resistance of the wall, and it should never be counted on to.

Why does timber cladding need a ventilated and drained cavity?

Because timber wets and then has to dry from both faces. Rain reaches the front, and condensation and driven water reach the back, so a board fixed flat against insulation or masonry holds moisture on its rear face where it cannot evaporate. That is where rot starts, and it starts invisibly, which is why a facade can look sound for years and then fail all at once along one elevation.

The cavity solves it in two ways at the same time. Air movement dries the back of the boards, and the gap gives any water that passes the joints a drained path to run down and out. This is the same rainscreen principle that lets open joints between boards exist at all: the design assumes water gets past the cladding and answers it with drainage rather than with sealant.

Why is larch the usual Slovak choice, and what does it do over time?

Larch is locally available, naturally durable in its heartwood, dimensionally reasonable and moderately priced, which is why it is the default on Slovak timber facades. It is the worked example worth understanding, because everything it does, the other species do too, only faster or slower.

Left untreated, larch weathers. Ultraviolet light breaks down the lignin at the surface and the wood turns a silver grey, quickly on the south and west elevations and slowly where an overhang shelters it. Greying is a surface effect, not decay, and untreated larch on a properly ventilated cavity has a long service life in that state. What it is not is uniform: the same building will grey unevenly, and a client who has not been shown a weathered example will read that unevenness as a defect. That conversation belongs at the design stage, and it is the single most common source of disappointment with timber facades.

What are the detailing rules that decide the service life?

• Keep the bottom edge of the cladding clear of the ground and of any surface that splashes, so the end grain is never standing in water.
• Detail the head, the reveals and the sills so that water is thrown clear of the boards rather than run behind them.
• Use fixings compatible with the species: tannin in larch and oak reacts with the wrong metal and stains the board around every screw.
• Fix boards so they can move, because timber swells and shrinks across the grain with every season.
• Seal or protect end grain, which absorbs many times faster than a face.
• Keep the cavity continuous past every horizontal interruption, or the ventilation stops where the water collects.

None of these is expensive at the time and each of them is the reason a facade either lasts decades or needs boards replaced within ten years. The material rarely fails on its own; the junctions fail, and they take the material with them.

How does timber cladding compare with a rendered facade?

Because the cavity behind timber cladding behaves like a chimney in a fire, and because timber itself is combustible, mineral wool at reaction to fire class A1 is the usual insulant behind it, with cavity barriers designed in. Fire safety is governed by STN 92 0201 and signed off by the fire authority for the specific project, and on anything larger than a detached house that assessment can decide the facade before any aesthetic question is reached.

What are the environmental arguments for timber cladding?

They are real but narrower than they are usually presented. Timber stores carbon that was bound during growth, and processing a board is far less energy-intensive than manufacturing a render system, so the embodied carbon comparison favours timber clearly. On a timber frame house the two decisions reinforce each other.

Two qualifications belong with that. The carbon benefit lasts only as long as the boards stay on the building, so a facade replaced after twelve years because it was badly detailed has spent its advantage. And the origin of the timber matters: certified timber gives an auditable answer to where the wood came from, which uncertified material of unknown provenance simply does not.