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Lacuna Passivhaus suitable bi-fold doors

Heat treated timber…and why it is such a good idea…

Heat treatment is not a new invention – whenever a Viking ship has been found at the bottom of Roskilde Fjord in Denmark, it has nearly always only been the keel that has been intact. To bend the bow and stern keel, the Vikings heat-treated the wood in hot embers, using water cooling to ensure that it did not catch fire. Once the wood was warm and smooth, the keel was bent into the desired shape. In this way, the sugars (Lactose and Glucose etc.), which the decomposition bacteria would otherwise live off, were burnt away and the wood achieved the unintended effect of a lifespan of many centuries.

The Danish company Lacuna makes use of this old method and manufactures the majority of their doors with heat treated wood. Being placed in large 198° C hot ovens for 24 hours, the structure of the wood changes permanently and gains a number of advantages, which ordinary soft or hard woods do not have.


Natural durability, dimensional stability and low maintenance requirements

Not only does the heat treatment increase the natural durability (and becomes equal to teak - Class 1) it also makes the timber very stable, and accordingly it is unable to absorb moisture and expand in humid weather. This is obviously an essential attribute for folding doors, where many panels in succession need to keep their dimensional stability in order to function properly all year around. It furthermore means that the paintwork does not crack and maintenance therefore is at a minimum.

Lacuna bi-fold doors - Thermal Performance

Whole window U-value (Uw)  -  as low as 0.85W/m²K
Frame U-value (Uf)  -  average 1.02W/m²K (centre leaves) - 0.92W/m²K (outer frame)
Spacer psi-value  -  as low as 0.029W/mK
Glazing U-value (Ug)  -  0.775W/m²K or 0.725W/m²K
g-value  -  0.5 (with the 0.775 glazing) or 0.38 (with the 0.725 glazing)

Lacuna bi-fold doors - Security

With internally beaded glazing (fixed with butylene sealant), toughened glazing as standard (laminated can be supplied), double rebate in frame, locks by Ruko/Assa on main door with 3-point locking system, 6-lever locks used as standard (and available in different configurations), 5.5mm marine-grade stainless-steel (316) hinges with security pins, hinges kept together by a bolt (which cannot be dismounted at the top) and all secondary door-blades being locked individually into both the top frame and the threshold – the Lacuna bifold doors are burglar-proof to the highest degree.


Individual .dwg and .pdf files can be downloaded from Lacuna (opens in a new window): http://lakuna.dk/index.php/technical-drawings.html


Please see our terms and conditions for more information.

5 Years on material and manufacturing defects. Please read our Lacuna Terms and Conditions and download Lacuna's Guarantee, Operation and Maintenance guide - Feb 2015





Danish made, Lacuna Passivhaus suitable bi-fold triple-glazed doors, energy efficient and environmentally sound, with Class 4 airtightness and U-values from as low as 0.85W/m²K. Building to the passivhaus standard no longer means sacrificing the aspiration of many self-builders and designers to create an inside-outside space.

  • Danish design and manufacturing
  • Bespoke dimensions for each individual project
  • Large openings available; up to 2700mm height and 6000mm width
  • Top hung design uses no guide rail and makes threshold cleaning easy
  • Slimmest profile available of any timber bifold door;  Only 56mm per door frame giving a maximum frame- to-frame width of 117 mm
  • FSC® (Forest Stewardship Council)-certified hardwood only, and environment certified
  • Superb Insulation - With the German heat-treated beech combined with double weather-stripping in the frame and triple weather-stripping between the door sashes the doors have extremely low U-values; e.g. for triple-glazing whole window U-values down to under 0.85 W/m²K and an average of 1.15 W/m²K for double-glazing.
  • No repainting for 25-50 years in normal conditions. The paint on heat-treated beech lasts longer without maintenance than on timber-alu products, as the heat-treated beech adopts the characteristics of a composite or fibre-glass and does not extend or retract when exposed to moisture or temperature
  • Weather-tight and watertight - The double/triple-sealed doors have been tested and have passed the most demanding test available for wind and water permeability
  • Highest marine-grade acid-free brushed stainless-steel handles and hinges (316) to prevent rusting in even harsh seafront locations
  • Precision manufacturing - Lacuna uses advanced five-axial CNC-machines to cut out the wood to achieve a precision of one tenth of a millimetre
  • Skilled carpenters assemble and paint each door by hand, and every product is checked and provided with an approval number

Please contact us, sending details of your project (schedule, elevations and plans). Also, click on the Technical tab below for more product information and see our terms and conditions.

  • Brand: Lacuna
  • Product Code: Lacuna
  • Availability: Contact us for a QUOTE

Tags: triple-glazed, sash, window, doors, lacuna, bifold, passivhaus, suitable