Crosslam timber / CLT - Performance characteristics

Insulation performance

Thermal conductivity (λ lambanda value) : 0.13 W/mK (spruce). CLT insulation performance is similar to aircrete blockwork and better than concrete blockwork or brickwork. Additional insulation is usually required (see External cladding) to upgrade the element to a higher performance. Passivhaus standards are easily achievable.


Thermal mass in CLT

Density: 480–500 kg/m3 (spruce).  Though trumpeted as an effective thermal mass, exposed CLT panels only provide a low – medium level of effectiveness and well behind brick or concrete / concrete blockwork.


Decrement delay through crosslam

The combination of thermal conductivity, specific heat capacity and density of solid wood panels such as CLT together with a multi-layer construction format ensure a high level of decrement delay. This is particularly useful in high summer temperatures where 10 – 14 hours of decrement delay (phase shifting) can be achieved.



Crosslam / CLT construction generally has a reputation for being able to achieve high degrees of airtightness through the timber technology alone – rather than, say, with the addition of a vapour control layer.

Good airtightness is achieved through well-matched contact surfaces screwed together. In addition, its common to find additional sealing at junctions such as those of the floor / wall and wall / pitched roof types. Additional sealing can be achieved through using pre-compressed foam tape or breather tape or sheets of airtight material.

However, because of shrinking cracks near the edges of the panels and movements in the panels caused by shrinking and swelling, there is a risk that local sealing might break down and fail over time. If this risk is considered significant, it might well be that the designer includes a vapour control layer or vapour permeable ‘intelligent’ membrane (used with other vapour permeable cladding materials) over the external face of the panel to ensure long-term airtightness.


Moisture protection

Cross laminated timber is a vapour permeable and vapour retardant construction material. The vapor diffusion resistance value is between μ = 40-80, dependent on the moisture content.

Where vapour-permeable (‘breathing’) external cladding materials, such as cellulose, woodfibre, hemp etc., are used, a vapour control layer is not needed.



Though the acoustic performance of CLT is good and stands well in comparison with other materials, relying on floor or wall panels alone to provide required levels of sound reduction is difficult.

In addition to direct sound transmission reduction, care needs to be taken in ensuring that flanking sound is also addressed. Typical flanking paths include continuous CLT subflooring, leaks around the edges of partitions, joints between floors and surrounding walls and leaks through electrical outlets.

Floors and ceilings

The use of an air gap is the most effective measure in improving performance and can be located above or below the CLT floor.

A below the ceiling solution might typically be a double layer of plasterboard suspended below the CLT floor on resilient supports and rails along with sound absorbing insulation in the air gap,

Above the floor the standard European solution is a concrete floor screed laid over a sound absorption layer and rigid insulation. However other options would include combining the suspended ceiling with a floating floor or timber laminate flooring laid over a sound-absorbing underlay.

Compartment walls

Sound reduction can usually be found by either sandwiching one CLT panel between layers of natural fibre insulation and plasterboard OR by using two CLT panels separated by a gap containing sound absorbing material and plasterboard to external faces.