Inside the Treevia wall: two build-ups, one U ≤ 0.15
Cut a Treevia exterior wall in half and you won't find a single slab of insulation between two boards, the way a lot of people picture a timber-frame wall. You'll find nine distinct layers, each doing one specific job, stacked to a purpose. An exterior wall isn't just the line between "inside" and "outside" - it's the piece of engineering that decides your thermal comfort, your structure's durability and your heating bill for the next 30 years. Here's what's actually in it.
Why the thickness is what it is
The Treevia exterior wall comes in two build-ups: 38.4 cm with timber cladding or 41.0 cm with a rendered facade, both including the installation shaft and a double plasterboard finish. That's noticeably more than a typical timber-frame wall (28-30 cm) - but every extra centimetre is accounted for.
The payoff: a U-value of 0.15 W/(m²K) - 25% better than the WT 2021 standard. Zero condensation in any layer. An 11-hour phase shift. A 17× damping of the outdoor temperature swing.
Variant A - timber cladding, 38.4 cm
1. Timber cladding 21 mm - tongue-and-groove boards, factory painted. A ventilated facade.
2. Ventilated cavity 30 mm - air behind the cladding carries moisture away and protects the membrane for decades.
3. Dörken Delta-Vent S+ membrane 0.5 mm - highly vapour-permeable windproofing. Lets vapour out, keeps water and wind out.
4. Mineral wool 2 × 50 mm - on cross battens in front of the frame, λ = 0.036. Breaks thermal bridges at the studs.
5. Wool in C24 studs 145 mm - a 45 × 145 mm frame at 600 mm centres, fully filled. 295 mm of wool in total.
6. PE foil 0.2 mm - vapour barrier, sd = 26 m. Airtightness from the inside and zero condensation.
7. OSB/3 board 12 mm - internal sheathing, racking stiffness.
8. Installation shaft 50 mm with wool - cables and pipes stay on the warm side of the vapour barrier.
9. Plasterboard 2 × 12.5 mm - finish, acoustics and fire resistance.
Total: 38.4 cm, 295 mm of mineral wool, U ≤ 0.15 W/(m²K).
Variant B - rendered facade, 41.0 cm
1. Thin-coat render 5 mm - silicone render on a primer coat.
2. Adhesive with reinforcing mesh 5 mm - reinforced base layer for the render.
3. Rigid facade wool 100 mm - bonded and doweled, λ = 0.036.
4. Fermacell board 12.5 mm - external sheathing, non-combustible and rigid.
5. Wool in C24 studs 200 mm - a 45 × 200 mm frame at 600 mm centres. 350 mm of wool in total.
6. PE foil 0.2 mm - vapour barrier, sd = 26 m.
7. OSB/3 board 12 mm - internal sheathing.
8. Installation shaft 50 mm with wool - services routed without piercing the vapour barrier.
9. Plasterboard 2 × 12.5 mm - internal finish.
Total: 41.0 cm, 350 mm of mineral wool, U ≤ 0.15 W/(m²K).
The installation shaft - a Treevia signature
Cables, pipes and sockets run in the 50 mm shaft, not in the structural wall: servicing without touching the insulation, room for future upgrades without chasing walls, and an unbroken vapour barrier.
Why three insulation layers beat one
A typical timber-frame house has a single layer of mineral wool (say, 180 mm) between studs. The problem is that timber studs insulate worse than wool does, so a thermal bridge forms at every single stud.
Treevia instead runs three independent insulation layers (75 + 150+ + 50 mm) on three different sets of battens. The layers overlap, so the bridges get broken up rather than running straight through. The result:
- A better U-value at a comparable overall thickness
- More even temperature distribution across the wall surface
- Zero condensation - confirmed by ubakus calculations at -20°C / 80% humidity
What an 11-hour phase shift means for your afternoon
When July temperatures hit 35°C at 2:00 pm, that heat wave only reaches the interior around 1:00 am - by which point it's already 18-20°C outside. The wall effectively buffers the heat for you.
17× amplitude damping means a 30°C swing outdoors becomes roughly a 1.5°C swing indoors. In practice, that's a house that stays naturally cool through summer without leaning on air conditioning - a parameter that only gets more valuable as summers get hotter.
The shaft you won't appreciate for 15 years
Nobody notices the installation shaft on move-in day. But 15-20 years down the line, when you need to replace wiring, upgrade plumbing, add sockets or reshuffle a room layout, a 4-6 cm shaft is the difference between a simple job costing a few thousand and a costly renovation running €5,000-12,000, complete with broken walls, damaged insulation and a punctured vapour barrier.
In houses without a shaft, every installation job means cutting into the thermal envelope - and a pierced PE film never seals as well the second time.
Parameters at a glance
- Wall thickness: 38.4 cm (timber cladding) or 41.0 cm (rendered facade), including installation shaft and 2× plasterboard
- Wall U-value: 0.15 W/(m²K) - 25% better than WT 2021
- Condensation: zero in any layer (ubakus calculations, -20°C / 80%)
- Phase shift: 11 hours
- Amplitude damping: 17× (30°C outside = ~1.5°C inside)
- Structure: C24 timber, studs at 600 mm
- Membrane: Dörken Delta-Vent S+ (high-diffusion)
- Insulation: 3 layers of mineral wool on independent battens
Every layer earns its place. If you'd like the full technical drawing for either variant, ask our team - it's a five-minute read once you have the picture in front of you.