Building envelope & airtightness
Insulating a timber-frame wall
Tight insulation without voids and thermal bridges
The insulation provides most of an external wall's thermal resistance, but only if it fits tightly, without voids and without thermal bridges. The guide covers the U-value requirements, choosing insulation by λ, correct installation, wind barrier and vapour barrier, and why airtightness often matters more than the last centimetre of insulation.
Know the U-value requirements
BR18 sets the minimum U-values for new buildings: external wall U ≤ 0.30 W/m²K, roof/ceiling to an unheated space U ≤ 0.20 W/m²K and ground floor slab U ≤ 0.20 W/m²K. For renovation, the requirement is typically tightened to approx. 0.18 for external walls. A lower U-value means better insulation.
Exterior wallU ≤ 0,30 W/m²KRoof/ceilingU ≤ 0,20 W/m²KGround-bearing slabU ≤ 0,20 W/m²KChoose insulation according to λ from the data sheet
Mineral wool for timber-frame walls is typically declared with a thermal conductivity λ of 0.034–0.037 W/mK — always check the current product data sheet, as λ determines how thick the insulation must be to meet the U requirement. Lower λ = better insulation at the same thickness.
λ mineral wool (typical)0.034–0.037 W/mKCut the insulation to size
Cut the insulation boards/batts slightly wider than the distance between the studs so that they are held tight and do not leave gaps along the edges. A poorly cut board with air gaps along the side loses a large part of its effect — the air gap is a thermal bridge.
The insulation must be lightly squeezed between the studs — no gaps along the edges.
Build tight and without voids
Fill the entire cavity, stagger joints between boards/layers, and avoid compression that reduces the thickness. With several layers, lay them with staggered joints so no joint runs straight through the insulation plane. Voids and joints mean direct heat loss.
Fit the windbreak on the cold side
Fit windbreak board or membrane on the cold (outer) side of the insulation, so the wind does not blow into and through the insulation. The windbreak must be vapour-open, so moisture can escape outwards. Joints are sealed according to the supplier's instructions.
Fit the vapour barrier on the warm side
On the warm, inner side is the vapour barrier, which keeps indoor moisture out of the construction. See the separate guide «Fitting a vapour barrier» for overlap (min. 100 mm), sealing and airtightness. The order cold-side wind barrier / warm-side vapour barrier is fixed.
Understand the U-value calculation
U = 1 / R_total, where R_total = R_si + Σ(d/λ) + R_se. The surface resistances are R_si = 0.13 m²K/W internally and R_se = 0.04 m²K/W externally. Each insulation layer contributes R = thickness[m] / λ. It is the insulation layer that provides by far the largest part of R_total in a lightweight wall. Use the «U-value» calculator with your own layers and λ from the data sheets.
R_si (int.)0.13 m²K/WR_se (ext.)0.04 m²K/WInclude thermal bridges
Mechanical penetrations — studs, screws, bolts and brackets that break the insulation plane — are thermal bridges and must be included in the U-value documentation. A calculated thermal bridge correction is required for the building permit; never leave the thermal bridges out of the estimate.
Airtightness and thermal-bridge-free details can save more energy than an extra centimetre of insulation.
Source
Based on Discebo's carpentry article «Energy requirements in BR18 — U-values and energy frame», the product range's «Mineral wool — batts» and the calculation module's U-value method (R_si/R_se according to EN ISO 6946; BR18 minimum requirements via bygningsreglementet.dk ch. 11).