08 Material Considerations - Timber and CLT
17/11/2025 (home)
Week 1 : Timber, Processing and Properties. Timber Framing Introduction and Cladding
timber is wood prepared for use in construction
natural timber - some trees (species, which hardwood, most used, cladding) e.g., common oak (hw), sweet chestnut (hw), scots pine (hw), norway spruce (sw), western red cedar (sw).
softwood - 80% of UK timber in construction is pine and spruce
leaves absorb sunlight and carbon dioxide from the air and combines with water drawn up from the roots to produce sugars and release oxygen to the air (photosynthesis)
growth rings (annual rings but not always annual!) ~ bc wood at the start of the year (early wood) has different properties from wood at the end of the year (late wood)
early wood - conduction and later support is dominant (vessel diameter is smaller),
continuous growth = uniform ‘grain’
seasonal growth = growth rings
strength of natural materials is dependent on the grain angle, knots and defects, ratio of latewood to early wood
stronger when grain is parallel to the wood
knots (branch meets trunk): green knots (wood fibres of branch continuous with trunk), black knots (branch has died)
moisture content: { (weight - dry weight) / dry weight } x 100 = % of moisture content
external use - 15%
internal use - 10%
>20% risk fungal decay
no drying (seasoning) - green timber - shrink over the first years of building
grading timber: process of assessing and categorizing wood based on its strength, quality and sustainability for specific applications either visually or by machine.
then assigned a strength class
can look up characteristic values for design e.g., stiffness
structural design and sizing e.g., without species
visual grading:
softwood
GS = general structural
SS = special structural
hardwood (all UK hardwoods are visually graded)
HS = hardwood structural
machine grading is measuring stiffness related to timber strength
durability - rot:
fungus, moisture +20%, keeps timber dry by detailing.
overhangs, protecting end grain, protecting from rain splash, lifting away from ground, leaks - moisture from ground and condensation
timber in buildings
durable - if appropriately specified
natural appearance
workable
strong
low thermal conductivity
high sustainability
SUSTAINABILITY
how?
low carbon footprint (low embodied energy/carbon) buildings,
carbon sequestration,
low generation of other pollutants
lightweight
good thermal properties (cold bridging)
low toxicity,
benefits of offsite fabrication.
whole life carbon = operational carbon + embodied carbon
whole life co2 = operational co2 + embodied carbon co2
insulation thickness required to obtain insulation value - u value calcs
VCL vapour control layer: membrane installed within the fabric of a building to limit the movement of water vapour from the warm interior to the colder exterior - reduces risk of condensation forming in the walls - structural damage/mould growth/reduced insulation performance
thermal bridge - an area of less well insulated construction
WORKSHOP
1500 × 1500mm on concrete base (no new foundation)
hit u value of 0.15
primary timber frame 140mm deep w/ cavity insulation + PIR 65mm
25mm unventilated services void
external timber rainscreen cladding — 20mm (cedar board, orientation w/ 8–12 mm shadow gap)
vertical secondary battens / ventilated air gap behind cladding — 20 mm (creates ventilated rainscreen cavity)
breathable weather — typically a thin layer
OSB — 12 mm (structural sheathing, vapour-open)
continuous rigid insulation (PIR, high performance) — 65 mm (0.022 W/mK) — reduce thermal bridging
primary timber stud frame — 140 mm deep
cavity insulation (mineral wool) inside stud bay — 115 mm (stud depth - 25 mm services void)
services void (unventilated) behind lining — 25 mm (for wiring, small services)
internal lining (plasterboard) — 12.5 mm
internal surface thermal resistance and external thermal resistance are included
Rsi = 0.13
Rse = 0.04
material choice
timber species: accoya is recommended for longevity, low movement, high durability and low maintenance which is good for exposed cladding on uwe campus
finish: oil or water-based stain allows the grain to show and is breathable. also it means we can avoid impermeable paints that trap moisture
direction of cladding: vertical rainscreen boards with shadow gaps - helps rapid shedding and a contemporary appearance
fixing method: stainless steel clip system (hidden fix) and for visible fixing (if needed) - countersunk screws
secondary framing: battens 20 × 40 - 20 × 50mm to create rainscreen cavity - ensure battens are pressure-treated
u-value calculation:
R1 + R2 + R4 + R5 + R7 + R8 + R9
0.143 + 0.15 + 0.092 + 2.955 + 3.194 + 0.18 + 0.05 = 6.764 m²K/W
films: total R = 6.764 + 0.13 + 0.04 = 6.934 m²K/W
1 / ans = 0.144 W/m²K
R1 + R2 + R4 + R5 +R6 + R8 + R9
0.143 + 0.15 + 0.092 + 2.955 + 1.077 + 0.18 + 0.05 = 4.647 m²K/W
films: total R = 4.647 + 0.13 + 0.04 = 4.817 m²K/W
1 / ans = 0.208 W/m²K
overall: 0.075 × 0.208 + 0.925 × 0.144 = 0.149 W/m²K
construction
fix secondary battens to OSB with stainless screws into studs - 20mm vent gap behind cladding
install breather membrane over OSB before PIR
vapour control with mineral wool in cavity and continuous PIR outside
fasteners: stainless steel and secret clips (cleaner aesthetic)
I chose a rainscreen timber cladding system over a framed timber backing to combine durability, maintainability and thermal performance while retaining the natural aesthetic appropriate for UWE Frenchay. The first priority was to meet the thermal target of U = 0.15 W/m²K for the mock-up while keeping the primary frame reasonably shallow (you asked for 140 mm or deeper). Thermal bridging through studs is the single largest penalty in framed timber walls — standard stud cavities filled with insulation often struggle to meet 0.15 if studs are close-spaced. To mitigate this, I used two complementary tactics: (1) fill the stud cavity with high-quality batt insulation (mineral wool λ ≈ 0.036) and (2) place a continuous layer of rigid insulation (PIR, λ ≈ 0.022) outside the sheathing. The continuous PIR drastically reduces the effective thermal bridging by placing high R-value outside the studs so heat flow is forced through the insulation rather than directly through timber. This lets us keep the primary timber frame to 140 mm while still achieving the target U with about 65 mm PIR. A 25 mm unventilated services void is retained on the warm side to allow wiring distribution without penetrating the thermal layer repeatedly — it’s small enough to have limited thermal effect but large enough to fit wiring/trunking.
For species and finish I recommend Accoya for the rainscreen (or alternatively Western Red Cedar/Siberian Larch). Accoya’s dimensional stability and durability reduce maintenance and staining cycles in an exposed campus environment; microporous oils or translucent stains are recommended so the timber can breathe and the finish is repairable. I propose vertical boards with secret clips for a contemporary look and effective water shedding; vertical boards also reduce the risk of water pooling at laps. Battens should be ventilated to provide a rainscreen cavity behind the cladding (20 mm), to allow drainage and drying.
Structurally, the secondary battens attach back to OSB sheathing and the primary studs; battens sized ~20×40–20×50 mm are adequate. Use stainless steel fixings for longevity and avoid through-cladding penetrations except where required. Manage airtightness with a continuous internal airtightness tape and consider a vapour control layer if the client wants more robust moisture control (or ensure the PIR and the sheathing arrangement give the required vapour profile).
Finally, check fire classification of the chosen PIR (some PIR boards need protection) and validate details with a thermal modeller or a SAP / U-value calculator for formal compliance. The assembly above gives a robust, low-maintenance solution that meets the brief while keeping the mock-up buildable and illustrative of a real external wall detail.