Timber frame construction

How is a timber frame house built?

A stud system is a set of vertical timber members at a regular spacing with insulation between them, plus a sheathing board that braces the wall. The frame carries the load, the board provides racking stiffness and a substrate for the following layers, and each floor deck acts as a platform for the next storey, which is where the name platform frame comes from.

The difference between building on site and prefabricating is organisational rather than structural. On site the frame is cut and assembled in place, which is flexible when things change but leaves the timber exposed to the weather for longer. Prefabricated panels are made in a factory where timber moisture and dimensional accuracy can be controlled, and the finished envelope goes up in a few days. Prefabrication locks decisions in earlier, though: a change after the panels are made costs more than the same change in blockwork.

Why is the wall thinner and the process dry?

The insulation sits inside the structural depth, between the studs, so wall thickness is not the sum of a structural layer plus an insulation layer as it is in insulated masonry. With glass wool at roughly 0.032 to 0.040 W/(m·K), stone wool at 0.035 to 0.045 or flexible wood fibre batts at 0.038 to 0.040, a fairly thin wall reaches both levels of STN 73 0540 Part 2: the recommended value of 0.22 W/(m²·K) for the external wall of a new dwelling and the target value of 0.15 W/(m²·K) that applies to new buildings from 1 January 2021.

The second advantage is the dry process. No water from mortar or wall concrete enters the structure, so there is nothing to dry out and follow-on trades can start at once. The winter break shrinks and the overall construction period gets shorter. A dry build is not a weather-free build, however: frame and board materials must be protected from rain during erection, soaked timber must not be closed in, and weather protection during erection belongs in the programme rather than in the site manager's improvisation.

Why do airtightness and the vapour control layer decide the outcome?

A masonry wall is reasonably airtight in itself and holds a reserve of material that forgives moisture. A timber frame has neither. Its airtight layer and its vapour control layer are structural components in the functional sense, not accessories, and if they leak then humid indoor air reaches insulation held inside a structure that was not built to get wet.

A vapour retarder has an equivalent air-layer thickness of roughly 0.2 to 10 m, and variable membranes move between about 0.25 and 10 m with seasonal humidity. External layers must be diffusion-open, at 0.3 m or less and commonly under 0.1 m for roof underlays. The falling order applies: tight inside, open outside, by roughly a factor of five or six across the build-up. Services belong in a service void in front of the control layer so that nobody drills through it, and the result is measured by a blower door test while the layer is still accessible for repair. Airtightness is therefore not a certification exercise here but the mechanism that keeps the structure dry.

What has to be designed in that masonry gives you for free?

A timber frame has little thermal mass, so it warms up fast, which is pleasant with heating on demand and unhelpful during a heatwave. Summer overheating has to be solved in the design: glazed area and orientation, external shading, night ventilation. Acoustics work the same way. A lightweight structure has no mass of its own, so mass is added deliberately, through additional board layers, resilient mountings and a floating floor over the deck. Building acoustics requirements are set by STN 73 0532, and meeting them on the drawings is far cheaper than in a finished house.

How does a timber frame compare with masonry and with solid timber?

If mass matters to you, the timber alternative is a solid cross-laminated panel: heavier, structurally active in two directions, with the insulation added on the outside. It costs more and builds up thicker, but it behaves closer to a heavy structure and it removes much of the taping risk, because a solid panel has far fewer joints than a framed wall.

What should a client ask before choosing a timber frame?

Three questions, and none of them is about the timber. Who is responsible for the airtight layer, and when will it be tested while it is still repairable? How is summer overheating being answered, given that the structure will not help? And what protects the frame from rain between erection and closing the envelope?

A supplier who answers all three specifically is offering a timber frame house that will outlast its owners. One who answers by describing the species of the studs and the speed of erection is selling the two things that were never in doubt. The material is not the risk; the sequence and the membrane are, and both of them are decided on paper.