LGS build-ups: acoustics, vapour control and services

Will the house sound hollow?

This is the question we hear most often, and the answer is disappointingly simple: every material can be designed and built well or badly. Poor acoustic walls have been built in masonry and poor floors in concrete. The decisive factor is not the material, but the design and execution.

LGS actually provides more opportunities to achieve a good solution than conventional construction, provided they are used correctly.

Walls

These are not plasterboard sections

This distinction matters because it causes many misconceptions. Plasterboard sections are generally 0.4–0.6 mm thick. Structural thin-walled sections start around 0.8 mm and commonly reach 1.6 mm in residential work. Their structural behaviour is entirely different.

The lining determines how the wall feels

Several layers of higher-density board can create a wall that feels more solid and sounds less hollow than a thin masonry partition. If only one layer of the cheapest plasterboard is installed, the same quality cannot be expected — and the structural system is not the cause.

Party walls between apartments

Masonry party walls normally need to be doubled, consuming considerable space. With LGS, separate frames, an appropriate build-up and sufficiently dense lining boards can achieve comparable performance at roughly half the thickness. The exact result always depends on the specified assembly.

Floors

We normally propose one of two systems, distinguished primarily by mass.

The economical option: floating dry floor

A resilient layer separates the floor from the structure. The system works, but contains little mass, and without mass the achievable airborne and impact-sound insulation is always limited.

The higher-performance option: Lewis dovetail steel deck

For higher requirements, such as apartment floors, we design the Lewis system. The dovetail deck acts as bottom reinforcement, so a strong and acoustically effective floor can be built with a thin concrete layer of roughly three centimetres. Rubber strips isolate the concrete from the steel sections, an economical solution because full-surface isolation is unnecessary.

---

External walls — our preferred build-up

Board material goes on the outside

We use a doubled wall structure with cement board outside. The outermost layer is therefore hard and impact-resistant rather than exposed insulation, creating a noticeable difference in the completed house. A common alternative — OSB outside, polystyrene over it and mineral wool between the studs — reverses that logic. If a board already provides hardness, why not put it at the outside?

We avoid water-absorbing organic material in external walls

That means neither OSB nor gypsum-based boards. Moisture cannot disappear within a lightweight wall as it can within a thick masonry wall. Every material built into the assembly must remain suitable throughout the structure’s full service life.

One discipline insulates, not three

Spray foam can fill the frame and insulate the exterior in one operation. It expands into gaps, making quality less dependent on how carefully batts are fitted, and provides excellent thermal performance for its thickness. Fire regulations can restrict its use; where it is not permitted, mineral wool can achieve equally good results, but careful filling of every gap becomes critical.

---

Services — the most important point

Machine manufacturers often promote punched holes for cables, and many builders use them. We advise against this for two reasons.

Fire protection

Approved components exist on paper for protecting penetrations through the structure. In practice, their cost means they are almost never installed, so the solution exists in the specification but not in the wall.

Vapour control

This is the more serious issue. A 300 mm masonry wall can be chased 60–80 mm and still remain relatively airtight because the masonry itself contributes to the enclosure. In lightweight construction, the membrane alone provides vapour control. Any penetration creates a point at which moisture can accumulate, and every 65 mm opening drilled for an electrical switch does exactly that.

Our preferred solution places fire-rated board and the vapour-control membrane on the inside of the structural frame. Mechanical and electrical services then run within a second internal plane formed with battens or spacer profiles. This is not a complete independent service wall and is therefore considerably more economical.

What this gains

The vapour-control membrane remains intact, acoustic performance improves, and future maintenance requires opening only the inner lining rather than disturbing the structure, insulation and membrane.

What it costs

The complete wall uses an additional 3–4 centimetres. Larger-diameter drains require a true service wall along one or two short sections; with well-planned building services, this amounts to only a few square metres in an entire family home. It is not necessarily more expensive than routing through the studs because the sections do not need to be punched.

For architects: discuss it from the first line of the concept

When the designer understands the system’s strengths and limitations at concept stage, work progresses faster and fewer compromises remain at the end. We are happy to explain these principles to architects even before we work on a project together.