Villa Castelli

The Building

Building pre-intervention

Building post-intervention

Villa Castelli is a listed building from the 19th century located at the riverside of Lake Como (Italy). The owners set the ambitious goal of renovating the Villa, which had belonged to the family for about 140 years, to the lowest possible energy demand while maintaining the original use of the rooms and the external appearance. The renovation achieved a 90% energy demand reduction and a significant increase in comfort, demonstrating that also a listed building can become nZEB. The villa is under formal protection in two respects: the building is listed in the land-use plan as a building worth preserving (vincolo architettonico) and as part of the riverside landscape, it is situated in a protected area (vincolo paesaggistico). In general there was no detailed assessment prior to the retrofit planning and no description of possible retrofit interventions from heritage office side. The only document the building owner had, was a vague description that all vertical structures and ceilings had to be maintained and that no intervention from outside was possible. During the planning phase the planning team searched for a direct contact and exchange with the heritage office. Usually, the process for developing heritage compatible retrofit solutions was that the planning team was proposing a solution to the heritage office and in case they declined, the planning team proposed a new solution (often several times) until it was approved.

More Details

The Solution

Solution post-intervention

What is the Solution?

The insulation of the floor slab against ground was carried out with 20 cm XPS lying on a 15 cm thick layer of lean concrete. Above the insulation layer, the installation level is located in a light, fast-drying subfloor (Lecacem) of 12 cm on which the floor heating system with expanded polystyrene (ISODOMUS) and the parquet are posed.

Why Does it work?

The ground floor solution was chosen to provide high thermal insulation and integrate underfloor heating, significantly improving energy efficiency and indoor comfort. Technically, the XPS layer and lightweight concrete prevent heat loss to the ground and avoid moisture issues. From a heritage perspective, the intervention is compatible because it preserves the original spatial layout and does not alter visible historic features, while all changes remain reversible and do not impact the building’s appearance or structure.

PROS

U-value drops from 1.49 to 0.15 W/m²K thanks to 20 cm XPS under the slab, sharply cutting heat loss to the ground.
Bituminous and PFU waterproof membranes plus hydrophobic XPS form a continuous barrier that limits rising damp.
12 cm fast-drying Lecacem screed speeds installation and keeps the added weight moderate.
Integrated ISODOMUS under-floor heating delivers uniform, low-temperature comfort without affecting the historic façades.

CONS

Total build-up (≈56 cm) raises internal floor levels, forcing adjustments at doors and thresholds.
The 15 cm lean-concrete slab is a “wet” layer; its removal would require demolition, so reversibility is limited.
XPS is petro-based and non-breathable, offering high performance but a less sustainable material profile.

Insulation material

Extruded polystyrene (XPS) panels

Insulation thickness

275

Thermal conductivity

0.032 - 0.036 (ANIT)

Health issue

information not available

Technical drawings

Installation Method

20 cm of hydrophobic XPS panels (separated from the structure by a continuous bituminous waterproof membrane) were installed on top of a newly poured 150 mm lean-concrete slab. Above the insulation, a 120 mm lightweight cementitious Lecacem screed (accommodating services) was cast, followed by a PFU separation membrane, an EPS under-floor heating panel and the new parquet finish. Only the layers above the screed can be lifted without demolition, so the build-up is partly reversible

Moisture Management and Technical Compatibility

information not available

Airtightness

An airtight building envelope was important in two respects: firstly, to ensure that the interior insulation was not damaged in the long term and secondly to limit ventilation losses in view of the location exposed to the wind. The installation of a comfort ventilation system ensures optimum air hygiene (primarily CO2 concentration, but also room humidity). Particularly when using internal insulation, a continuous layer airtightness layer all around the building envelope is crucial in order to avoid condensation of warm room air behind the internal insulation or in construction nodes. In case of the external wall, the internal plaster forms the airtight layer, whereas on the underside of the roof the vapor barrier under the internal gypsum plasterboard and on the baseplate a foil under the subfloor on the XPS insulation forms the airtightness layer. In all points where this continuous air-tightness layer is interrupted/penetrated, e.g. by ceiling beams, detailed solutions were be sought individually to guarantee air-tightness.

Before retrofit

After retrofit

Before retrofit

Floor thickness

After retrofit

Floor thickness

562

Before retrofit

Floor build-up

Parquet [15], Substructure in wooden slats [40]

After retrofit

Floor build-up

Parquet/decorative concrete [15], Floor heating system with expanded polystyrene (ISODOMUS) [75], PFU membrane [1], Light fast-drying subfloor (Lecacem) + installation [120], XPS Insulation [200], Bituminous membrane [1], Lean concrete [150]

Before retrofit

U-value

1.49

After retrofit

U-value

0.15