The thermally insulated and airtight building envelope must be designed with as few gaps as possible. With regard to the position of the insulation layer, there are several options in old buildings, which must be evaluated differently with regard to thermal bridge effects.

• Course of the insulation layer in the area of the cold cellar
• Course of the insulation layer in the area of the upper building closure: top storey ceiling or roof
• Dealing with the heated staircase in the area of the cold cellar and the unheated roof space

For the position of the airtightness layer, see the separate airtightness checklist.

• at the perimeter in combination with the basement ceiling
• at verge and eaves,
• at the transition of the façade element ─ to the top storey ceiling. Only provide fire protection plates in the rear ventilation space of the outer skin. They must not pass through the insulation level.

• Position of the supporting elements (timber frame) in the area of joints and transitions
• Optimization of the window installation (with and without external venetian blind box)
• Position and construction of the anchors
• Minimization of the weight of the refurbishment modules (timber content, clinker brick slips instead of clinker brick facing shell) to minimize anchorages
• The elements must be constructed in such a way that no gap remains between the existing wall and the element. This prevents convection in the gap and sound transmission.
• The windows should be connected to the façade element. This prevents sound and odour transmission. This applies regardless of where the airtight layer runs.
• Façade elements that already have the outer skin integrated as far as possible are advantageous. Scaffolding downtimes for completing the façade can thus be shortened or the scaffolding can be dispensed with altogether.

• Advice on insulating the heat distribution pipes in the ground
• Advice on the insulation and position of the heat distribution pipes within the façade elements (backpackers)
• Insulation and airtightness of external riser shafts (backpacker)

Smaller ventilation units with heat recovery can be easily integrated into the façade elements (backpackers). This allows them to be arranged outside the apartment to save space. From there, only ventilation ducts need to be routed into the apartments. Nevertheless, detailed planning must be carried out in advance for this air distribution ducts:

• The air distribution ducts must be planned in detail and, if necessary, in several variants for different apartment layouts so that the installation in the individual apartments can be carried out quickly and with as little inconvenience to the tenants as possible.
• It must be expected that tenants will have individual built-in furniture in their apartments, which must be taken into account when routing the air ducts. Tenants may be able to refuse the installation of ventilation. This can then only be done when the next tenant moves in. However, this is not detrimental to the concept because the ventilation units will simply not be in operation until then.
• The connections of exhaust air ducts from the bathroom, toilets and kitchen must be planned in detail. Silencers must be provided in the exhaust air ducts. Space in the bathroom and kitchen may be very limited in small apartments. Detailed planning is therefore particularly important.
• The supply air ducts are best routed directly from the appliance to the hallway of the apartments. In the hallway, the air ducts can be concealed behind a suspended ceiling if necessary. Sufficient silencers must be provided in the supply air ducts. Openings from the corridor into all living rooms and bedrooms must be defined in the planning. Only supply air valves from the corridor must be placed in the living rooms. No further ventilation ducts are required in the living rooms.
• Alternatively, the supply air and exhaust air ducts can also be routed on the outside wall in the newly installed wall elements during serial refurbishment. This means that air ducts in the apartments can be largely avoided. The air ducts run along the outer wall and each living space is equipped with a supply air or exhaust air element.
• However, the arrangement of the ventilation units and the ducts in the façade elements must be planned with particular care.
• The length of the warm supply air and exhaust air ducts in the outside wall elements must be kept as short as possible to minimize heat loss. This is because covering the air ducts with around 20 cm of thermal insulation is good. However, the remaining heat losses are noticeable and may significantly reduce the heat recovery rate.
• Even if the ducts are routed outside, individual flats may not remain connected for the time being: the ducts are laid and sealed at both ends. The pipes are only connected to the central ventilation units and the rooms when new tenants move in.