Airtightness approaches for deep renovation

Author: Søren Peper

An airtight building envelope is one of the cornerstones of energy efficient buildings. It is necessary to implement an adequate level of airtightness in every building for the following reasons in particular:

For Passive House buildings, n₅₀ values ≤ 0.6 h ^-1 are required; for deep retrofits with Passive House components (EnerPHit), the n₅₀ value should not exceed 1.0 h ^-1 . The installation of a serially produced thermally insulating building envelope usually includes the creation of an airtight layer and is therefore assessed according to the requirements for new builds.

The energy demand of an exemplary Passive House building demonstrates the huge effect airtightness has: the energy demand of the building is doubled if the building is only implemented with n₅₀ values = 3.0 h ^-1 instead of n₅₀ values = 0.4 h ^-1 , which is the usual value for Passive House buildings, even though all other components of the building remain the same. From the still good airtightness value of n₅₀ = 1.0 h ^-1 for a building retrofit (EnerPHit), the heating demand increases by almost 12 kWh/(m²a) if the building is inadequately planned and executed with n₅₀ = 3.0 h ^-1 .

Fig. 1: Heating demand of identical buildings with variation of the airtightness level [Peper 2024].

If we look at deep retrofit projects in the Passive House project database (www.passivehouse-database.org) which were executed using Passive House components, we see excellent results with an average n50 value of 0.7 h ^-1 (simple average value of 459 projects). The EnerPHit requirement of n₅₀ ≤ 1.0 h ^-1 for airtightness of a building after a deep retrofit is therefore generally easy to achieve. Only 12 of the included retrofit projects have a measured value that is higher than the EnerPHit requirement of n₅₀ ≤ 1.0h ^-1 .

Fig. 2: Evaluation of measured airtightness of 459 retrofit projects in the PHI Project Database [Peper 2024].

An important basic rule applies for airtightness that must generally be observed: there must always be one single airtight layer. Two partly airtight layers do not lead to the required result and are generally more complicated and expensive. For example, a conventional ‘vestibule door’ behind the non-airtight front door will not lead to an adequate level of airtightness of the building. Imagine a leaky bucket that is placed in another leaky bucket: water continues to leak out (water corresponding to air). It is therefore not advisable for example, to consider the internal plaster as an airtight layer if weak points exist here (e.g. inaccessible wood beam ceilings without plaster on the masonry in this area).

In general, the following methods are available for consistent airtightness during renovation:

• Improving the internal plaster, possibly also the ground slab or top floor ceiling and the roof

More likely to be considered in the case of extensive reconstruction in the uninhabited state, or if it can be ensured to the greatest possible extent that the internal plaster is intact and tightly connected from the load bearing floor slab to the ceiling, enclosing the entire space. In the process, particular attention should be given to the following:

• power sockets, cable routing, penetrations in exterior walls/ ceilings/floors,
• power sockets, cable routing, penetrations in exterior walls/ ceilings/floors,
• ensuring of airtightness in stairwells or exits of cold areas such as basements or attics

Improving the external plaster (before installing the exterior insulation or the insulated façade module) Can be applied in conventional renovations in the inhabited state, in which case very little work will then be carried out inside the building. This solution also makes sense if the airtightness of the internal plaster cannot be guaranteed (e.g. wood ceilings etc.). In such case:

• the external plaster should be improved (repairing, also conceivable with a new layer of adhesive).
• the connection to the new window should be planned in detail,
• attention should also be given to the penetrations caused by pipes or ducts.
• Connections of ascending components to the basement wall, gable wall or eaves should

Integration of the airtight layer in new pre-wall elements.
In the process, consistent improvement of the internal plaster (surface, installations, windows, storey ceilings) in an inhabited building probably won't be possible and neither can the external plaster be improved because scaffolding cannot be used.The following must first be clarified:

• How the airtight layer should be routed in the new wall element.
• As a possible alternative to the classic solution on the warm side of the module, the airtight layer could also be routed on the outside, under cladding. In this case however, it must be designed to be diffusion-open.
• How sealing between the modules, at the intersections and top and base points can be achieved so that installation can be carried out without any scaffolding and in the shortest possible time and with minimum materials
• How airtight integration of the windows can already take place during the manufacturing process.
• Details such as sealing at mechanical supports for the modules as well as cable and pipe penetrations are must also be covered in the design.

The following matrix shows the possible ways of ensuring airtightness of the building for the renovation process.

[Peper 2024] Peper, S.: Solutions for an airtight building envelope. In: Serial energy retrofit according to Passive House principles. Protocol Volume No. 61 of the Research Group for Cost-effective Passive Houses, Passive House Institute, Darmstadt, 2024

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