ᐅ Renovation of Older Buildings :: Vapor Retarder on Uninsulated Exterior Walls

Hello everyone,

I just registered here in the forum because I am currently dealing with the topic of renovating an old building and still have some questions.

Background:
I own a semi-detached house (built in 1936) that is generally in very good condition. A few years ago, double-glazed windows (PVC) were installed, and the house is heated centrally with gas. The annual heating costs are about 1,200 euros. A consultant from an association of private homeowners advised me to create a thermal envelope in the house using a vapor retarder. This means applying one to all exterior walls, the upper-floor ceiling, and the ground-floor floor. In addition, he recommended installing a mechanical ventilation system.

Why? The masonry is completely uninsulated. An energy-efficient renovation of the facade is not an option for me. I am rather skeptical about the cost-benefit ratio of this. Of course, I confronted the consultant with my question but received an answer that I could not really make sense of. It was: "[...] the thermal envelope concerns the location of the insulation boundary. The insulation concept can either be a vapor-resistant system (with vapor retarder) or a vapor-open system with capillary-active building materials. A mechanical ventilation system regulates, among other things, the humidity but not the diffusion resistance of building components.[...]"

The question:
Should I just do it, or should I better leave it? (to quote Fettes Brot).

Thanks in advance for your advice!

pouhwl schrieb:
advised to create a thermal envelope inside the house using a vapor barrier. That means applying a suitable one to all exterior walls, the ceiling of the upper floor, and the floor of the ground floor. He also recommends installing an automatic ventilation system

How a vapor barrier can "create" a thermal envelope is unclear to me. The thermal envelope is the boundary of the enclosed and heated space. You can at most adjust its position, but not create it with a vapor barrier. A vapor barrier itself is not a tangible object. There are vapor-retarding materials and vapor-open materials—but no vapor barrier as such. I only know vapor diffusers and handbrakes. If he means the continuous installation of a vapor-retarding membrane along the thermal envelope—then that makes sense—but it is still unnecessary and I have never seen it done! No solid masonry building has an additional vapor barrier installed in the exterior wall! Only with timber frame constructions with cavity insulation (e.g., prefab houses, "Swedish houses") does this make sense and is commonly done.

Adding external wall insulation afterward is often the most expensive and least cost-effective measure for saving energy costs in many cases. I understand that you do not want to add such insulation unnecessarily. For buildings from 1936, depending on the construction type, it can make sense. For example, with only 24cm (9.5 inches) thick solid brick exterior walls, it would be beneficial. Likewise, with cavity walls, one can consider blown-in insulation.

For insulating roof slopes and the upper floor ceiling, installing a vapor barrier is advisable. Generally, improving insulation in these areas is usually cost-effective and efficient.

For a building from 1936, it might also make sense to renew the screed if it has not been done yet. Due to height restrictions, a dry screed with a vapor barrier and insulation underneath could be an option. For timber floors on wooden joists over sand, filling the cavity is an option. Whether to include a vapor barrier depends on the type of insulation used.

You already have a ventilation system: it is called window ventilation and has worked quite well in existing buildings for nearly 1,000 years.

Thank you for your response. Your explanation definitely helps me a lot and roughly confirms my assumption. I will start planning the insulation of the roof and ceiling floors as well as the renewal of the screed. These measures also seem quite reasonable to me.

This brings up another question. What are the advantages and disadvantages of a floor structure with dry fill and screed panels compared to a traditional screed floor? This seems like a convenient solution. And what exactly does the build-up look like (vapor retarder, dry fill, screed panels, impact sound insulation, parquet)?

Regards...

pouhwl schrieb:
What are the advantages and disadvantages of a floor construction using dry fill and screed panels compared to a traditional screed floor?

Advantages:
"Dry" – no need to mix screed,
no waiting time, immediately finished and walkable,
lighter, especially relevant in upper floors of older buildings,
the screed layer itself is thinner, resulting in lower overall floor height or better thermal insulation at the same height.

Disadvantages:
Not as load-bearing as concrete screed (e.g., wood-burning stove or similar installations not possible).

Construction (from bottom to top starting at the structural floor):

Moisture barrier against rising damp (if no basement below)
Leveling fill if needed (to even out irregularities or cover installed pipes)
Suitable insulation panels for dry screed (EPS, XPS, or mineral wool)
Vapor barrier / PE foil
Impact sound insulation for dry screeds (not necessary on ground floors)
Dry screed panels or 24mm (1 inch) chipboard (moisture resistant)
Floor covering

In wet areas such as kitchens and bathrooms, dry screed can also be used. However, if tiling is planned anyway, there is an even simpler solution: leveling compound on the structural floor (if needed) and then bonding Wedi construction boards – combining insulation, vapor barrier, and screed in one. Moisture resistant and suitable for direct tiling. Even underfloor heating can be installed directly within the panels.