Old building renovation :: Vapor barrier on uninsulated exterior walls

Hello everyone,

I just registered here in the forum because I am currently dealing with the topic of old building renovation and there are still some things unclear to me.

The background: I own a semi-detached house (built in 1936) which is generally in very good condition. A few years ago, double-glazed windows (plastic) were installed in the house and heating is done centrally with gas. The heating costs are about 1,200 euros annually. Now a consultant from the Association of Private Builders advised me to create a thermal envelope in the house using a vapor barrier. So to equip all exterior walls, the upper floor ceiling, and the ground floor floor with a corresponding one. In addition, he advised me to install an automatic ventilation system.

Why? The masonry is completely uninsulated. An energetic renovation of the facade is out of the question for me. I am rather skeptical about the cost-benefit ratio of this topic. Of course, I confronted the consultant with my question. But I received an answer that I can’t do much with. It was: "[...]the thermal envelope concerns the location of the boundary of the thermal insulation. The insulation concept can be a diffusion-retarding system (with vapor barrier) or a diffusion-open system with capillary-active building materials. With a mechanical ventilation system, among other things, the humidity is regulated, but not the diffusion resistance of components.[...]"

The question: Should I just do it, or better leave it? (to put it in the words of Fettes Brot).


Thanks in advance for your brainpower!!!

he advised to create a thermal envelope in the house by means of a vapor retarder. So to provide all exterior walls, the upper floor ceiling, and the ground floor floor with an appropriate one. In addition, he recommends installing an automatic ventilation system

How a vapor retarder can "create" a thermal envelope is beyond me. The thermal envelope is the boundary of the enclosed and heated space. You can at most shift it, but not with a vapor retarder. A vapor retarder itself is not even an object. There are vapor-retarding materials and permeable materials—but no vapor retarder as such; I only know chatterboxes and handbrakes. If by that he means the continuous application of a vapor-retarding foil along the thermal envelope—that makes sense—but it is still unnecessary and I have never seen it! No solid construction has an additionally applied vapor retarder on the exterior wall! Only in timber frame constructions with cavity insulation (e.g., prefabricated houses, "Swedish houses") is something like that sensible and done.

Retrofitting exterior wall insulation is in many cases the most expensive and least profitable measure for saving energy costs. That you don’t want to retrofit such unnecessarily, I can understand. For a building from 1936, it can make sense depending on the construction method. For example, only 24 cm thick solid brick exterior walls would make it sensible. Similarly, in the case of cavity wall construction, one might consider blown-in insulation.

When insulating the roof slopes and the upper floor ceiling, installing a vapor retarder is reasonable. In general, repairing the insulation at this point is usually cost-effective and efficient.

For a building from 1936, it would definitely make sense to replace the screed if not already done. Due to the build-up height, possibly dry screed with vapor retarder and insulation underneath. For plank floors on wooden beams in sand, filling in is advisable. Whether with or without a vapor retarder depends on the type of insulation.

You have a ventilation system: it’s called window ventilation and has worked quite well for nearly 1000 years in existing buildings.

Thank you very much for your reply. Your explanation definitely helps me quite a bit and roughly confirms my suspicion. Regarding the insulation of the roof and floor ceilings as well as the renewal of the screed, I will start planning. These measures also seem quite reasonable to me. But this also brings to mind another question. What are the advantages and disadvantages of a floor construction with dry fill and screed boards compared to a classic screed floor? This seems like a comfortable solution to me. And what exactly does the construction look like (vapor barrier, dry fill, screed boards, impact sound insulation, parquet)? Regards...

What are the advantages and disadvantages of a floor construction with dry screed filling and screed boards compared to a traditional screed floor?

Advantages:
"Dry" - therefore no mixing of screed,
no waiting time, immediately finished and walkable
lighter, especially relevant in old buildings on upper floors
the screed itself is thinner, lower construction height or better thermal insulation with the same construction height

Disadvantages:
not as load-bearing as concrete screed (wood stove or similar not possible)

Construction (from bottom to top from the raw floor)

Moisture barrier against rising damp (if no basement below)
Leveling filling if necessary (unevenness or installed pipes)
Suitable insulation boards for dry screed (EPS, XPS or mineral wool)
Vapor barrier / PE foil
Impact sound insulation for dry screeds (not necessary on ground floor)
Dry screed boards or 24mm chipboard (moisture-resistant)
Covering

In wet areas such as kitchen and bathroom, dry screed can also be used. However, if tiling is planned there anyway, there is an even simpler solution: leveling compound on the raw floor (if necessary) and then glued with Wedi construction boards -> insulation, vapor barrier, and screed in one, moisture-resistant and directly tileable. Even underfloor heating can be installed directly into the boards.