ᐅ Wood Fiber vs. Mineral Wool vs. Polystyrene in Timber Construction

Which of these insulation materials is better in the long term? With expanded polystyrene (EPS), a vapor barrier is definitely used because any moisture that penetrates cannot escape through the EPS. For a mix of mineral wool and wood fiber, a vapor retarder is applied, while with wood fiber alone, usually only a windproof sealing membrane is used.

Basically, from inside to outside:
- Gypsum board
- Vapor barrier / vapor retarder / vapor-permeable windproof membrane
- Timber frame + mineral wool or wood fiber
- Gypsum board
- EPS or wood fiber
- Plaster

a) Vapor barrier + mineral wool + EPS
b) Vapor retarder + mineral wool + wood fiber
c) Vapor-permeable windproof membrane + wood fiber + wood fiber

What are the advantages and disadvantages of each construction? How significant is the absence of an installation layer in each case? Is it less critical in c) since any moisture that enters can potentially escape again?

I would guess U-values and price are the main factors. Personally, I would lean towards wood fiber or mineral wool. However, expanded polystyrene is not that bad either.

Thermal insulation usually needs to be adjusted according to the wall thickness to achieve a similar level of performance. Expanded polystyrene sometimes offers advantages over wood fiber or mineral wool insulation in terms of thermal resistance, allowing for thinner walls. However, when it comes to protection against summer heat, the wall’s mass plays a major role, where styrofoam performs worse. Sound insulation is also often a matter of mass, but the specific characteristics of the building materials used (including windows and ventilation systems, as well as the actual wall construction) also have a significant impact.

Installation layer: Why skip it? The biggest issue without an installation layer is maintaining the airtightness of the entire system.

Expanded polystyrene is acceptable where justified. However, sometimes better alternatives exist, although these are usually more expensive.

Woodpeckers love styrofoam...

Yes, I mean that the biggest issue with omitting an installation cavity in a timber frame construction is that moisture can penetrate the insulation. In the otherwise vapor-permeable construction method c), this is, in my opinion, not a problem.

Would small airtightness defects be a major issue? As far as I know, no house is completely airtight. Every solid structure has tiny cracks in the interior plaster. That’s often why people use painter’s fleece...

The cracks in the plaster have nothing to do with the air tightness...

Not really? Why? As far as I know, the airtightness layer in a solid house is the interior plaster. Cracks in the plaster make the house airtight, in my opinion, at this point. However, as mentioned, no house is completely airtight, and such cracks, considering the total surface area, don’t really have much impact.