ᐅ Assessment of Wall Construction – Steel Frame Prefabricated House

Hello everyone,

We are currently considering choosing a prefab house supplier for the construction of our single-family home, which uses prefabricated steel frame modules.

From the perspective of the future homeowner, this offers several advantages for us:

• Very short construction time on the plot itself, which is located far from our current residence.
• More house for the same budget.
• The necessary deep foundation and earthworks will be significantly cheaper, as a slab foundation is omitted (point foundation possible) and the house’s weight is comparatively low.

Of course, I am skeptical because there are hardly any other suppliers offering this, and I want to avoid any major fundamental flaws hidden in the construction itself. Subjectively, as laypersons, we could not identify any obvious drawbacks (such as poor sound insulation) during a visit to a customer’s house.

Therefore, the question is whether anyone here could evaluate the wall construction used. A KfW40 standard is promised, which is said to be achieved as standard.

Exterior wall (from inside to outside) (U=0.091):

• Gypsum board 12.5mm (0.5 inches)
• OSB board 12mm (0.5 inches)
• Subframe 75mm (3 inches)
• ISUM foil
• Zinc
• Isopan F90 PIR panels (120mm) (5 inches)
• Zinc
• Air gap
• Zinc
• Facade cladding (60mm) (2.5 inches)
• Zinc

Roof (flat roof from inside to outside) (U=0.134):

• Gypsum board 12.5mm (0.5 inches)
• OSB board 12mm (0.5 inches)
• Installation level 50mm (2 inches)
• ISUM foil
• Air gap
• PU foam (100mm) (4 inches)
• Steel (2mm) (0.08 inches)

Floor (elevated on point foundations from inside to outside) (U=0.132):

• Parquet flooring (14mm) (0.5 inches)
• Rigid foam XPS (40mm) (1.5 inches)
• MDF (20mm) (0.8 inches)
• ISUM foil
• Zinc
• PU foam (120mm) (5 inches)
• Zinc

What matters to me in evaluating the construction is not necessarily whether this is the absolute best setup in terms of insulation, since we cannot influence that and KfW40 is fully sufficient for us, but rather whether it contains any pitfalls or no-gos that might lead to expensive renovations later on.

Also, any potentially critical questions that could be asked to the manufacturer about the construction would be of interest to me.

Here are a few additional conditions:

• Construction location: southern Bavaria
• A shallow pitched roof will be added on top of the flat roof for the building permit, but it will have no further function.
• A decentralized ventilation system with heat recovery is to be installed.

Thank you in advance!

Best regards,
Markus

MarkusK8 schrieb:

• Gypsum board 12.5mm (0.5 inches)

• OSB panel 12mm (0.5 inches)

• Stud wall 75mm (3 inches)

• ISUM membrane

• Zinc

• Isopanel F90 PIR (120mm) (4.7 inches)

• Zinc

• Air gap

• Zinc

• Facade cladding (60mm) (2.4 inches)

• Zinc

So, the construction sequence is hard to understand based on the description. I strongly assume that there is no zinc sheet on the exterior (even though zinc is listed last in your breakdown). Also, the repeated listing of zinc is confusing... It probably isn’t pure zinc, right?

Is there underfloor heating?
And is the building a bungalow, or does it have multiple stories?

So, the zinc components ultimately consist of two galvanized sandwich panels. One is a sandwich panel with F90 PIR insulation (120mm (4.7 inches)) and the other is a sandwich panel with the facade cladding (60mm (2.4 inches)) serving as additional insulation and for aesthetic purposes (similar to the gray facade shown on the homepage of Kingspan).

The company currently recommends installing an infrared heater on the wall or ceiling. (I am aware of the disadvantages of this heating method, including its efficiency being several times lower compared to a heat pump.)

The above construction refers to a bungalow (i.e., only exterior walls). We are indeed planning to have an upper floor. However, I have not yet received any information about the structure of the floor between levels.

MarkusK8 schrieb:
Elevated on point foundations from inside to outside

I don’t understand that. Aren’t the PU panels placed directly on the ground or a gravel bed?
Personally, I would never skip underfloor heating, simply for comfort reasons.

The insulation values sound good, but I’m concerned that acoustics and soundproofing could be an issue. That is usually compensated for by mass.

Have you had the chance to visit a house built using this method?

Caspar2020 schrieb:
I don't understand that. Aren't the PU panels placed directly on the ground/gravel base?

No, the entire house is basically elevated on a steel frame supported by piles, since a deep foundation is necessary anyway. The ground beneath the house does not have a load-bearing function, and directly under the PU sandwich panel there would be air.

I agree with you regarding the underfloor heating. Ultimately, this is a budget issue, as a lot of money can be saved with this construction method if most of the modules do not have any hydronic heating systems installed.

As I mentioned, we were in a bungalow where we didn’t notice anything unusual. The goal is to arrange a timely inspection of a two-story single-family house.

I’m trying to get an independent assessment to find out if the wall construction can actually work. I am somewhat concerned that the builder might be trying to impress a layperson with insulation values, but that the wall structure might not be sensible from an expert’s point of view or could be prone to mold or similar problems.

MarkusK8 schrieb:

• Parquet flooring (14mm (0.55 inches))

• Rigid foam XPS insulation (40mm (1.57 inches))

• MDF board (20mm (0.79 inches))

• Vapor barrier film (ISUM foil)

• Zinc sheet

• Polyurethane foam insulation (120mm (4.72 inches))

• Zinc sheet

MarkusK8 schrieb:
No, the entire house is basically elevated on piles, resting on a steel frame skeleton.

That must require a lot of piles or a very close grid spacing. Normally, polyurethane or extruded polystyrene insulation tends to be laid continuously over the entire surface.