ᐅ Structural Engineering – Shifting a House with a Basement Due to Insulation of Poroton Bricks

During the construction phase, we noticed that the basement insulation is somewhat insufficient. The plan was for 6cm (2.5 inches) of external perimeter insulation, but now we want to increase it to 8cm (3 inches) with a better thermal conductivity rating (WLG). The general contractor intends to move the 36.5cm (14.4 inches) Poroton blocks 2cm (0.8 inches) outward to accommodate this.

I am a bit concerned about the structural stability because the load will be distributed over a smaller surface area.

However, the site manager and construction supervisor said this can be done.

What do you think?

@11ant Could you maybe share your opinion on this as well?

Thanks in advance!

Here’s a picture again so you know what I mean. The basement walls are made of 30cm (12 inches) concrete C25/30.

How much overlap do you have between the concrete wall and the first row of bricks? About 28.5cm (11 inches), plus or minus the mason’s tolerance?

WilderSueden schrieb:

How much overlap do you have between the concrete wall and the first row of blocks? 28.5cm plus or minus the mason's tolerance?

Since the basement exterior wall is 30cm (12 inches) thick, I assume about 28cm (11 inches), plus or minus the mason's tolerance. Construction is done using Poroton T9 36.5.

Spiderman1982 schrieb:

During the construction phase, we noticed [... / reason secondary] The general contractor wants to shift the 36.5cm (14.4 inches) Poroton blocks 2cm (0.8 inches) outward. [...] @11ant Could you maybe comment on this as well?

The general contractor wants to build the house 4cm (1.6 inches) longer and wider than approved? – I can already hear my loyal readers guessing what I think about that.

Spiderman1982 schrieb:

I’m a bit worried about the structural integrity, since the load will be transferred over a smaller area.

It seems to me you’re drawing oversimplified conclusions, and I don’t know the foundation details.

Spiderman1982 schrieb:

During the construction phase, we noticed that the insulation of the basement is somewhat insufficient. Originally, 6cm (2.4 inches) perimeter insulation was planned on the exterior, and now we want to increase it to 8cm (3.1 inches) with a better thermal conductivity rating. [...] The basement walls are made of 30cm (11.8 inches) concrete C25/30.

The red flag for my assessment “thinking started too late” is in your introduction: “during the construction phase we noticed” (wrong timing, you realize that, right?). What exactly did you notice: that the basement’s intended use has changed, or why is the insulation on its exterior walls suddenly no longer appropriate as planned?
I clearly lack sufficient knowledge of your building project to comment in a well-founded way. From what I understand from your fragmented description, we are apparently talking about a usable basement and a house with monolithic walls of about 36.5cm (14.4 inches) thickness above it. So far, so good. The basement is now supposed to have thicker insulation on its exterior walls—2cm (0.8 inches) thicker—while the basement shell remains unchanged. This creates a difference in the “matching” exterior walls of the ground floor, milkmaid’s calculation understood so far. If the ground floor is still built to the original dimensions, the issue of tolerance gaps remains the same—still suboptimal, just slightly different. Whether legally or technically more important, does this imply that different building dimensions are now supposed to be created? – Without detailed project knowledge, it’s hardly possible to provide a meaningful technical comment.

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11ant schrieb:

The general contractor wants to build the house 4cm (1.5 inches) longer and wider than approved? – I can already hear my loyal readers thinking about what I have to say on that.

It seems to me you are drawing overly simplistic conclusions, and I’m not familiar with the foundation details.

The giveaway phrase for my diagnosis "brain switched on too late" is right there in the introduction: "during the construction phase we noticed" (wrong timing, you realize that yourself, right?). What exactly did you notice: that the basement is now supposed to be used differently, or why is the insulation on its exterior walls suddenly no longer suitable as originally calculated?
I definitely lack sufficient knowledge of your construction project to give a well-founded explanation here. From what I follow in your fragmented descriptions, we’re apparently talking about a habitable basement and a house with monolithic walls of around 365mm (14 inches). So far, so good. The basement is now supposed to get, among other things, thicker insulation on its exterior walls—2cm (0.8 inches) thicker—while the basic basement shell remains unchanged. This creates a difference on the corresponding exterior walls of the ground floor, a simplistic calculation is understood this far. If the ground floor is still built according to the imaginary changed dimensions, the quality problem from sloppy workmanship will "just be the same in green"—so this is already less than ideal. Legally, and even more than technically, different building dimensions are now supposed to be created here??? – technically, as mentioned, meaningful comment is almost impossible without knowledge of the project.

The basement was planned as a habitable basement. Usage class A and design principle A. Only the insulation was relatively weak at 6cm (2.4 inches) WLG 035.

Yes, different building dimensions are arising here. We have at least 0.5m (1.6 feet) of space to the property boundary on all sides of the house. It is a development area with a zoning plan / development plan.

The question is: what do you do?

6cm (2.4 inches) insulation WLG 028 with a U-value of about 0.4 (then we build the house as originally planned) or 8cm (3.1 inches) insulation WLG 028 with a U-value of about 0.3 and move the walls accordingly.

Spiderman1982 schrieb:

Yes, the house dimensions are different here. We have at least 0.5 m (20 inches) space on all sides of the house from the property boundary.

A house built differently from what was approved is considered an unauthorized construction, even if there is still some clearance within the building envelope. Of course, the consequences vary in severity.

Spiderman1982 schrieb:

The question is, what should you do?

I thought I made that clear:
To those whose qualified opinion you seek, present the basic facts.
We do not know your house design in general, nor the details of the base plate or footing.
Therefore, even with the best intentions, your question cannot be answered.

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