ᐅ New Construction on an Existing Basement: Is Insulation Worthwhile?

Hello everyone,

We are planning a new build on an existing basement with a granny flat, replacing the old prefab house.

The basement is about 50% underground and 50% exposed.

Now we are facing the question of whether we should insulate the basement. We have asked around two people and got three different opinions (a classic situation).

We have to weigh the pros and cons: no insulation vs. insulating the basement.

• Insulation: Approximately €9,000 (about $9,700) additional construction costs because the house becomes larger plus around €4,000 (about $4,300) for the insulation if done as DIY.
→ Total of about €13,000 (about $14,000)
• No insulation: Within the family, there is concern that this could lead to mold – an issue that, however, has not occurred in nearly 50 years.

Architect:
Considers the effort and costs disproportionate to the expected benefit. Although a positive effect would be present, in his view it is not necessary for the new house and not critical for the basement, since there have been no problems in the last 50 years.

Plumbing specialist in the family:
Sees a possible mold risk.

Question:
What is your opinion?
Is the mold risk real?

Hutschnurr schrieb:

Is the mold risk real?

Where exactly do you see a mold risk? In the basement or in the house?

The question of insulation depends on how the space is used. If the basement is only for storage or utility purposes, you don’t need to insulate it from the outside. However, some form of insulation will become mandatory at some point. The basement ceiling, for example, will need to meet new construction standards—either insulated from above or below. But the architect will take care of that.

nordanney schrieb:

Where do you see a risk of mold? In the basement or the main house?

The question of insulation depends on the intended use. If it’s a purely functional basement, you don’t need to insulate it from the outside.
One type of insulation will become mandatory in some form: the basement ceiling. For new builds, this needs to meet current building standards—either from above or below. But the architect will take care of that.

Thanks for the reply.

According to the architect, the new house will be very well insulated at the top with excellent thermal protection downward, so there is no issue with the upper part of the house.
The basement will still have a separate apartment, and that seems to be where people see the risk. The basement ceiling will therefore be better insulated than before. The thermal bridge remains the exterior wall, which is partially exposed.

Hutschnurr schrieb:

There will still be a basement apartment downstairs, and people seem to see that as a risk. So the basement ceiling is insulated better than before. The thermal bridge would still be the exterior wall, which is partly exposed.

Don’t you also have to meet the current requirements of the Building Energy Act for the new basement apartment in a new build? How does your architect plan to avoid that?

I find it exhausting that you create a separate thread for every single question within the same project.

Hutschnurr schrieb:

The basement is 50% underground and 50% exposed.

What does that mean: hillside buried and valley side exposed in a sloping site, or raised ground floor with a basement half above ground level all around? Generally, pictures would be helpful, including of what is to be removed. We cannot work with the secret knowledge in your head.

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