Basement insulation interior wall new construction

Hello everyone,

we are now at the end of our factory planning and I have one important question before we finalize the sampling and factory planning.

The following situation:

[*]we are having a prefab house with a prefab basement built

[*]the basement has a double-shell exterior wall with a thickness of 20cm

[*]additionally, the basement walls are provided on the outside with 100mm perimeter insulation (WLS035)

[*]under the basement floor slab there will be 120mm insulation (WLS040)

[*]the rooms Basement(1) and Basement2 are to be used as living space

[*]the room Basement(1) will have underfloor heating, the room Basement2 will have infrared heating

[*]the basement hallway is open upwards to the ground floor via the basement stairs and is also heated by underfloor heating

[*]we also have a ventilation system (Basement1: supply air, Basement2: supply air, laundry room: exhaust air)

[*]on the ground floor almost the entire living area is also open (without doors)

For better insulation of the living area in the basement, we have additionally booked a package for the interior insulation of the exterior walls in the basement. In the attached drawing, you can see the areas which now have such additional interior insulation (53mm).

The package includes:

[*]40mm polystyrene insulation boards (sandwich panels)

[*]125mm drywall panels

[*]In the basement hallway, according to the offer, only drywall panels will be installed on the interior walls (except the exterior wall behind the basement stairs - there also sandwich + drywall)

For better understanding, I have also attached a sketch of the basement.

My question would be:
1. is this additional (not exactly cheap) interior insulation measure sensible/necessary or is the existing perimeter/footing insulation sufficient? The two living rooms are only hobby rooms but they will probably be used 2-3 times per week (each time for several hours).

2. do you have better suggestions regarding the interior wall construction? / Would it also be sufficient here to simply chisel slots in the concrete wall for the electrical wiring and then putty/wallpaper it?

Many thanks in advance for your tips and advice.

Regards

This insulation on the inside has another purpose, namely to hide electrical wiring. Simply chasing the concrete walls as you plan will not work. How deep is the box supposed to be, for example? Or you do everything surface-mounted.

Our basement apartment is also designed this way, but what I would change is to add another board; mounting a wall cabinet, for example, becomes much easier this way.

In order not to compromise the watertightness of the WU basement, slots and flush-mounted boxes in the bare exterior basement walls are to be avoided. This means only surface-mounted or flush-mounted boxes in the internal insulation, as mentioned above.

and Zaba12: Thank you very much for your input. In the meantime, I was able to find out that my basement builder also offers the possibility to have the recesses for the electrical installations made at the factory for the individual concrete elements (our prefabricated house provider did not inform us about this). So, I am currently facing 3 options:

1. Concrete elements with electrical preparation already at the factory. This is probably the cheapest and easiest option, as basically you would only need to wallpaper afterwards and no additional internal insulation is required. In addition, I find this solution the cleanest in terms of electrical installation. However, my basement builder admitted that this would result in poorer room climate and acoustics compared to options 2 and 3. Since at least one room will be a home theater, I would at least consider option 2 or 3 for this room.

2. Plasterboard with vapor barrier foil. This allows flush-mounted boxes and switches. Furthermore, the room climate and acoustics would probably be better. This option would also be cheaper than option 3.

3. The already mentioned sandwich panels in combination with plasterboard. The most expensive option but possibly the best option in terms of sound, insulation, and living room climate.

Which option would you ultimately recommend? / What experiences have you had with basements used as living space?

Additional question: my prefabricated house provider might also equip the area under the basement stairs with underfloor heating, even though the stairs are completely "closed off" downwards by risers and surrounding walls. He admitted that this is technically not necessarily required, but it would bring more heat into the ground floor and also make the stair steps warmer. For this solution, I would of course install a door at the entrance to the storage room under the stairs. What do you think? Does this make (technical) sense?

Thank you in advance.

Hello khb70

I can only speak from my own experience as an installer but would still like to share this experience with you.

The second option seems to me to be the best, provided that care is taken to prevent mold from forming and everything is carefully taped and insulated behind the drywall panels. I am always very skeptical about interior insulation because the risk of mold is simply high. Are you sure about the wallpaper in the basement? Wallpaper prevents the walls from breathing, which strongly promotes mold. My recommendation is breathable paint, especially because of the drywall and the cool walls of the basement.

As an electrician, I can tell you that the option with drywall walls is easier for the tradesman anyway, which also speeds up the process significantly (saving money, time, and nerves).

As a plumber, I would like to ask why one side is heated with infrared heating and the other with underfloor heating (purely out of my own interest). However, you should definitely thoroughly inquire with the executing company whether they guarantee that no mold can form.

: Thank you very much! Of course, I want to avoid mold formation at all costs. A colleague simply wallpapered the walls in his newly built basement right away, as the electrical installation was already integrated into the delivered wall elements. From my perspective, that would have been the simplest and most cost-effective option.

Regarding Option2: I would not have insulated this construction but rather only considered a substructure for the electrical installation + vapor barrier + drywall panels.

Regarding the infrared heater in Hobby Room 2 (fitness room): we will spend at most one hour a day in this room. The temperature shouldn’t be too high here anyway, since we mainly do sports in this room. Hobby Room 1 will be used significantly longer during the day, so I have planned underfloor heating there.

I am currently very uncertain about which option to choose. I think (and hope) the basement is hopefully sufficiently insulated with the 120mm insulation under the basement floor + 100mm perimeter insulation on the exterior walls. Additional internal insulation might be better for the indoor climate, but in the end, I believe the underfloor heating will provide enough warmth.