Usable basement: Insulate, heat, raise ceiling?

Hello,

Our heads are a bit spinning, so I would like to use this opportunity to sort out my thoughts and at the same time hear what other opinions are. Unfortunately, the architect and cellar builder have not been much help so far, more than "You have to know what you want" somehow doesn't come out. Our building inspector won't be back for another two weeks.

In order:

Since we would exceed the floor area ratio with our space requirements, we planned with a partial-use cellar, outside the thermal envelope of our KFW55 house, for technology and storage room and a small sports cellar. The cellar is completely underground.

This was the initial plan (room layout was/is not final yet, please ignore)

[*]Room height 2.40m
[*]Technology downstairs
[*]Insulation under cellar slab (€5,605)
[LIST]
[*]120 mm load-bearing perimeter insulation (XPS) beneath the cellar slab
[*]120 mm perimeter insulation (XPS) single layer on the end face in the base area

[*]Cellar exterior walls as core-insulated walls with 120 mm insulation. U-value: 0.25 W/m²K. The exterior walls are made of concrete C25/30, wall thickness approx. 36.5 cm, including built-in reinforcement, here the wall structure
[*]
[*]Water exposure class W 1.1-E
[*]Price €96,800


If we take the insulation underneath the cellar slab, then the separation in the thermal envelope would no longer be necessary and the interior cellar staircase wouldn’t have to be closed and insulated for KFW.

Remains the question about heating. The cellar builder swears by underfloor heating, but that seems oversized to me – the cellar is used too little for that: no heating is needed in the technology room, and for the few hours per month workshop and sports maybe an infrared heater is enough? In my current sports cellar room, which is uninsulated and has 2 exterior walls, I always have between 16 and 19 degrees which is totally okay. But the room adjoins heated and insulated interior rooms.

I just don’t know what to do here for KFW and what really makes sense. Intuitively, I would have said:

- Choose the core-insulated construction but without the insulation under the cellar slab
- Keep the interior cellar staircase insulated
- Plan infrared heaters (or similar) in the rooms where you might be more often. I mean, right now our workshop is also in the garage and my wife, the main user, just stands there in a thick sweater

Bonus question: Exterior cellar staircase: Is the investment worth it – I have not received an offer yet, only a rough estimate that it could be €7,000–10,000.

So, I think that’s my knowledge state. Opinions?

Either insulate everything and include it in the thermal envelope or insulate nothing at all and close the stairwell. Intermediate solutions with wall insulation but without floor insulation are nonsense. For the intended use, the uninsulated version is also sufficient. Then without underfloor heating and, for example, infrared for workshop / gym / playroom. If you are not really planning rooms with usable windows and for longer stays (office / guest room), I would probably spend the 30k additional cost elsewhere. For example, for the outdoor stairs. That is really a good thing if, for example, they lead into the laundry room with a sink. Then the dirty kids can go in there, or if you have done something in the garden or on the car, you can go straight in there to wash your hands. But it also depends heavily on the plot situation. Such a staircase takes up a lot of space directly at the house, which you also have to have or not need for something else.

I have a different opinion: if there is money, insulate everything, no exterior stairs. This way you can use the basement much more flexibly and it increases in value. I would also have underfloor heating installed everywhere. Infrared heats the wall surface or so close to the head with the low ceilings, I wouldn’t want to have only that. However, I would do without the 2.40m height, 2.25m is enough in the basement. Isn’t 2.40m possibly counted as living space?

Well, it is beyond question that with freely available budget the insulated basement is the better choice. The question is whether the additional costs correspond to added value. In my opinion, they do not for the planned use. Whether the insulation then also increases the resale value by 30k? I don’t believe so. Living space is not created only by ceiling height; appropriate window areas are also a prerequisite. But as far as I understand, that is not an issue here anyway. Sure, an infrared heater requires some space, but there is definitely more than enough available. I mean, there is a 30m² hallway and still 4 reasonably sized rooms for the desired use.

Planned use today! At 14°C in the home theater, watching Batman vs Pumuckl 3 is not so comfortable or the children simply do not like to play there with their train and it has to be placed upstairs after all. But yes, for 30 TEUR, long direct heating can also be operated.

We have infrared heating as the main heating besides the wood stove, and what I have experienced with it so far over the past few weeks pleases me (so far). Our manufacturer offers glass elements that you can recess into the concrete ceiling, so from below you only see a stylish smoked glass pane. In the open-plan kitchen-living area, we also only have 240 or 242, and my concern that my non-existent bald head would get burned seems to have been completely unnecessary. In addition, the elements are placed so that nobody lies or stands underneath them. Basically, a separate basement entrance is of course something nice; that’s how I had it in the former house. Regarding insulation and the building envelope etc., I am not too familiar, but considering your planned use, I would probably insulate under the concrete ceiling to the maximum, but definitely not use underfloor heating in the basement, as it heats up excessively, is slow to adjust, and you only need it sporadically or spontaneously. These are conditions that underfloor heating doesn’t like very much :D Our former house from 1990 also had a basement and certainly did not have excessive slab insulation back then. We used the rooms in various ways, including as a bedroom, and lastly there was a granny flat there. The tenant probably still lives there... the heating costs are "normal," the new owner would have told me otherwise. Back then, carpet flooring was common and maybe because of that I never had the feeling it was too cold there; we also walked around barefoot. Calculations are sensible, but I wouldn’t neglect my personal feeling about it; I have now shared mine with you.