ᐅ Is the basement offer consistent (waterproof concrete structure / energy-efficiency requirements)?
Created on: 17 May 2021 13:30
K
KBthmnnDear community, I hope you can help me!
We are building a KfW 55 house with a basement. According to the development plan, the basement should be constructed as a "white tank" due to occasional high groundwater levels in the building area. We contacted a basement contractor who was recommended to us, providing these two requirements (KfW 55, white tank), and received the following offer:
The foundation of your residential building will be made according to individual planning using concrete footings or a load-bearing basement slab. The following deliveries and services are included in your offer:
The following deliveries and services are included for the basement shell:
The following basement waterproofing and thermal insulation are planned for your residential building:
The following other deliveries and services are included for the foundation works in your offer:
Does this offer include everything that is important for us? Is the basement perhaps not actually constructed as a white tank? Is the planned insulation sufficient to meet the KfW standard?
Thanks in advance for your answers!
We are building a KfW 55 house with a basement. According to the development plan, the basement should be constructed as a "white tank" due to occasional high groundwater levels in the building area. We contacted a basement contractor who was recommended to us, providing these two requirements (KfW 55, white tank), and received the following offer:
The foundation of your residential building will be made according to individual planning using concrete footings or a load-bearing basement slab. The following deliveries and services are included in your offer:
- Base slab under the house, reinforced concrete, concrete grade C25/30 WU XC2, approx. 25 cm (10 inches) thick including a 0.2 mm (0.008 inches) thick polyethylene sheet over the entire basement floor area between the lean concrete layer and the concrete slab (reinforcement separately)
- Finishing all edges of the slab
The following deliveries and services are included for the basement shell:
- Supply, cutting, bending and installation of reinforcing mesh for solid reinforced concrete components such as floor slabs, foundations, ceilings, walls, beams, lintels, columns, etc.
- Exterior walls in reinforced concrete as hollow wall elements with joint sealing tape between the slab and the concrete wall
- Interior walls; masonry made of perforated brick units d = 17.5 cm (7 inches), compressive strength according to structural design
- Solid reinforced concrete ceiling C25/30 above the basement d = 18 cm (7 inches)
The following basement waterproofing and thermal insulation are planned for your residential building:
- Basement waterproofing as thick coating in the earth-contact area (two-component coating) with fabric reinforcement at wall joints
- Protection mat as a safeguard for the basement waterproofing or perimeter insulation in the earth-contact area
- Complete exterior insulation of the basement with perimeter insulation "Styrodur 3000 s" 100 mm (4 inches) thick WLZ 035 or equivalent
The following other deliveries and services are included for the foundation works in your offer:
- Lean concrete and frost protection layer made of capillary-breaking material under the foundation slabs d = 0.15 m (6 inches)
Does this offer include everything that is important for us? Is the basement perhaps not actually constructed as a white tank? Is the planned insulation sufficient to meet the KfW standard?
Thanks in advance for your answers!
Hello,
First of all – regarding insulation sufficient for KFW requirements, I cannot provide advice – this needs to be checked by a qualified planner.
Topic: Waterproof Basement (WU - waterproof construction)
1) At the moment, you are only getting a waterproof floor slab with a sealing tape between the wall and slab (joint tape).
2) Since a bituminous coating was offered for waterproofing and there was no mention of joint tape in the wall joints, this area is not considered a waterproof structure but instead receives a conventional external waterproofing.
3) The transition of the joint between the wall joints and the slab-to-wall joint is unclear.
4) It is unclear where the joint tape is located. This overlaps with point 3 – all waterproofing layers must be coordinated.
5) There is no information about the joint between ceiling and wall (also reinforcement fabric?).
6) There is no statement about whether the contractor independently prepared the structural engineering for the basement – how does the provider know that exposure class XC2 is sufficient? What happens if the exposure class needs to be increased?
7) There are no details on how to deal with unforeseen joints.
Therefore, I would ask yourself two questions beforehand:
1) Where is the actual groundwater level? Do I have permanently standing water or mostly stagnant seepage water, meaning there may be temporary periods without groundwater? – Soil survey?
2) Who is planning the basement? The general contractor themselves?
If the development plan requires the construction of a “white tank” (fully waterproof concrete shell), the walls should also be waterproof (WU). Your current offer would not meet this requirement.
If you still want to proceed as planned, I would have further questions regarding the wall-side waterproofing depending on the groundwater level.
My recommendation would be to get a soil report (or reference to groundwater levels) and involve someone knowledgeable (e.g., me 😉 ) so that the design can be based on the foundation plan. For KFW, consult an energy advisor.
Then, do not build the walls as hollow elements but monolithic, and close the connections with joint sheets rather than tapes (this costs less but is sufficient).
Best regards,
Jann
First of all – regarding insulation sufficient for KFW requirements, I cannot provide advice – this needs to be checked by a qualified planner.
Topic: Waterproof Basement (WU - waterproof construction)
1) At the moment, you are only getting a waterproof floor slab with a sealing tape between the wall and slab (joint tape).
2) Since a bituminous coating was offered for waterproofing and there was no mention of joint tape in the wall joints, this area is not considered a waterproof structure but instead receives a conventional external waterproofing.
3) The transition of the joint between the wall joints and the slab-to-wall joint is unclear.
4) It is unclear where the joint tape is located. This overlaps with point 3 – all waterproofing layers must be coordinated.
5) There is no information about the joint between ceiling and wall (also reinforcement fabric?).
6) There is no statement about whether the contractor independently prepared the structural engineering for the basement – how does the provider know that exposure class XC2 is sufficient? What happens if the exposure class needs to be increased?
7) There are no details on how to deal with unforeseen joints.
Therefore, I would ask yourself two questions beforehand:
1) Where is the actual groundwater level? Do I have permanently standing water or mostly stagnant seepage water, meaning there may be temporary periods without groundwater? – Soil survey?
2) Who is planning the basement? The general contractor themselves?
If the development plan requires the construction of a “white tank” (fully waterproof concrete shell), the walls should also be waterproof (WU). Your current offer would not meet this requirement.
If you still want to proceed as planned, I would have further questions regarding the wall-side waterproofing depending on the groundwater level.
My recommendation would be to get a soil report (or reference to groundwater levels) and involve someone knowledgeable (e.g., me 😉 ) so that the design can be based on the foundation plan. For KFW, consult an energy advisor.
Then, do not build the walls as hollow elements but monolithic, and close the connections with joint sheets rather than tapes (this costs less but is sufficient).
Best regards,
Jann
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