L
LComputer29 Nov 2025 13:16Dear forum,
As part of a renovation, our raised basement was uncovered and (hopefully correctly) coated with a new two-component liquid waterproofing membrane (cleaning, removing protrusions, correcting the fillet, grinding off the foundation overhang, sanding the foundation edge due to cement residue, priming with bituminous primer 901, scratch coat with 2K liquid waterproofing membrane, 3 mm (0.12 inch) wet layer of 2K liquid waterproofing membrane, fabric reinforcement, second layer with 3 mm (0.12 inch) wet thickness of 2K liquid waterproofing membrane).
Now, the corresponding 24 cm (9.5 inch) perimeter insulation (XPS, tongue-and-groove, glued at the joints) and base insulation (XPS, straight edge) are to be installed, as long as the wall temperature remains above 5°C (41°F) (full surface application with 2K liquid waterproofing membrane, possibly using the buttering and floating method, if there are significant unevenness issues). As I understand it, this should not be a problem because the wall temperature in the adhesive area should actually rise due to the insulation.
The perimeter insulation is 24 cm (9.5 inch) thick (tongue-and-groove). Unfortunately, the base insulation was not available in this thickness from my building materials supplier (limited supplier network), so I could only get it in 12 cm (4.7 inch) thickness and need to install it in two layers. Here, I probably cannot apply the second layer with the 2K liquid waterproofing membrane, since the surface would then be exposed to the outside temperature, correct?
I was already advised that we must ensure the waterproofing membrane is fully and evenly dried.
There will then be weather protection (rain and UV) provided by tarpaulins (200 µm). These are appropriately long (about 4 m (13 feet), and attached at a maximum height of 2 m (6.5 feet) above ground level, so there is a considerable overhang beside the trench. The tarpaulins are weighted down to prevent them from blowing away.
Now, it would be better not to reinforce the base insulation, right? Because the surface temperature would then become too low. Another point I was told, if no reinforcement is applied immediately, is that a thermally decoupled zone between the wall and the unprotected insulation could develop, allowing moisture to accumulate for longer. Unfortunately, I did not understand this point and would appreciate a brief explanation (maybe even with a drawing?). Due to the sloping site of the building, we also make use of the fact that the base insulation boards may extend slightly below ground level, although I wonder whether securing the base boards in the soil with a mineral waterproofing slurry would be advisable.
Installing the perimeter insulation would have the advantage that the drainage could still be laid and at least partially backfilled. This drainage would then end in a sump, from which I could (temporarily) pump out the drainage water.
I am also somewhat concerned about leaving the strip footing exposed during the cold season (last weekend it got significantly colder, so I at least placed insulation boards in front of it).
What do you think?
As part of a renovation, our raised basement was uncovered and (hopefully correctly) coated with a new two-component liquid waterproofing membrane (cleaning, removing protrusions, correcting the fillet, grinding off the foundation overhang, sanding the foundation edge due to cement residue, priming with bituminous primer 901, scratch coat with 2K liquid waterproofing membrane, 3 mm (0.12 inch) wet layer of 2K liquid waterproofing membrane, fabric reinforcement, second layer with 3 mm (0.12 inch) wet thickness of 2K liquid waterproofing membrane).
Now, the corresponding 24 cm (9.5 inch) perimeter insulation (XPS, tongue-and-groove, glued at the joints) and base insulation (XPS, straight edge) are to be installed, as long as the wall temperature remains above 5°C (41°F) (full surface application with 2K liquid waterproofing membrane, possibly using the buttering and floating method, if there are significant unevenness issues). As I understand it, this should not be a problem because the wall temperature in the adhesive area should actually rise due to the insulation.
The perimeter insulation is 24 cm (9.5 inch) thick (tongue-and-groove). Unfortunately, the base insulation was not available in this thickness from my building materials supplier (limited supplier network), so I could only get it in 12 cm (4.7 inch) thickness and need to install it in two layers. Here, I probably cannot apply the second layer with the 2K liquid waterproofing membrane, since the surface would then be exposed to the outside temperature, correct?
I was already advised that we must ensure the waterproofing membrane is fully and evenly dried.
There will then be weather protection (rain and UV) provided by tarpaulins (200 µm). These are appropriately long (about 4 m (13 feet), and attached at a maximum height of 2 m (6.5 feet) above ground level, so there is a considerable overhang beside the trench. The tarpaulins are weighted down to prevent them from blowing away.
Now, it would be better not to reinforce the base insulation, right? Because the surface temperature would then become too low. Another point I was told, if no reinforcement is applied immediately, is that a thermally decoupled zone between the wall and the unprotected insulation could develop, allowing moisture to accumulate for longer. Unfortunately, I did not understand this point and would appreciate a brief explanation (maybe even with a drawing?). Due to the sloping site of the building, we also make use of the fact that the base insulation boards may extend slightly below ground level, although I wonder whether securing the base boards in the soil with a mineral waterproofing slurry would be advisable.
Installing the perimeter insulation would have the advantage that the drainage could still be laid and at least partially backfilled. This drainage would then end in a sump, from which I could (temporarily) pump out the drainage water.
I am also somewhat concerned about leaving the strip footing exposed during the cold season (last weekend it got significantly colder, so I at least placed insulation boards in front of it).
What do you think?
Hi,
One thing stands out: you have carried out the waterproofing preparation remarkably thoroughly, but the interface between the liquid-applied waterproofing membrane (KMB), perimeter insulation, and later base treatment often hides subtle pitfalls. Your approach of full-surface adhesion is basically sound, but you should carefully monitor how evenly the two-component waterproofing membrane has actually cured. A slightly sticky residual film, which can sometimes persist longer at cooler wall temperatures, may cause micro-movements when pressing on the XPS boards, leading to unnecessary stress later on as the backfill material settles. Have you checked on site whether the layer has become consistently firm-elastic even in the smallest recesses? This is often underestimated.
The question of whether the base insulation should be left unreinforced at the current temperature window is valid and is often answered reflexively with yes. However, this creates a thermally decoupled zone between the wall and the unprotected insulation where moisture can remain longer. A thin preliminary reinforcement layer can sometimes be more stabilizing than exposing bare XPS to the weather. The crucial question is: do you really trust that the temporary cover will remain windproof and permanently dry? Even a small slip is enough for cold air to reach the boards. I wonder if you really want to install the drainage now with sufficient slope and drainage layer before the entire surface is closed. Wouldn’t it be more practical to lay all the insulation boards first so the work flows in a logical sequence?
One small tip: buttering-floating tends to be overused. If you do it, only apply it where the irregularities are objectively measurable. Otherwise, you create defined hollow spaces that later act as thermal traps. Did you notice, when grinding the foundation edge, any area where the transition to the base slab is slightly frayed? That would be a spot where capillary-bound residual moisture could accumulate, which the waterproofing membrane does not tolerate.
Good luck!
One thing stands out: you have carried out the waterproofing preparation remarkably thoroughly, but the interface between the liquid-applied waterproofing membrane (KMB), perimeter insulation, and later base treatment often hides subtle pitfalls. Your approach of full-surface adhesion is basically sound, but you should carefully monitor how evenly the two-component waterproofing membrane has actually cured. A slightly sticky residual film, which can sometimes persist longer at cooler wall temperatures, may cause micro-movements when pressing on the XPS boards, leading to unnecessary stress later on as the backfill material settles. Have you checked on site whether the layer has become consistently firm-elastic even in the smallest recesses? This is often underestimated.
The question of whether the base insulation should be left unreinforced at the current temperature window is valid and is often answered reflexively with yes. However, this creates a thermally decoupled zone between the wall and the unprotected insulation where moisture can remain longer. A thin preliminary reinforcement layer can sometimes be more stabilizing than exposing bare XPS to the weather. The crucial question is: do you really trust that the temporary cover will remain windproof and permanently dry? Even a small slip is enough for cold air to reach the boards. I wonder if you really want to install the drainage now with sufficient slope and drainage layer before the entire surface is closed. Wouldn’t it be more practical to lay all the insulation boards first so the work flows in a logical sequence?
One small tip: buttering-floating tends to be overused. If you do it, only apply it where the irregularities are objectively measurable. Otherwise, you create defined hollow spaces that later act as thermal traps. Did you notice, when grinding the foundation edge, any area where the transition to the base slab is slightly frayed? That would be a spot where capillary-bound residual moisture could accumulate, which the waterproofing membrane does not tolerate.
Good luck!
L
LComputer29 Nov 2025 13:39The KMB (bituminous waterproofing membrane) should be fully cured; we will pay special attention to this (overall, the drying time is quite long and not, as is common on many construction sites, where the surface no longer feels sticky and insulation is immediately glued on). This is also due to the fact that the work can only be done at certain times.
I’m not entirely clear on the thermally decoupled zone—could you maybe provide a sketch of it? :-)
The plastic sheeting cover (200 µm) definitely keeps the insulation dry permanently, as it extends well beyond the trench with a wide overlap (total length of the sheet about 4 m (13 feet) for a wall height of approximately 2 m (6.5 feet), so it slopes nicely over the trench).
Regarding the drainage, the idea was to install it now in sections (of course, only after the respective sections have been insulated) between the flushing shafts (using a pump that pumps out of the flushing shaft), to allow for backfilling step by step.
I have some concerns about frost during winter affecting the slab foundation. Here, there is only a slab foundation with no strip footings or frost protection barriers underneath.
I haven’t really noticed any “fraying” at the described location. On the outside of the slab foundation, any larger defects have been sealed with a fillet mortar, while smaller defects were repaired with the scratch coat of the two-component bituminous waterproofing membrane (2K KMB).
What do you think about applying a second layer to the base plates regarding surface temperature?
The problem with fully completing the areas is, of course, that the entire house would need to be exposed to do so. This would be too risky for me, especially with the current temperatures—the front side of the house (approximately 2.5 m (8 feet) deep) won’t be accessible before spring anyway.
I’m not entirely clear on the thermally decoupled zone—could you maybe provide a sketch of it? :-)
The plastic sheeting cover (200 µm) definitely keeps the insulation dry permanently, as it extends well beyond the trench with a wide overlap (total length of the sheet about 4 m (13 feet) for a wall height of approximately 2 m (6.5 feet), so it slopes nicely over the trench).
Regarding the drainage, the idea was to install it now in sections (of course, only after the respective sections have been insulated) between the flushing shafts (using a pump that pumps out of the flushing shaft), to allow for backfilling step by step.
I have some concerns about frost during winter affecting the slab foundation. Here, there is only a slab foundation with no strip footings or frost protection barriers underneath.
I haven’t really noticed any “fraying” at the described location. On the outside of the slab foundation, any larger defects have been sealed with a fillet mortar, while smaller defects were repaired with the scratch coat of the two-component bituminous waterproofing membrane (2K KMB).
What do you think about applying a second layer to the base plates regarding surface temperature?
The problem with fully completing the areas is, of course, that the entire house would need to be exposed to do so. This would be too risky for me, especially with the current temperatures—the front side of the house (approximately 2.5 m (8 feet) deep) won’t be accessible before spring anyway.
For the second layer of base plates, I would be less concerned about the surface temperature and more about the adhesive bond strength during the cold phase. PU adhesive often works more reliably than a bituminous waterproofing membrane (KMB), which on the outside is practically in immediate contact with the cold. I am a bit surprised that you want to install the drainage in sections before all plates are firmly seated and locked in place. Would it not be more sensible to first ensure a clean alignment of the perimeter and base plates so that the load transfer during backfilling does not concentrate on an unsecured insulation edge? The “thermally decoupled zone” simply means that a temperature buffer forms between the wall and the unreinforced insulation, which slows down the moisture movement and slightly compresses the bituminous membrane.
L
LComputer29 Nov 2025 14:28Maybe I didn’t express myself clearly regarding the sections – by section, I meant one side of the house. The insulation boards obviously need to be securely fixed beforehand, that's obvious.
Since the base insulation boards are partially embedded in the ground and I plan to fully bond the perimeter insulation (and at least the first layer of the base insulation boards) with the two-component adhesive, including at the joints, it doesn’t seem intuitive to me to use a PU adhesive for bonding the two layers of base insulation boards – so would it be better to wait for warmer weather before applying the second layer?
Does the bottom edge of the perimeter insulation boards actually need to be sealed with a waterproofing membrane? I will need to cut this bottom edge to insulate the foundation laterally.
Best regards,
Chris
Since the base insulation boards are partially embedded in the ground and I plan to fully bond the perimeter insulation (and at least the first layer of the base insulation boards) with the two-component adhesive, including at the joints, it doesn’t seem intuitive to me to use a PU adhesive for bonding the two layers of base insulation boards – so would it be better to wait for warmer weather before applying the second layer?
Does the bottom edge of the perimeter insulation boards actually need to be sealed with a waterproofing membrane? I will need to cut this bottom edge to insulate the foundation laterally.
Best regards,
Chris
If you wait for warmer weather to install the second layer of base plates, it’s better than applying a semi-effective waterproofing membrane in cold conditions. However, the first layer must be perfectly level and firmly in place, otherwise even a millimeter of misalignment will transfer. You should protect the cut lower edge with a thin waterproofing membrane strip to prevent colder micro-areas from developing there.
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