Challenge: How to build a basement bathroom on lightweight concrete wall?

[KJD]

I have numerous threads about my basement bathroom, which has interior walls made of Siporex lightweight concrete elements. I've consistently been told by contractors providing quotes that these are perfect conditions since everything is concrete: the floor is a concrete slab, and the walls and ceiling are lightweight concrete.

Now I've heard concerned comments that lightweight concrete absorbs and retains water, even though structurally it's insensitive, it apparently stores water.

Someone mentioned sticking Jackon-type wet room boards on top before tiling or whatever you choose. Then I think you're sealing it and thus unable to let the moisture from the surrounding ground ventilate out of the basement?

What does the collective experience here say about this? Many lightweight concrete houses are being built today, although they are not uninsulated like my house, of course. But I'm thinking if newly built houses with lightweight concrete walls that are plastered, both the plaster and the lightweight concrete should absorb moisture, right?

Highly confusing.

 

[Rickard.]

Lightweight concrete is moisture-resistant but not waterproof, an example is lightweight concrete roofs that have collapsed due to extensive leakage. A thick layer of plaster does a good job of protecting lightweight concrete walls, but it's not the same as in a shower.

Now, I know very little about your specific basement, but if I think about my basement, which is built with lightweight blocks or whatever they are called, which is quite similar, I think that a moisture barrier is an absolute necessity, at least in the shower. Instead of insulation, shouldn't one be able to plaster/spackle the walls straight before the moisture barrier?

 

[KJD]

Lightweight concrete is moisture-resistant but not waterproof; an example is lightweight concrete roofs that have collapsed due to extensive leakage. A thick layer of plaster does a good job protecting lightweight concrete walls, but it's not the same as in a shower.

Now, I know very little about your specific basement, but if I think of my basement, which is built with lightweight blocks or whatever they're called, which is quite similar, I think a waterproofing layer is absolutely necessary at least in the shower. Instead of insulation, you should be able to plaster/spackle the walls straight before the waterproofing layer?

So direct water on the surface is not tolerated by the lightweight concrete, but "indirect" moisture is okay? I don't think I saw this in any product brochure for lightweight concrete, but it might be true. I'd like to believe you, but do you have somewhere you can point to where this is stated? (I want to avoid guessing )

A waterproofing layer is a necessity in the shower/wet zone 1. However, the waterproofing manufacturer/s say that you shouldn't have a waterproofing layer outside the shower if there is penetrating moisture. I have one wall in regular concrete, an uninsulated wall, and two insulated or interior walls.

I might be able to use the rollable waterproofing layers that are diffusion-open there, but the question is how does it work for the lightweight concrete? Does it protect it enough from getting wet?

I don't have room to plaster the wall, but I was thinking of spackling. Is the spackle enough protection for the lightweight concrete then?

In extreme cases, I could also demolish the uninsulated lightweight concrete wall and replace it with, for example, Leca.

 

[BirgitS]

When the ground beneath the foundation is wet, moisture will be absorbed into the concrete slab and then rise into the (lightweight) concrete walls standing on the concrete slab, so inner and outer walls in the basement are made to withstand moisture. It's only when you introduce organic materials on the basement walls that it becomes sensitive to moisture (and unsuitable).

There is a difference between walls/slabs frequently getting wet and being exposed to a humid environment and occasional wet instances such as during cleaning. This is why, according to industry regulations, you should only have waterproofing in the shower area when it comes to a basement with penetrating moisture.

 

[Rickard.]

To clarify a bit, lightweight concrete is not sugar and does not melt with water splashes, but it doesn't like to be soaked repeatedly. I don't have brochure knowledge, just experience from many water-damaged lb-houses, and it's generally quite a durable material, but it's not concrete either.

Except in the shower where it gets really wet every time, I wouldn't be worried about the lightweight concrete at all, but some form of plaster is probably needed for airtightness (insulation value). Or can you skip that with siporex ?

 

[KJD]

To clarify a bit, lightweight concrete isn't sugar and doesn't melt from water splashes, but it doesn't like to be soaked repeatedly. I don't have brochure knowledge, only experience from many water-damaged lb-houses, and it's generally quite a durable material but it's not concrete either.

Except for in the shower where it gets really wet every time, I wouldn't be worried about the lightweight concrete, but some form of plaster is needed for airtightness (insulation value), or can you skip that with siporex ?

I'm thinking of using A46 (concrete filler) to fill in uneven areas.

The insulation value should be provided by the lightweight concrete, with all its air-filled cavities, as I understand it.

I think proper ventilation (e.g., moisture-controlled) will be important though.

Meanwhile, the shower room has managed with woven wallpaper on the lightweight concrete outside the shower for 50 years before I tackled it.

I'll call Xella and Kiilto tomorrow.

 

[KJD]

Xella (equivalent to today's Siporex): Works in basement wet rooms with "penetrating moisture," but requires a cement-based filler/plaster. The supplier of the filler, waterproofing, and adhesive is responsible for proper adhesion and base for tiles/equivalent.

Kiilto:
* Kiilto SR DF concrete filler to fill holes/pores in lightweight concrete
* with penetrating moisture: rollable waterproofing only in wet zone 1.
* walls with external insulation => no penetrating moisture = internal walls => regular waterproofing on fabric or no waterproofing at all depending on the amount of penetrating moisture.

To "measure"/estimate penetrating moisture, use a dehumidifier and read the volume of liquid that condenses per day.

 

[BirgitS]

To "measure"/estimate incoming moisture, use a dehumidifier and read the volume of liquid condensed per day.

Since the amount varies throughout the year—more when it rains or thaws a lot and very little during a heatwave—it seems to be a questionable answer.

 

[cpalm]

To complicate matters further, there is a third option for moisture ingress, known as a diffusion-open waterproofing layer, which allows moisture to pass through but not water.

I have been considering this for my basement bathroom which has a wall with moisture ingress, but is otherwise "insulated."
It seems silly to only waterproof in "wet zone 1" as I don't want moisture to seep into the insulated floor.

 

[BirgitS]

To complicate things further, there is a third option for penetrating moisture, so-called diffusion-open waterproofing, which allows moisture through but not water.

I've been considering this for my basement bathroom which has a wall with penetrating moisture, but is otherwise "isolated."
It seems silly to only waterproof in "wet zone 1" since I don't want moisture getting into the insulated floor.

The exception for basement rooms only applies when it's an uninsulated concrete slab, right?

 

[cpalm]

The exception for basement rooms only applies when it is an uninsulated concrete slab, right?

I have understood it as the same thing applies to walls with incoming moisture. And why would there be a difference (between walls and floors)?

 

[BirgitS]

I have perceived that the same applies to walls with pressing moisture. And why would there be a difference (between wall and floor)?

Hehe, the question "why" doesn't always belong in industry regulations. It doesn't explain why it's one way but not the other.

Figures 2 and 3 only deal with ground moisture, and the text in 3.1.1 is difficult to interpret regarding your case, I think. It might be like this:

The distribution of areas with or without waterproofing is assessed in the individual case depending on the construction design together with the chosen waterproofing supplier.

Regarding diffusion-open, it says the following:

It may also be possible to use a cement-based so-called diffusion-
open waterproofing. These products are normally developed for use in
applications other than residential wet rooms, and therefore do not meet the requirements
for approved waterproofing systems according to §6. The use of these products
in accordance with BBV may only occur after the manufacturer of the product
has issued project-specific approval and instructions.

https://apibkrse.cdn.triggerfish.cloud/uploads/2021/12/20091034/BKR_BBV_branschregler_2021.pdf
https://www.bkr.se/regler-material/regler/branschregler-bbv211

 

[cpalm]

Figures 2 and 3 are only about soil moisture

It's soil moisture I'm dealing with in my case. The wall is below ground and not drained.