Sub-slab depressurization

[Nygge72]

Hello, I was wondering about a house built in 1972, which stands on a slab with a raised floor (I'm not sure if it's over-insulated). Are there ALWAYS problems with these floors? Reading online, it seems like doomsday articles all the time.

I went and looked at this house, and it's really beautiful even though very little has been renovated since it was built. Anticimex has conducted an inspection, and the wet rooms and attic show great results regarding moisture. But then you come to the floor construction. I have been in the house on three different occasions over a month's time and have not in any way noticed any bad smell or seen signs of moisture damage. Nor did the inspection guy feel anything.

So the question is, do these houses always suffer from moisture issues or not? I would think that if it's been standing for 45 years and should have moisture problems, it should have already appeared as a smell or visible damage, or am I wrong?

For the second question, has anyone installed a negative pressure ventilation system, and what did it cost in that case?

 

[justusandersson]

What you are talking about is a house with an uninsulated concrete slab on the ground. It can be considered a risk construction if one is not aware of how it is built. The main problem is that the water vapor present in the warm indoor air can condense in the concrete and contribute to the concrete always being a bit damp. If this is not taken into account, problems can easily arise. Today, the conditions are well-known and there is really no reason to refrain from buying. However, this should be reflected in the price.

 

[TheGame]

Live in such a house from '74.
The slab sits on a bed of sand then it's insulated on the slab with polystyrene or something. At least from what I managed to feel when I tore down the walls in a room, I could feel down to the slab.
The rest of the house has chipboard over all the insulation.
Before we moved in, the inspector drilled a hole in one spot to check.
Everything was dry.
I think the risk is exaggerated but the method is probably not optimal, so if built on the wrong land, it could cause problems.
In the 70s, not much consideration was given, and houses were thrown up here and there because they needed to be built.

 

[Nygge72]

What you're talking about is a house with an uninsulated concrete slab on the ground. It can be considered a risk structure if you're not aware of how it's built. The main issue is that the water vapor present in the warm indoor air can condense in the concrete, contributing to the concrete always being slightly damp. If this isn't taken into account, problems can easily arise. Today, these conditions are well-known and there's really no reason to refrain from purchasing. It should, however, be reflected in the price.

Hi, if hypothetically there’s no problem based on what I've felt/seen, and there is a good balance between humidity/ventilation, is there then no risk of disrupting this by drilling holes in the floor to measure the moisture?

 

[Mikael_L]

One should probably try to restore after the hole-drilling as best as possible. But there is theoretically a small risk. However, if that's the case, it was probably already very close to the limit beforehand.

But the 70s slabs on the ground, this risk construction, do not cause problems for all slabs. Generally, those standing on dry fine ground manage quite well. But in the 70s, two things happened: first, they started building houses without basements with this construction to a much greater extent than before, and at the same time, good dry buildable land became a clearly more scarce resource, and residential areas began to be developed on old farmland, etc., that is, lower-lying land with a lot of soil and clay. This contributed to quite a few problem houses from that time.

 

[justusandersson]

No. Moisture is not a genie in a bottle but follows the laws of physics. Moisture has three states (like other matter), solid = ice, liquid = water (and droplets), gas = water vapor. It takes heat for ice to melt and water to turn into water vapor. The moisture in an uninsulated concrete slab is in liquid form and can only affect organic material it is in contact with. Of course, one should plug up drill holes.

 

[justusandersson]

The early 70s houses were built largely as part of the so-called miljonprogrammet, 1965-75. The design of the houses was heavily controlled by the state through favorable interest-subsidized loans. These were administered by the National Housing Board, which issued detailed regulations on how to build, resulting in what was then called loan architecture. A fundamental idea was that everything should be done as cheaply as possible. I don't remember if uninsulated slabs were a requirement, but one might suspect so.

 

[Nygge72]

You'd probably try to restore after drilling as much as possible. But there is, theoretically, a small risk. But if that's the case, it was probably already very close to the limit.


But the 70s ground-level slabs, this risk construction, not all slabs have problems.
Generally, those on dry, fine land manage pretty well.
But in the '70s, two things happened, partly began building houses without basements with this construction much more extensively than before, and at the same time, good dry buildable land became a clearly more scarce resource, and they started developing residential areas on old farmland etc., i.e., lower-lying land with a lot of soil and clay in. Which contributed to quite a few problem houses from this time.

This house is located on a gravel/sand ridge. So moisture shouldn't be standing in any case.