Hello!
It's time to start building up the floor after we replaced the joists that were directly against the concrete. (they were quite furry)
We will install a sorption dehumidifier that blows dry air under the floor, so we want a small air gap between the sub-floor and the joists.
A small sketch:
As it says on the image; vapor barrier on the slab, air gap, polystyrene as the sub-floor, insulation, vapor barrier, tar paper, and floorboards.
Would this work? It's just over five cm air gap but it should be enough to get the air moving.
Has anyone solved it in another way?
It's time to start building up the floor after we replaced the joists that were directly against the concrete. (they were quite furry)
We will install a sorption dehumidifier that blows dry air under the floor, so we want a small air gap between the sub-floor and the joists.
A small sketch:
As it says on the image; vapor barrier on the slab, air gap, polystyrene as the sub-floor, insulation, vapor barrier, tar paper, and floorboards.
Would this work? It's just over five cm air gap but it should be enough to get the air moving.
Has anyone solved it in another way?
The question is whether to use two layers of plastic? And shouldn't it be possible to dry the slab with just a ventilated Skirting board and a fan connected to a channel on the opposite side that pulls down indoor air. Like GIHA or nivell
But why do you have a dehumidifier?
As previous posts point out.
There are ready-made systems, e.g. Nivell, where air is drawn over the slab in the air gap and the moisture is ventilated out.
I am skeptical about plastic at all; the ventilation air will carry all the moisture out.
As previous posts point out.
There are ready-made systems, e.g. Nivell, where air is drawn over the slab in the air gap and the moisture is ventilated out.
I am skeptical about plastic at all; the ventilation air will carry all the moisture out.
Based on what we've read, it's a bit questionable to draw air from the kitchen and bathroom down towards the cold slab. The air there is often humid, and the moisture will condense on the slab. In 2/3 of the house, there's a crawl space, so we will have a dehumidifier there anyway.
Someone suggested that you could place the styrofoam directly against the slab and possibly vacuum at several points in the insulation?
Is there any actual disadvantage to adding an extra layer of plastic at the bottom? Better to add a little extra now than to rebuild everything again in ten years.
Someone suggested that you could place the styrofoam directly against the slab and possibly vacuum at several points in the insulation?
Is there any actual disadvantage to adding an extra layer of plastic at the bottom? Better to add a little extra now than to rebuild everything again in ten years.
I believe it's not exactly crucial to draw air from the exact right room. Perhaps you can draw air from an adjacent room.
If you put plastic on the slab, you will definitely get water condensing between the plastic and the slab.
An experience.
In a house where there were problems, we cut a hole of about 40x40cm. On the concrete, we found a soaking wet piece of cardboard.
The next day, when we tore out the rest, it was "completely dry" on top of the concrete. It doesn't take much to ventilate away the moisture that comes up through the concrete slab.
If you put plastic on the slab, you will definitely get water condensing between the plastic and the slab.
An experience.
In a house where there were problems, we cut a hole of about 40x40cm. On the concrete, we found a soaking wet piece of cardboard.
The next day, when we tore out the rest, it was "completely dry" on top of the concrete. It doesn't take much to ventilate away the moisture that comes up through the concrete slab.
I have no opinion on how dangerous it is.Vidar said:
However, I know from experience that you should choose the compromise whose consequences are easiest to understand.
I understand the consequences of ventilating away the moisture.
However, I don't understand the consequences of having water standing on the slab.
Difficult discussions about concrete and organic material. It depends on so many things.
In our own house from 1960 (the same applies to our approximately 30 neighboring properties), we have a concrete slab on the ground with plastic film under the slab. The floor joists with insulation between them sit directly on the slab, followed by a jointed subfloor. The baseboards are ventilated, but in most houses, they have probably installed regular baseboards when the floors were replaced. Despite this, I haven't heard of anyone having mold problems. A faint smell may arise occasionally during the year when the concrete slab becomes a little damp from temperature fluctuations. But it disappears and hasn't been a major issue in this 50-year-old house. However, the construction is risky, and the reason they have fared so well is that they are on quite well-drained ground.
When we converted our garage into a relaxation area 10 years ago, we first tested the moisture in the slab by taping plastic film to see if there was any moisture in the slab. The house is on a small hill and is also well-drained, so there was no moisture at all. Asked several carpenters/designers how they would have done it, and this determined how we joisted the floor. I laid a plastic film on the slab and laid 45x70 joists, which we fastened to the concrete with plastic shims as spacers to adjust the heights. IF there had been even the slightest risk of moisture in the slab, I would have used ventilated baseboards on two sides (in the direction of the joists).
So, in your case with high moisture in the slab, I would not hesitate to have a well-ventilated baseboard and skip the plastic film on the slab to get rid of the moisture. You must, however, place the joists on some inorganic material. A leveling system, plastic shims, or the like.
In our own house from 1960 (the same applies to our approximately 30 neighboring properties), we have a concrete slab on the ground with plastic film under the slab. The floor joists with insulation between them sit directly on the slab, followed by a jointed subfloor. The baseboards are ventilated, but in most houses, they have probably installed regular baseboards when the floors were replaced. Despite this, I haven't heard of anyone having mold problems. A faint smell may arise occasionally during the year when the concrete slab becomes a little damp from temperature fluctuations. But it disappears and hasn't been a major issue in this 50-year-old house. However, the construction is risky, and the reason they have fared so well is that they are on quite well-drained ground.
When we converted our garage into a relaxation area 10 years ago, we first tested the moisture in the slab by taping plastic film to see if there was any moisture in the slab. The house is on a small hill and is also well-drained, so there was no moisture at all. Asked several carpenters/designers how they would have done it, and this determined how we joisted the floor. I laid a plastic film on the slab and laid 45x70 joists, which we fastened to the concrete with plastic shims as spacers to adjust the heights. IF there had been even the slightest risk of moisture in the slab, I would have used ventilated baseboards on two sides (in the direction of the joists).
So, in your case with high moisture in the slab, I would not hesitate to have a well-ventilated baseboard and skip the plastic film on the slab to get rid of the moisture. You must, however, place the joists on some inorganic material. A leveling system, plastic shims, or the like.