Hello!
I have dug up the floor and excavated two rooms in the basement, totaling about 12 sqm. The house has a slab on grade and was built in 1947. In the rest of the basement, the old slab remains. After browsing the internet and this forum, I have concluded that for our situation, it seems appropriate to fill with at least 10 cm of gravel (or macadam, I assume it doesn't matter as long as it is capillary-breaking), then 20 cm of insulation (cellular plastic), and then pour a new slab. I am also planning to install water-based underfloor heating. Now to my thoughts that I hope someone can help me with:
Should/must a geotextile fabric be laid before laying the gravel? Since the area I have excavated was full of stones, the ground is not completely level, and there are quite large stones protruding in places. It feels like the geotextile fabric risks getting damaged...
How should I proceed with the casting? Should I insert rebar into the slab of the adjacent rooms and then pour the new slab against it? I read somewhere that there is a risk of the slab cracking if you cast the slabs together since the new slab will settle a few millimeters before everything sets. I really don't want any cracks since we have some ground radon. I hope the latter isn't true!
My final question concerns the underfloor heating. Can this be arranged after the slab has been cast, or should it be, so to speak, baked into the slab?
Grateful for all answers!
//Johan
I have dug up the floor and excavated two rooms in the basement, totaling about 12 sqm. The house has a slab on grade and was built in 1947. In the rest of the basement, the old slab remains. After browsing the internet and this forum, I have concluded that for our situation, it seems appropriate to fill with at least 10 cm of gravel (or macadam, I assume it doesn't matter as long as it is capillary-breaking), then 20 cm of insulation (cellular plastic), and then pour a new slab. I am also planning to install water-based underfloor heating. Now to my thoughts that I hope someone can help me with:
Should/must a geotextile fabric be laid before laying the gravel? Since the area I have excavated was full of stones, the ground is not completely level, and there are quite large stones protruding in places. It feels like the geotextile fabric risks getting damaged...
How should I proceed with the casting? Should I insert rebar into the slab of the adjacent rooms and then pour the new slab against it? I read somewhere that there is a risk of the slab cracking if you cast the slabs together since the new slab will settle a few millimeters before everything sets. I really don't want any cracks since we have some ground radon. I hope the latter isn't true!
My final question concerns the underfloor heating. Can this be arranged after the slab has been cast, or should it be, so to speak, baked into the slab?
Grateful for all answers!
//Johan
Yes, you should place geotextile fabric under the gravel bed - the fabric definitely holds up! If you’re going to have a constant same temperature in the basement, you can choose whether or not to dowel into the existing slab. If you choose not to dowel in, you’ll need to make a soft joint between the new and old slab. To avoid having to apply self-leveling compound on top of the new concrete slab, it's better to cast the underfloor heating into the concrete. If the basement floor is at least 1.5 m below ground level, then 10 cm of polystyrene under the concrete slab is more than sufficient.
Thanks for the quick response! Those were exactly the answers I needed, superb! Thank you so much, Tyresö!
I should have mentioned that it is a souterränghus and these rooms are not below ground level. So I assume that 20 cm insulation might be good?
I should have mentioned that it is a souterränghus and these rooms are not below ground level. So I assume that 20 cm insulation might be good?
Then a minimum of 20 cm is required. Preferably 30 cm, so that you reach the normal villa standard.
I made a "soft joint" around the new slab by gluing (construction adhesive) thin foam (1 cm) around all the outer edges where I was going to pour. In this way, the new floor becomes "floating," meaning it is not attached to the edges against existing walls or remaining floors. Once the new concrete cured, I scraped away a couple of cm deep along all edges and filled with elastic construction sealant to prevent any radon gas from seeping up along the seam. I also placed vapor-tight plastic between the foam layers under the slab for the same reason (to prevent radon).
Ok b8q, and I assume you're satisfied with the solution so far? Have you tested radon afterwards or do you know that the solution will work against it? I was a little discouraged by the fact that you used plastic under the entire concrete slab; doesn't that risk trapping any moisture that comes from condensation or moisture migration in that way? I've heard that constructions in basements should be able to breathe, but I may be wrong. Maybe the system breathes at the edges?
In the adjacent room, the previous owner made a floor heating solution with insulation over the slab and then poured self-leveling compound over the floor heating. Just pure laziness. This has led to the slab in these rooms becoming very damp since it isn't heated. The moisture has then seeped into the rest of the basement. It also means that the floor in these rooms is quite high above the slab, which could be a problem for me in the new rooms. It will be hard to seal against radon if I pour the new slab higher up. Perhaps I should simply pour it thicker than in the adjacent rooms? Or live with the floor in the new rooms being a few decimeters lower? Any suggestions?
In the adjacent room, the previous owner made a floor heating solution with insulation over the slab and then poured self-leveling compound over the floor heating. Just pure laziness. This has led to the slab in these rooms becoming very damp since it isn't heated. The moisture has then seeped into the rest of the basement. It also means that the floor in these rooms is quite high above the slab, which could be a problem for me in the new rooms. It will be hard to seal against radon if I pour the new slab higher up. Perhaps I should simply pour it thicker than in the adjacent rooms? Or live with the floor in the new rooms being a few decimeters lower? Any suggestions?
I haven't measured the radon level again yet, that'll happen later when I've progressed further in the renovations, but I find it hard to believe that my actions have resulted in a HIGHER level. So far (about 5 months) I am very satisfied with the measures, my basement is now both warm and dry. I don't think the plastic layer beneath the concrete should be a problem from a moisture point of view. This is "how it's done," according to the builders I've spoken with. There aren't any materials that could be damaged. If there's ground moisture from below, the capillary-breaking layer with macadam should be able to channel away moisture that possibly transitions into liquid phase, and if it's from above (if you clean the floors, for example), both concrete and tiles can dry out upward. The slab handled the water I sprayed on it daily the week after pouring, and it is dry and in good condition today.
Even though it's not so common to have plastic between the styrofoam panels, I don't see that this plastic could cause any problems since it's under the slab, and moisture can, as you say, be transported away both below and above the plastic.b8q said:
we place the plastic between the Styrofoam sheets and it's no problem, when the Styrofoam warms and removes the moisture... The plastic is supposed to remove moisture from underneath, and what then happens is that the moisture evaporates between the Styrofoam.....