Insulation for new AND existing concrete floor slab
My basement is comprised of two areas:
1) Old section (100 years old), approximately 11′ x 23′, had a rubble base with thin concrete “slab” over top. This “slab” has been removed and the whole area is now down to the sand.
2) New section (15 years old), approximately 11′ x 15′ which has a conventional slab with ceramic tile over top. This area is in fine shape.
There is a difference in height between the old and new sections of approximately 1-1/2″, with the old section being higher than the new section. The foundation walls are 14″ rubble, with footings comprised of even wider stones, generally protruding approx. 2″ horizontally into the basement.
I need to have the slab, in the old section, replaced. So I now have a number of questions about the best way to do this.
1) Is a layer of gravel necessary? I have read varying discussions on the installation of gravel below the slab. Some say, why take out the existing sand and then add a bunch of materail (i.e., gravel) back in? My understanding is the gravel acts as a capillary break between the soil and concrete. However, does this matter if a poly barrier is also used?
2) Does the new floor slab have to go on top of the footings? I was hoping the new slab could be slightly lowered and essentially go between the footings on each side, thus giving me a little extra headroom. I am already quite limited with headroom and want to maximize where I can. My concern here is the foundation wall stones are not attached to the footing stones (i.e., NOT keyed in as with new construction). Thus nothing is resisting the lateral soil presuure at the base of the wall, other than the dead weight of the house itself. However, the old “slab” clearly played no structural role and the house is still standing…………
3) Is it preferable to insulate below the slab as opposed to on top? Since I am doing the slab from scratch in the old part, I can do it either way. I would prefer to do it above as then I will be insulating both sections in the same manner so it will likely be a little easier. However, if I do it below, #2 becomes less of an issue as the slab is then at a higher level, relative to the footings, than if the insulation goes on top.
4) In the New section, I was thinking I would lay down a poly-barrier, then a 1″ layer of XPS, then a subfloor of 5/8″ plywood tapconned down into the tile/slab. This would then bring the level up to match the floor level in the slightly higher Old section. Although, how much higher depends on #2 above.
Any obvious issues with doing any of this?
Thanks.
NOTE: In the picture the NEW section is the NEW portion of the house, NOT the new slab.
Replies
Particularly interested in opinions about #2 and the slightly exposed footings, possibly and inch or so at the one end as the slab slopes down.....
Thanks.
Any time you have a rubble foundation and a relatively shallow footing things get a little iffy with regard to basement slabs. There is very likely going to be some motion in the foundation that will tend to crack the slab, and similarly some motion between the old and new sections. You need to allow for that.
The moisture situation is difficult to evaluate. It could be that you have an inherently dry setup (where are you located, and what's the terrain like?) in which case most moisture control steps are unnecessary. Or it could be you have constantly damp or wet/dry subsoil conditions that make both heaving and moisture infiltration an issue. (Presumably you don't have an actual water seepage problem or you would have mentioned that.)
old cellars and slabs.
I've worked in a number of old cellars like you describe, from the time when hand dug cellars and large rock foundations were typical, especially in farm houses. A lot of them were dug shallow, just enough to get below frost. Another factor was the average height of our ancestors was 4"-6" less than today, mostly due to the lack of good year round nutrition. If there was enough head room to stand between the large floor beams, that was good enough for them.
1. I wouldn't spend any time or money, replacing a well established, compacted soil base with gravel. Poly provides an excellent barrier and it allows the new concrete to cure more slowly too, a significant benefit to its strength.
2. This question is slightly more difficult to answer with any certainty, not without seeing the situation BUT I've dug out old cellars to get more headroom, digging right up next to and lower than the bottom layer of big rocks, then poured the concrete. My theory, that the new concrete slab would give needed lateral support to soil base under the foundation, worked out fine. Those old foundations have had many years to settle, compacting the soil to the max. There's nothing to fear when digging out near them, as long as care is taken to return lateral support as I've described.
3&4. I don't have any long term experience with using XPS, either over or under slabs.
Best wishes and take good care of your back while you're working in that situation. Doing any heavy work while bent over can put a lot of strain on the back, abs too, the effects from which you probably won't feel until the next day.
Peter
Just had a well-recommended concrete contractor in to give me a quote and he agrees with NOT removing an extra 4" of sand just to replace it with gravel. He also said "there is no need to tie the new slab into the existing slab as there is no frost at that level to cause any heaving". I'll admit, this one makes me a little more nervous but I really can't see there being any significant movement. Given that I am covering both areas with flexible insulation and then flexible sub-floor if there is a limited amount of movement it shouldn't be noticeable anyway. We aren't going to do finished flooring at this point due to budget so we can live with it and see how it goes for a few years before putting down the finished flooring. Thanks for your input everybody.