In thickened edge insulated slab, I am looking for info
Driven_5EM
| Posted in Energy Efficiency and Durability on
In most application’s, you see the insulation follow the slope of the thickened edge to where the nominal slab is placed. Would there be a problem with insulating/slab seal on the flat then add the rock to build up the slope to where the slab will be?
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I think it's a good idea that will work as long as you insulate the exterior edges and tie it into your insulation placed below the crushed stone. The hardest part will be to keep from damaging the foam while you are placing the stone. Stone would typically be installed with a small dozer or skid loader, either of which would destroy the foam. On a big area you could convey the stone in place or place it with an excavator. Both options would require hand work to rake and grade it out. Using smaller aggregate like 3/8" clear chips would make raking easier.
Marty,
A few potential problems come to mind:
- Placing the poly on the foam means the fill will absorb a lot of water from the slab when it is poured. To avoid this, typically the poly is placed above the fill.
- It will be difficult to compact the fill - especially the sloped portions.
- You have a note about the slab thickness depending on type of heat. If you are thinking of using radiant heating, the lag time will be quite long if it has to heat the fill as well as the slab.
- Is the barn going to be kept to the same temperature as the suite? If not there isn't any insulation between the two slabs.
Marty,
I've always placed the rigid foam on a slope at the area where the slab makes the transition from thin to thick.
However, a GBA guest blogger named Rob Myers did it the way you propose. I don't know if he had difficulties placing the crushed stone, but he says that he managed to compact it. Is it a good approach? I'm not sure. I'll post a photo below showing what he did.
To read more, see A Timber-Frame House for a Cold Climate — Part 1.
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Thanks for the comments. The foundation is small enough to use an excavator to place the small crusher chips (which shouldn't need compaction, though we'll run a small one over it). I was also thinking that those same chips would expand the heat sink of the slab, so the response would be slower but it would hold heat extremely well. The insulation between the barn/suite is the one area that I'm a little unclear on yet. To avoid the fill absorbing water another layer of poly could be used?? It'll get some holes poked in it from the rebar mat etc but as a bulk moisture retainer, could work well.
Marty,
I should have prefixed my first answer by saying I don't want to dissuade you, just point out potential problems.
- You definitely need to compact the crusher chips, whether they include fines or not. If it does have fines, which will make forming the slope a lot easier, at 2'-0' deep I'd be inclined to do a proctor test, but that's your call. The problem is that to properly compact the fill you will need to add moisture to it. Moisture that has no where to go except up over time.
- Why not run 2" of rigid insulation vertically under the wall separating the suite from the barn?
- Poly placed directly below a slab can get small holes or tear, but it doesn't really matter much. Unlike air-barriers, vapour barriers are effective proportionate to the percentage of the area they cover. A few gaps don't represent a problem. The place for the vapour barrier is under the slab. A second one over the foam may do more harm than good.
Marty,
The photo Martin posted is of my slab. I have done slabs both ways and the jury is still out on whether this last one was a good idea or not. My main reason for putting the filled area above the insulation was thermal mass and ease of installation. I am heating the house with wood and I felt that extra mass would even out the wild swings in temperature that you can get with wood heat plus it would also allow being absent from the house for a period of time in winter without it cooling too much. This has pretty much worked as expected this winter when I ran a wood stove while working, but the house is not completed, there is no interior insulation (just the outer REMOTE shell) and we are not yet living in it. We will know a lot more next year.
My understanding is that any fill smaller than 3/4 clear should be compacted and if it is a small compactor then use small (3" thick) lifts. Compacting the edges etc. is a problem whether it is on top of or below the foam. I used a hand tamper on the edges and it worked fairly well (but is really tedious) I do think it is easier to maintain the continuity of the foam if it is under the fill and because I used high density XPS foam I just lay sheets of OSB on the surface and it wasn't damaged at all by normal small equipment traffic.
As to the placement of the vapour barrier, I used a Canadian guide for frost protected slabs and put the vapour barrier under the foam. About a week after I poured the slab Martin did an article outlining why the vapour barrier should be immediately under the concrete (timing is everything!). In my case, the fill was fairly wet and the slab did take a while to dry but because it is a very well drained area, there should be no long term consequences.
If I was to do it again I think I would use the more normal structure consisting of foam on top of the fill/ vapour barrier immediately under the concrete (note that the concrete guys won't like this placement of the poly because it then takes longer for the concrete to set up before they can do the finishing.).
All fill needs compacting. Clear crushed aggregate of similar size will not technically "compact" the way fill with fines does, but needs to be vibrated to get particle interlock. You can test this yourself by filling a jar with the aggregate (or something like dried peas from your kitchen) and gentle tapping it. The level will drop as it fills the voids more efficiently.
Malcolm,
Thanks for the clarification. I did compact the 3/4 clear stone that I laid down but I thought that it was overkill.
Rob,
First I should say I am eagerly anticipating future episodes of your blog. You are building an enchanting and beautifully crafted house!
To expand on my last post:
Materials that can't be compacted can be consolidated. Some, like washed sand, benefit most by being saturated with water. Larger aggregate needs vibrating. The fill that consolidates least is uniform, round aggregate like pea-stone. The problem with using it in Marty's case is that its natural angle of repose is very shallow, so you couldn't form a 45% slope. it would slough when you walked near the edge or poured the slab.
For some years my warranty provider required proctor tests on fill before I poured. Several times I was surprised at how low the results were on what I thought was diligently compacted aggregate. Unlike suspended structural slabs where the forces are easy to anticipate and reinforce against, slabs on grade with unevenly uniform substrates can become subject point loads from below as the fill subsides. A well compacted base is a great starting point.
Malcolm,
Thanks for the additional information (and for your kind comments regarding my house). There are endless details to work out during construction and it is very difficult for a non-professional builder to get them all right. You are one of many people on GBA that take the time to help with this process. It is much appreciated.
Rob
Rob,
That's why I am so impressed by your build. You have taken on not only all those necessary details, but also thrown on a whole other layer with all the complexity of timber frame joinery. That's something well beyond me.