Polystyrene where I want to lay ground slab for deck foundation - how do I do it?

[Laverda1999]

Hi, my first post here, hope you have some good ideas insights into this 'problem':

I moved into a newly built house in October 2020. Now spring is starting and with it the first outdoor project is underway: building the deck!

I have put gravel where the deck will be built. The gravel is positioned so that there are 28 cm until the panel on the facade. This is so that after I've laid the ground slab + frame + decking there's only a few cm left to the panel (which I think looks best).

Ground slabs (35*35) are used as a 'foundation' where I then place the bearer. Now here is my concern: in certain spots around the house (where the downpipes go into the ground), there is Styrofoam. That's exactly where I need to place the bearer, and therefore the ground slabs (see image).

Now I wonder:

1. Why is Styrofoam there? Is it to ensure that the ground doesn't freeze, and thus the pipes/water in the ground doesn't freeze?

2. Can I place the ground slab on it without problems? Mostly wondering if the Styrofoam will just get pressed down and not really function as a foundation?

3. If I can't place them on it, can I cut out a 35*35 piece, put gravel in it, and then place the ground slab?

4. If I were to do no. 3: Then if the Styrofoam is there for frost protection, wondering mostly if I need to do anything about this? Insulate it somehow?

Thanks for the help!

 

[Matti_75]

The foam plastic is there to prevent cold from entering under the house and freezing the ground, which can cause frost heave. Depending on the quality, you might be able to lay your tiles on it.

 

[TheGame]

Why is the foam plastic so visible? Or did you dig it up?

It probably won't make any difference if it looks like that.

 

[pysen78]

That foam plastic is likely there to protect the downspout pipe from freezing. Now, such boards can indeed withstand some pressure, so it might work to lay some stone dust and then flatten it anyway. It depends a bit on how wide the spacing is between the posts and thus how much pressure your slabs will generate against the ground, and of course, the quality of the foam plastic.

If there are site plans available, they might provide answers. Personally, I would probably aim to minimize the load right there, and then hope for the best.

A last resort could have been to remove the insulation completely. Replace it with gravel that you pack carefully, and then insert a heating cable into the pipe.

 

[Laverda1999]

That cell foam is likely there to protect the downpipe from freezing. While those panels can indeed withstand some pressure, it might work to lay some fine stone dust and then compress it anyway. It depends a bit on how wide the space between the posts is and hence how much pressure your tiles will generate against the ground, and of course what quality the cell foam is.

If there are site plans for the ground, maybe they can provide answers. Personally, I would try to minimize the load right there and then hope for the best.

A last resort could be to remove the insulation entirely. Replace with gravel that you pack carefully, and then insert a heating cable in the pipe.

Thanks for the answer. That's exactly how I solved it! Good to know you think the same, it feels like a solid solution.

 

[Laverda1999]

The foam insulation is there to prevent the cold from coming in under the house and freezing the ground, which can cause frost heaving. Depending on the quality, you might be able to lay your tiles on it

Thank you for the response. This led me to find a solution to the problem. Which turned out to be exactly what pysen78 describes below.

 

[bossespecial]

The solution was hopefully not to remove the insulation? The insulation is there to prevent frost from creeping under the house.

 

[Mikael_L]

That foam board is likely there to protect the downspout line from freezing. Now, those boards can withstand some pressure, so it might work to add some crushed stone and then flatten it out anyway. It depends a bit on how wide the gap is between the posts and thus how much pressure your tiles will generate against the ground, and of course, the quality of the foam board.

If there are any site plans, they might provide answers. Personally, I would try to minimize the load right there, and then hope for the best.

A last resort might have been to remove the insulation entirely. Replace it with gravel that you pack carefully, and then insert a heating cable into the pipe.

No, the insulation is likely there as perimeter insulation to prevent frost heave under the slab, which could otherwise crack the bottom slab and cause settlement in the entire house structure.

If it is perimeter insulation, it's serious to remove it.

I have perimeter insulation along the entire foundation, but I probably could have managed by insulating only the corners, that was my assessment when I decided how much to insulate... but... better safe than sorry... And it doesn't cost much, not compared to what the whole house cost, or what damage might cost...

You can probably check if it's perimeter insulation, as it would either go around the entire house, or it's only in all the corners of the bottom slab, about 1.2-2 meters in each direction. If it's to prevent freezing of the stormwater pipe, then it would only be over the pipe, not in the other direction.

 

[Mikael_L]

Edge insulation is never found in old houses. It first appeared when people began insulating more thoroughly under the house's foundation slab. Previously, there was no insulation or so little that all the heat leakage from the house kept the frost at bay. But when the insulation under the house started to reach 20-30cm of expanded polystyrene, or even more, the problem arises with the frost creeping under the slab.

When choosing edge insulation (or deciding to skip it), factors such as the frost susceptibility of the ground, foundation depth, and climate zone are considered. Snow cover insulation effect in northern Sweden can also be factored in.

 

[Matti_75]

I like how you get to choose the best answer based on how you want to interpret things even though in this case it is completely crazy.

 

[Markarbetaren]

Edge insulation is never found in old houses. It first appeared when more extensive insulation was introduced beneath the house's foundation slab. Previously, there was either no insulation or so little that all the heat leakage from the house kept the frost at bay. But when insulation under the house started to reach 20-30cm of foam plastic, or even more, the issue of frost creeping under the slab arose.

When choosing edge insulation (or deciding it can be skipped), one considers the ground’s susceptibility to frost, the foundation depth, and the climate zone. Insulation effects from snow cover in northern Sweden can also be factored in.

Is this something new or is it related to the climate zone? I’ve made house foundations since the 80s, in southern Sweden, and seen how development has progressed from uninsulated slabs to Rockwool and today 40cm of foam plastic under the slab. But insulating outside the slab was something new to me. However, I have edge insulated large buildings, where we placed a row of 240cm on the inside of the wall and then uninsulated slab on crushed stone for the remainder.

But I can understand the principle and its purpose, even though in my parts of the country we have never had issues with frost heave under houses.