Insulation of concrete slab

[solvebring]

Hello!

We have recently bought a house where someone built a conservatory on an older concrete patio slab - just over 100 cm above the ground. I am planning to insulate the conservatory so it can be used in more seasons than just summer. The question is how to insulate the floor. Since the roof is sloping and the ceiling height is fairly minimal, there's no point in insulating directly underneath the decking. In other words, I should insulate the concrete slab from underneath. To do this, like many others, I plan to use foam insulation boards. However, since I have never done anything like this before, I have a few questions.

1. Do I need some type of moisture barrier, either between the foam and the concrete or between the foam and the ground?

2. What would be an appropriate way to attach the foam? An acquaintance made a similar insulation solution and sectioned with studs between which he placed the foam and then screwed the boards from the side. However, his entire construction was in wood.

Thanks in advance!

 

[klaskarlsson]

Hello!

We recently bought a house where someone built a conservatory on an older concrete patio slab – just over 100 cm above ground. I'm planning to insulate the conservatory so that it can be used in more seasons than just summer. The question is how I should insulate the floor. Since the roof is sloping and the ceiling height is quite minimal, there's no point in insulating directly under the decking. In other words, I should insulate the concrete slab from below. To do this, like many others, I'm planning to use foam insulation boards. However, since I've never done this before, I have a few questions.

1. Do I need some type of moisture barrier, either between the foam and the concrete or between the foam and the ground?

2. What should I use to attach the foam? An acquaintance did a similar insulation solution and sectioned it with frames between which he placed the foam and then screwed the boards from the side. However, his entire construction was in wood.

Thank you in advance!

1. Normally you use a capillary-breaking layer on the ground, and foam as insulation then concrete, so nothing more than that
https://www.byggahus.se/bygga/borja-ordentlig-dranering
I get a bit unsure when you say "100 cm above ground" - if there's air under the concrete, you naturally don't need any gravel, then you can insulate on top of the concrete slab. If you remove the old one and cast a new one (you said that otherwise the ceiling height might become low) it should also be above ground, and no such problems should arise?

2. `I'm also unsure here. Normally you lay the foam on the capillary-breaking layer (gravel). If you're building "in the air," you'll probably have to build a mold for the concrete and cast it in.

But if you're casting "in the air," it would be better to insulate on top I think? That is, cast a concrete floor slab, if you don't want wood for some reason, and then insulate on top with, for example, foam between frames where you can then mount the floor:

Ref: Wood guide - between floor slabs
That construction should not be used if the concrete has ground contact, though, as the wood will absorb moisture...

 

[solvebring]

1. Normally, a capillary-breaking layer is used on the ground, with foam insulation followed by concrete, so nothing more than that.
[link]
I'm a bit unsure when you say "100 cm above ground" - if there is air under the concrete, you obviously don't need any gravel, then you can insulate on top of the concrete slab. If you remove the old one and pour a new one (you said the ceiling height might otherwise be low), it should also be above ground, and thus no such problems should arise?

2. I'm also unsure here. Normally, you lay the foam on the capillary-breaking layer (gravel). If you are building in "the air," you probably have to construct a form for the concrete and pour it in.

But if you're pouring "in the air," it should be better to insulate on the top, I think? That is, pour a concrete slab, if you don't want wood for some reason, and then insulate on top with foam between joists where you can then install the floor:
[image]
Ref: Träguiden - intermediate floor
That construction should not be used if the concrete is in contact with the ground, as the wood will absorb moisture...

I think we're talking past each other a bit here. That is, I'm not going to build a new structure—not pour anything at all. The conservatory already exists, as does the concrete slab, since the conservatory, as it is, is built on an old patio slab. However, this is not ground-built but is about a meter above ground. Since the conservatory's ceiling height is low, I can't insulate the slab from above, so I have to do this from below. The question then is how to best perform this insulation, likely primarily with foam.

 

[klaskarlsson]

I think we're talking past each other a bit here. That is, I'm not going to build any new construction—no casting at all. The sunroom already exists—as does the concrete slab since the existing sunroom is built on an old patio slab. However, this is not ground-built but is about a meter above ground. Since the sunroom's ceiling height is low, I cannot insulate the slab from above but have to do it from underneath. The question then is how I perform this insulation, probably primarily with foam boards, in the best way.

Ah...you want to keep the slab but insulate from underneath, and you can also access it down there?
Got it.

The easiest way would probably be to simply glue foam boards under the slab, wouldn't it?

 

[solvebring]

Ah...you want to keep the slab, but insulate from underneath, and you can access underneath?
Got it.

The simplest way would probably be to glue foam board under the slab, right?

Yes, exactly.

Okay, do you think the glue will hold/last over time?
Foam board is quite porous and doesn't hold up well against chemicals, so PL400 might be a bit too much, but perhaps PL200 or equivalent...?

Has anyone else tried such a method? Advice from experience is greatly appreciated!

 

[klaskarlsson]

Yes, exactly.

Okay, do you think glue will hold/last over time?
Cellplast is quite porous and doesn't hold up well against chemicals, so PL400 might be a bit much, but perhaps PL200 or equivalent...?

Has anyone else tried this method? Advice based on experience would be greatly appreciated!

I imagine that PL200 should work fine if it's not exposed to direct water.
I have insulated from below with cellplast myself, but then against wood, so I used screws with washers... I think it would be very tricky with concrete though.

 

[Lutte]

Plastic anchor plugs are what you need, then you can glue round 2 with sealant foam and cellplastspikar.

 

[solvebring]

Plastic anchor plugs are what you should use, then you can glue the second layer with foam adhesive and polystyrene pins

I've seen/heard those anchor plugs mentioned on other occasions but don't really know what they are. I've tried googling but found a ton of variations and suggestions. How do they work by the way – are they supposed to be glued into the concrete with downward-facing pins/barbs to hold the polystyrene in place?

 

[Lutte]

I have seen/heard those nail plugs mentioned on other occasions but don't really know what they are. I've tried googling but find a ton of variations and suggestions. By the way, how do they work – are they supposed to be glued into the concrete with downward-hanging nails/barbs to which the foam board is attached?

It might be a bit difficult for us as laymen, but all professional traders should be able to get something like these.
https://www.essve.com/sv/produkter/...lerhallare/isolerhallare/essve-isolerhallare/

You press the board with your hand and then you simply drill straight through the board and into the concrete. Once you've reached the right depth, you just push in the plug and then hammer in the plastic nail so it grips tight.

I would probably use 4 plugs per board if they are 60x120 and 5, meaning one in the middle also on larger boards.

In the second round, as mentioned, you can glue with foam, preferably low-expanding. A few dabs of foam, then up with the board, and then it wouldn't hurt with mechanical fixing as well. Either with insulation nails into the first layer, or with one insulation holder placed in the middle of the board and up into the concrete.

 

[solvebring]

It might be a bit difficult for us as laymen, but all professional traders should be able to handle these.

[link]

You press the board with your hand and then just drill straight through the board and into the concrete. When you reach the right depth, just press in the plug and then knock in the plastic spike so it clamps tight.

I would probably set 4 plugs per board if they are 60x120 and 5, i.e., one in the middle as well on larger boards.

For the second layer, you can, as mentioned, glue with foam sealant, preferably low-expanding. A few clicks of foam then up with the board and it doesn't hurt to also have mechanical fastening. Either with insulation nails into the first layer or with 1 insulation holder that sits in the middle of the board and up into the concrete.

Okay! Great, thanks! =)