ᐅ Construction of a slab-on-grade with frost protection edging but without a traditional foundation
Created on: 10 Sep 2012 11:23
S
Singerl
Hello everyone,
I would like to ask the experts for advice. So far, I haven’t found the optimal solution:
We are building a single-family house in 2013 with a footprint of 17 x 9 meters (56 x 30 feet).
The entire structure will be on a slab-on-grade with a thickness of 25 cm (10 inches), possibly increased to 30 cm (12 inches) — no basement!
I want to install a 60 mm (2.4 inches) thick Styrodur insulation layer under the slab.
My builder initially proposed strip footings 40 cm (16 inches) wide with insulation placed on the inside.
However, I want to embed the entire slab into this Styrodur insulation — essentially creating an insulated slab “box” without thermal bridges. A footing would create a thermal bridge where the slab meets the footing surface. The builder refuses to formwork the footing. Therefore, insulation on the footing side is not possible.
Now I am considering installing only a frost wall (frost skirt) with a width of 20 cm (8 inches) and a depth of 80 cm (32 inches). The slab should not be supported by this frost wall, which would run outside the slab’s perimeter.
I hope some experts understand what I mean and can share their opinions.
Thank you very much for your help.
I would like to ask the experts for advice. So far, I haven’t found the optimal solution:
We are building a single-family house in 2013 with a footprint of 17 x 9 meters (56 x 30 feet).
The entire structure will be on a slab-on-grade with a thickness of 25 cm (10 inches), possibly increased to 30 cm (12 inches) — no basement!
I want to install a 60 mm (2.4 inches) thick Styrodur insulation layer under the slab.
My builder initially proposed strip footings 40 cm (16 inches) wide with insulation placed on the inside.
However, I want to embed the entire slab into this Styrodur insulation — essentially creating an insulated slab “box” without thermal bridges. A footing would create a thermal bridge where the slab meets the footing surface. The builder refuses to formwork the footing. Therefore, insulation on the footing side is not possible.
Now I am considering installing only a frost wall (frost skirt) with a width of 20 cm (8 inches) and a depth of 80 cm (32 inches). The slab should not be supported by this frost wall, which would run outside the slab’s perimeter.
I hope some experts understand what I mean and can share their opinions.
Thank you very much for your help.
B
Bauexperte13 Sep 2012 11:25Hello,
Kind regards
Projekt 2012 schrieb:For shallow foundations, meaning buildings without basements, a frost skirt is poured around the perimeter of the slab. The purpose of this is to prevent water from flowing under the slab, which could freeze in winter and cause the slab to lift. The skirt is either formed and poured in place or directly cast into an earth trench. It is usually made of lean concrete and a layer of clean gravel.
I always thought frost skirts are flush with the edge of the slab???
Kind regards
B
Baustratege15 Sep 2012 11:57Singerl schrieb:
I want to install 60mm (2.4 inches) thick Styrodur insulation under the concrete slab... may I ask why? ... are you absolutely sure that no moisture will ever penetrate this insulation later on?Baustratege schrieb:
.. please, for what purpose? ... are you absolutely, absolutely, absolutely sure that no moisture will ever penetrate this insulation later?With proper planning and execution, insulation below the foundation slab is not a problem. From a building physics perspective, it is even the correct approach.
P
Projekt 201215 Sep 2012 22:28Hello,
Another related question on the topic of frost protection:
Do the pipes for utilities such as water and sewage necessarily have to be routed downward through the foundation slab and then outside to be frost-proof, or is it also possible to run them through the wall or at the joint between the wall and the foundation slab and then downward?
Reason given by the building engineer for the option through the wall: "That way, the entire foundation slab does not have to be broken up if there is ever a leak."
Regards
Projekt 2012
Another related question on the topic of frost protection:
Do the pipes for utilities such as water and sewage necessarily have to be routed downward through the foundation slab and then outside to be frost-proof, or is it also possible to run them through the wall or at the joint between the wall and the foundation slab and then downward?
Reason given by the building engineer for the option through the wall: "That way, the entire foundation slab does not have to be broken up if there is ever a leak."
Regards
Projekt 2012
V
Vielfrager18 Sep 2012 23:51Hello,
In 2011, we built our house (12 x 11.5 m) based on the soil report (clay soil, possible swelling and shrinking → potential structural damage) using the following foundation design:
- Geotextile of GRK 3
- At least 30 cm (12 inches) of gravel base to provide the required “bearing modulus,” with compaction verified by plate load tests
- 10 cm (4 inches) of Floormate 500-A
- 30 cm (12 inches) rigid concrete slab made of waterproof concrete (WU concrete)
(We then decided not to apply a bituminous membrane under the slab because the WU concrete of this thickness does not allow water penetration anyway.)
----------
Our garage had to be founded separately (strip footings), even though the architect initially planned a continuous foundation, in order to
a) avoid thermal bridges and
b) create a neat boundary closure (the gravel must protrude at least 30 cm (12 inches) beyond the rigid slab and maintain a specific slope)
--------------
Important: The grounding system must be routed out of the now well-insulated slab!
---------------
The insulation cost over €4,000, but we saved some money on the membrane application.
--------------
The exterior walls are made of Poroton blocks, the interior walls of calcium silicate bricks, all without insulating blocks on the ground slab.
-----
Ground floor construction: 20 cm (8 inches) thickness (underfloor heating), of which 10 cm (4 inches) is insulation
-----
Why all this?
At first, nobody could directly explain it to us, so we did our own research and found the following:
Energy savings:
It looks good in theory; when you look at the calculations available online, insulation under the slab usually does not pay off economically. Last winter, I had not yet completed the perimeter insulation around the slab (I still needed to pour a footing for the entrance step), and guess what: the air was freezing cold, whereas the concrete at the open edges of the slab felt comfortably warm. Clearly, a lot of heat is lost, since the whole slab is heated even though there is 10 cm (4 inches) of insulation inside!
More interesting is the effect on dew point formation!
By insulating under the slab, the dew point shifts outward. This means that mold formation at the floor level can be almost completely avoided. For us, this might not be a big issue (solid construction), but it is certainly more important for timber-frame construction.
--------------
I hope this information from a non-expert builder helps with your future decisions.
In 2011, we built our house (12 x 11.5 m) based on the soil report (clay soil, possible swelling and shrinking → potential structural damage) using the following foundation design:
- Geotextile of GRK 3
- At least 30 cm (12 inches) of gravel base to provide the required “bearing modulus,” with compaction verified by plate load tests
- 10 cm (4 inches) of Floormate 500-A
- 30 cm (12 inches) rigid concrete slab made of waterproof concrete (WU concrete)
(We then decided not to apply a bituminous membrane under the slab because the WU concrete of this thickness does not allow water penetration anyway.)
----------
Our garage had to be founded separately (strip footings), even though the architect initially planned a continuous foundation, in order to
a) avoid thermal bridges and
b) create a neat boundary closure (the gravel must protrude at least 30 cm (12 inches) beyond the rigid slab and maintain a specific slope)
--------------
Important: The grounding system must be routed out of the now well-insulated slab!
---------------
The insulation cost over €4,000, but we saved some money on the membrane application.
--------------
The exterior walls are made of Poroton blocks, the interior walls of calcium silicate bricks, all without insulating blocks on the ground slab.
-----
Ground floor construction: 20 cm (8 inches) thickness (underfloor heating), of which 10 cm (4 inches) is insulation
-----
Why all this?
At first, nobody could directly explain it to us, so we did our own research and found the following:
Energy savings:
It looks good in theory; when you look at the calculations available online, insulation under the slab usually does not pay off economically. Last winter, I had not yet completed the perimeter insulation around the slab (I still needed to pour a footing for the entrance step), and guess what: the air was freezing cold, whereas the concrete at the open edges of the slab felt comfortably warm. Clearly, a lot of heat is lost, since the whole slab is heated even though there is 10 cm (4 inches) of insulation inside!
More interesting is the effect on dew point formation!
By insulating under the slab, the dew point shifts outward. This means that mold formation at the floor level can be almost completely avoided. For us, this might not be a big issue (solid construction), but it is certainly more important for timber-frame construction.
--------------
I hope this information from a non-expert builder helps with your future decisions.
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