The advantages of over-slab insulation on a slab on grade include greater comfort, hardwood compatibility, and ensuring an effective thermal break. We take a more detailed look at these benefits below.
While it is possible to insulate after pouring your concrete (above the concrete) we recommend insulating below the concrete layer. This allows the concrete slabs to actually absorb the heat too, which makes overheating unlikely and is our preferred method of concrete insulation.
On most projects where insulation meets the under-slab vapor barrier, we have found that the project teams elect to install their insulation first and to lay the vapor barrier over top. Various industry leaders have commented on the advantage of placing the vapor barrier directly beneath the slab.
The sequence of construction steps is up for interpretation and may vary depending on your contractor or construction manager. Some builders swear by installing the vapor barrier first, while others claim that laying the vapor barrier over the insulation is better. Both may be correct.
Positioning the thermal insulation directly below the underfloor heating system improves the responsiveness of the heating system by isolating the thermal mass of the concrete slab from the building interior, meaning the internal rooms will heat up quicker.
Where should the insulation be positioned? All insulation should be installed below the underfloor to prevent downward heat loss. On a solid floor, this can be between the concrete slab and the underfloor heating layer or it can be under the concrete slab.
Insulation can be placed vertically outside the foundation wall or grade beam. This approach effectively insulates the exposed slab edge above grade and extends down to reduce heat flow from the floor slab to the ground surface outside the building.
That's why nearly every expert in the concrete industry recommends a below-slab vapor barrier to stop the vapor drive of water upward and into your building envelope. The group of industry experts at the American Concrete Institute committed this to their guide, ACI 302.2R in 2006.
The common taboo against a double vapor barrier arises from a very real concern: If you have a Class I vapor retarder (less than 0.1 perm) on both sides of a wall, that wall has virtually no drying potential in either direction.
4.505. 2 Concrete Slab Foundations
A 4-inch-thick (101.6 mm) base of 1/2 inch (12.7 mm) or larger clean aggregate shall be provided with a vapor retarder in direct contact with concrete and a concrete mix design, which will address bleeding, shrinkage and curling, shall be used.
FLOOR INSULATION THICKNESSES
In the case of PIR Rigid Insulation Boards, thickness of 120mm is normally used. In terms of XPS boards such as Soprema SL or STS, 50mm thickness is sufficient for the floor insulation.
Insulate all exterior walls, including (3A) walls between living spaces and unheated garages, shed roofs, or storage areas; (3B) foundation walls above ground level; (3C) foundation walls in heated basements. 4. Apply insulation to floors above unconditioned spaces, such as vented crawl spaces and unheated garages.
Using EPS and PIR insulation under concrete floors
Polystyrene insulation, also known as EPS (expanded polystyrene) or Styrofoam insulation, is a type of plastic foam that is commonly used to insulate buildings. It is lightweight, moisture resistant, and has a high R-value, making it an effective insulator.
Add Your Insulation
If you can't cover the whole area with one sheet, you can lay more but make sure you fill any gaps with foam and tape the sheets down to avoid moisture seeping into the concrete. Add your insulation on top of the DPM and around the edges of the surface, and then it's time to pour the concrete.
Vapor barriers are installed along, in, or around walls, ceilings, and floors. Of course this is done to prevent moisture from spreading and potentially causing water damage.
The insulation can be applied outside (Figure 2) or inside (Figure 3) the foundation wall. Exterior applications require a metal flashing or durable finish for protection. The insulation can also be placed vertically along the foundation. wall (Figure 3) or horizontally under the slab (Figure 4).
It provides an additional layer of insulation by trapping air and helps to prevent moisture buildup by allowing for ventilation. So, it's always recommended to leave an air gap when installing insulation, whether it's in your walls, roof, or floors.
As a general guideline, it's advisable to install double layers of thermal insulation throughout your house. Nevertheless, this may not be feasible in all areas due to space limitations. Typically, it's not practical to add two layers of insulation within the walls of your home.
You should always put down the vapor barrier before installing insulation. That way, you don't end up with gaps in the insulation that allow moisture to get into the wall cavity, floor joist, or on a crawlspace floor.
By adding insulation underneath your slab on grade, you are providing a cushion that allows minor expansion and contraction, while also providing a barrier that separates your concrete slab from direct contact with the Earth's fluctuation in temperatures.
We don't recommend you seal your vapor barrier down to the concrete, as that can damage your subfloor. We recommend layering the seams of the vapor barrier rolls together (approx. 6 in of overlap) to create a watertight barrier and use the seam sealer there.
It's better not to pour concrete directly onto the dirt. Instead, use a layer of gravel underneath your concrete slab. Because concrete is porous, it absorbs water.
Typically the insulation would be situated below the slab and below the damp proof membrane for heavy duty use, whilst normal domestic buildings would have a higher performing, lower compressive strength material above the slab. How much space do you have? How much space you have is important.
Rigid insulation placed on the exterior surface of a concrete or masonry basement wall has some advantages over interior placement in that it (1) can provide continuous insulation with no thermal bridges, (2) protects and maintains the waterproofing and structural wall at moderate temperatures, (3) minimizes moisture ...
Since the IECC building code recommends R10 continuous insulation under slab for most climate zones, and more for heated slabs—code compliance can be achieved with just over 1.5” of polyiso.