Should insulation go above or below slab? (2023)

Should insulation go above or below slab?

Slab insulation, typically foam board, is installed either directly against the exterior of the slab and footing before backfilling or under the slab and along the inside of the stem wall of the foundation. Construction details vary widely, so it's best to consult a building professional in your area.

Should insulation go under or over concrete slab?

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.

Can you put insulation under a slab?

Insulation can be installed under concrete slabs to improve thermal efficiency, with around 30 percent of all heat in homes lost through the floor.

Does vapor barrier go above or below slab insulation?

Under-Slab Insulation Above the Vapor Barrier 🡹

Similarly, under-slab water vapor can accumulate beneath a nearly impermeable, under-slab material like a vapor barrier and further increase the localized amount of moisture surrounding/contacting any insulation installed below said barrier.

Is under slab insulation necessary?

Slab insulation is important not only to save on energy bills, but also to improve comfort. Cold concrete slabs are one of the most notorious sources of discomfort in a home. Installing slab insulation around the perimeter of the slab will reduce heat loss and make the slab easier to heat.

Can I put insulation on top of a concrete slab?

Placing insulation on top of the slab does have additional benefits and cautionary notes to consider. Provides a thermal break against the slab - slows the rate of heat transfer through the slab compared to under-slab insulation. Creates a more comfortable walking surface.

Can you put insulation directly against concrete?

It's not recommended that you apply fiberglass insulation directly to a concrete wall. If you must use fiberglass, there needs to be a space between the insulation and the wall. If not, the place where the insulation touches the concrete will be overtaken by mold and mildew.

What is the best insulation under slab?

Expanded Polystyrene, commonly referred to as EPS, is what you would most likely recognize as Styrofoam and is often used for under slab insulation. EPS comes in various sizes and compressive strengths and has a meager water absorption rate, which makes it an excellent option for under slab insulation.

Why put spray foam insulation under slab?

Spray foam insulation works by expanding when exposed to air forming a thick layer of insulation that prevents heat transfer. This means that when applied beneath concrete spray foam can help keep the surface of the concrete cool during the summer months and warm during the winter months.

How many inches of insulation under slab?

Designers generally insulate both directly under the slab and around the perimeter as well. In cold climates it is advisable to use a minimum of 2 inches of extruded polystyrene under the slab. Many increase the under-slab insulating foam board thickness to 3 or 4 inches at the outer 2 to 4 feet of the slab.

What happens if there is no vapor barrier under slab?

Without a vapor barrier, moisture would continually wet the bottom of the slab, as vapor will continuously drive from the high humidity below slab to the relatively low humidity above the slab.

What goes first vapor barrier or insulation?

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.

Why do you put Styrofoam under concrete?

Rigid foam insulations, such as expanded polystyrene (EPS), have been used successfully under concrete slabs for more than 40 years. Such insulation helps reduce heat loss to the ground in residences, cold storage units, warehouses and other commercial, institutional and industrial structures.

What needs to go under a concrete slab?

Instead, use a layer of gravel underneath your concrete slab. Because concrete is porous, it absorbs water. If you place concrete directly onto dirt, water will collect under the concrete and slowly erode the bottom of the pad. Gravel prevents this erosion by allowing water to drain away from the concrete.

How thick should insulation be under concrete floor?

The edge insulation should be at least 25mm thick and height should extend from the top of the concrete slab to the top of the finished floor screed. Protect the exposed top of the perimeter insulation from any damage until covered by skirting or wall finish.

What kind of foam do you use for insulation under a slab?

The most common insulation materials for use under a concrete slab are extruded polystyrene (XPS) and expanded polystyrene (EPS). Of these two materials, green builders prefer EPS, because EPS is manufactured with a more environmentally benign blowing agent than XPS.

Do you need vapor barrier on top of concrete slab?

If you have a concrete floor in continuous contact with a source of moisture, you will have problems. This is why a vapor barrier under concrete is essential. Vapor barriers are a way to keep moisture from getting into the concrete.

What is the best way to insulate over a concrete floor?

There are two main ways of insulating concrete. You can cover the concrete with rigid foam insulation, add two layers of plywood and add the finish flooring or you can attach wood sleepers to the floor, fill the gaps with rigid foam insulation, apply a subfloor and finish flooring.

Does foam board insulation need to be covered?

Tests have shown that these laminated boards can withstand UV ray damage for a reasonably long time. That said, it's still advisable to cover any foam board insulation exposed to the Sun within 30-60 days at the most. On most projects, that's more than enough time to finish the exterior walls.

How do you insulate an exposed concrete slab?

Install non-moisture-sensitive rigid foam board insulation (such as extruded polystyrene [XPS] or mineral wool) along the edge of the slab of a slab-on-grade foundation to meet or exceed the insulation R-value required by code.

Do you need insulation under concrete?

Rigid insulation under a concrete slab is an essential part of home construction as it improves the energy efficiency of the building, controls moisture, and reduces energy costs.

What PSI is needed for under slab insulation?

The minimum acceptable compressive strength for underslab insulation is 16 PSI.

Why put plastic sheet under concrete slab?

What is Vapor Barrier? A plastic sheet that is placed between the base and the concrete slab. Always placed beneath the rebar or wire mesh and on top of the gravel base. The sole purpose of the vapor barrier is to prevent moisture from penetrating through the concrete.

When should you not use spray foam insulation?

Spray foam insulation of any type should not be applied to a substrate or in an air temperature above 120 degrees Fahrenheit. In storage, you also want to keep your spray foam insulation at least 3 inches from any heat source.

How long does foam last under concrete?

In most situations, our repairs are permanent. The polyurethane foam Liftech uses to complete repairs will last indefinitely – through your lifetime and well beyond. It cures within minutes as a solid, impermeable structure that adheres directly to the concrete.

What is the typical R-value of a slab?

The R value measures how well a building material (e.g. a concrete slab) resists the conductive flow of heat. The R value for an uninsulated concrete slab is approximately 0.1 per 150mm. A higher R value means that the material has better insulation properties.

What is the minimum gap between insulation and roof?

It is important that air is allowed to circulate between the roof covering and the insulation to avoid condensation - a gap of at least 50mm (2 inch) must be left between the insulation and the inside of the roof covering.

How thick should a vapor barrier be under a slab?

The American Concrete Institute's Guide for Concrete Floor and Slab Construction recommends that the thickness of the vapor retarder be at least 10 mils.

What should I put between 2x4 and concrete?

Anyplace where wood meets the ground or concrete, the lumber must be pressure treated. For additional moisture protection, a gasket or strip of closed-cell foam can be installed between the concrete foundation and the sill plate.

Does concrete slab need to breathe?

This means that in addition to groundwater working its way up, you also have a moisture-laden flooring surface slowly releasing water vapor for years. Concrete needs to breathe; it's the nature of the beast.

When should you not use a vapor barrier?

You may find that vapour barriers are often not required in warmer climates. And, if installed in the wrong climate or on the wrong side of building materials, a vapour barrier can cause more harm than good. This circumstance may prevent water vapour from drying, which in turn can cause rot and mold.

Can you install insulation without vapor barrier?

Not every type of insulation needs a vapor barrier. But if it does, the barrier should face inside in northern, heating climates, and outside in humid southern climates.

Should you put plastic over insulation?

Moisture can cause rot of the building structure and can contaminate the air. A vapor barrier helps regulate the moisture inside the home. You should always install a vapor barrier to protect exposed ceiling insulation even if it's temporary.

How do you insulate under a basement slab?

Evenly spread the base gravel under the basement slab, keeping it 1 inch lower than the conventional method. Place a 1-inch layer of 4'x8' extruded polystyrene foam board over the gravel, covering the entire floor area. Tape the joints between the boards with builder's tape; you want to create a tight insulation layer.

What happens if you don't put gravel under concrete?

Because concrete is a very porous material, it will absorb any moisture that it contacts. This can cause pooling. Without crushed stone, pooling water will settle under it and erode your slab. Adding a layer of crushed stone will add proper drainage, as well as create a barrier between your slab and the ground.

Should I put gravel under my concrete slab?

Whether you pour concrete for a walkway or patio, a strong gravel base is required to prevent the concrete from cracking and shifting. Gravel is especially important in clay soil because it doesn't drain well, which results in water pooling under the concrete slab and slowly eroding the soil as it finally drains.

What do you put between house and concrete slab?

Gaps against a house can direct water against the foundation, leading to more problems. Once a year, go around your home and fill these gaps and joints with urethane caulk (or polyurethane caulk) to prevent problems.

Is it worth insulating a concrete floor?

Generally speaking, you only need to insulate the ground floor. If you're on an upper floor, you don't usually need to insulate your floor space. However, you should consider insulating any floors that are above unheated spaces such as garages, as you could be losing a lot of heat through those.

How thick should concrete be above insulation?

It is recommended good practice that all joints should be lapped 150mm and sealed. Apply a sand/cement or self-levelling screed over the insulation boards to a minimum thickness of 65mm.

How much gap between insulation and concrete wall?

I'd create an air gap of about 3/4-inch between the back of the wall studs and the foam insulation. This air gap serves two purposes. First, it allows you to create plumb walls should the poured concrete be out of plumb. Second, the air space is a place for any rogue condensation to evaporate.

What foam fills voids under a concrete slab?

Polyurethane foam is injected into the voids through small holes drilled in the slab. The expanding foam fills the entire void space, creating a solid mass that bonds well to the concrete. A concrete mixture is also injected into the voids, but it usually requires a much larger hole to be drilled.

Do you need a vapor barrier between concrete and insulation?

Without a vapor barrier, moisture would continually wet the bottom of the slab, as vapor will continuously drive from the high humidity below slab to the relatively low humidity above the slab.

Should you insulate under basement slab?

Creating an insulation layer and a vapor barrier between the concrete slab and the ground beneath it is the best practice in insulating basement floors. The insulation barrier will help keep warm air in, and the vapor barrier will keep unwanted moisture out.

What is the best insulation for basement concrete floor?

How thick should insulation be above concrete slab?

The edge insulation should be at least 25mm thick and height should extend from the top of the concrete slab to the top of the finished floor screed. Protect the exposed top of the perimeter insulation from any damage until covered by skirting or wall finish.

Why do they put Styrofoam under concrete?

Foam insulation under concrete slab avoids thermal discrepancies caused by uncontrollable temperature variations, heat transfers and losses and variable environmental conditions. This will help to reduce energy costs.

What insulation to use under concrete slab?

One of the most common forms of insulation used under a concrete slab is rigid foam insulation, such as expanded polystyrene (EPS), extruded polystyrene, and Polyisocyanurate (Polyiso). Expanded polystyrene (EPS), also known as beadboard, is a lightweight, rigid, closed-cell foam insulation.

How do you insulate a pre existing concrete slab?