Foundations

Moisture and Vapor Management

Foundation walls are in direct contact with the soil, which is a major source of moisture. Managing this moisture is vital to prevent water intrusion, mold growth, and structural damage. The process involves a multi-layered system:

• Drainage: The first line of defense is a drainage system to move bulk water away from the foundation. This includes sloping the finished grade away from the building, installing gutters to direct rainwater, and a weeping tile or perforated drain pipe system at the base of the footing. This system is designed to collect water and move it away from the foundation.

• Waterproofing: The exterior of the foundation wall is typically treated with a dampproofing or waterproofing membrane. Dampproofing is a simple coating that resists moisture, while a waterproofing membrane is a thicker, more durable material that can withstand hydrostatic pressure—the force of water pushing against the foundation from saturated soil. This layer is crucial for preventing liquid water from penetrating the porous concrete.

• Vapor Barrier: Concrete is naturally porous and allows water vapor to pass through it via capillary action. A vapor barrier is installed to stop this moisture vapor from migrating into the basement or crawl space. On the interior, a vapor retarder is sometimes installed on framed basement walls, while in a crawl space, a heavy plastic sheet is laid over the dirt floor.

R-Value of Common Foundation Materials

The R-value of common foundation materials is very low, making insulation essential for an energy-efficient basement or crawl space.

• Poured Concrete: The most common foundation material. It has a very low R-value of about R-0.08 per inch. An 8-inch concrete wall would only have an R-value of roughly R-0.64. This is why an uninsulated basement is a major source of heat loss.

• Concrete Masonry Units (CMUs): Commonly known as concrete blocks. An 8-inch CMU wall has an R-value of about R-1.11. While slightly better than poured concrete due to the air pockets in the block's cores, it's still a very poor insulator.

• Insulated Concrete Forms (ICFs): A modern alternative where hollow foam blocks are stacked and then filled with concrete. The foam provides continuous insulation on both the interior and exterior of the wall, giving the finished assembly a high R-value, often ranging from R-20 to R-25.

• Wood Foundation: Also known as a Permanent Wood Foundation (PWF), it's a structural system of pressure-treated lumber. The R-value of wood is higher than concrete, at about R-1.25 per inch. The main benefit is that insulation can be easily installed between the wood studs.

Types of Insulation used beneath concrete slabs

The most common types of insulation used under concrete slabs are rigid foam boards because they have high compressive strength and resist moisture. They can withstand the weight of the concrete and the loads placed on it without being crushed.

• Expanded Polystyrene (EPS): Also known as beadboard, this is the most common and cost-effective option. It has an R-value of about R-3.6 to R-4.0 per inch.

• Extruded Polystyrene (XPS): This is a more durable and moisture-resistant rigid foam board, often colored pink, blue, or green. It has a slightly higher R-value of R-4.5 to R-5.0 per inch.

• Polyisocyanurate (Polyiso): This offers the highest R-value per inch, at R-6.5 to R-6.8, and is also very moisture and fire-resistant.

Vapor Barrier and Insulation

The roles of the vapor barrier and insulation are different but complementary. A vapor barrier (or vapor retarder) is a sheet of plastic, typically 6-10 mil thick, that is installed to prevent moisture vapor from the ground from migrating up into the concrete slab.

• Placement: It's critical to place the vapor barrier correctly. It should be laid directly on top of the insulation, just beneath the concrete slab. Placing it here prevents any water from the concrete mix from soaking into the insulation, which would compromise its R-value. It also ensures the slab is isolated from ground moisture, which can cause flooring failures and condensation issues.

• Without a Vapor Barrier: Even if you use a moisture-resistant insulation like XPS, a vapor barrier is still necessary. While the foam boards resist liquid water, they are not a perfect vapor barrier on their own. The vapor barrier is the final line of defense against soil gases like radon and moisture, preventing them from entering the building.

Mineral Wool Insulation

Mineral wool (also known as stone wool) is a viable option for under-slab insulation. Products like ROCKWOOL's Comfortboard are rigid and have the compressive strength to be used in this application. However, it's important to understand its properties:

• Vapor Permeability: Unlike rigid foam, mineral wool is vapor permeable, meaning it allows moisture to pass through it. This can be beneficial in some applications, as it allows the assembly to dry out, but it means a separate, high-quality vapor barrier is absolutely essential on top of the insulation to prevent moisture from reaching the slab.

• Moisture Resistance: While mineral wool resists water absorption and will not rot or grow mold, it can absorb some moisture. Therefore, it's not recommended for use in areas with high groundwater levels.

How Concrete Releases Moisture

When concrete is mixed, water is added to the cement and aggregates. The cement then undergoes a chemical reaction with the water, called hydration, which allows the concrete to harden and gain strength. This is a process that can take many months to fully complete.

• Evaporation: Initially, a significant amount of water evaporates from the concrete's surface. However, a lot of water remains within the slab's pores, and it continues to move slowly up through the concrete via capillary action.

• Moisture Vapor Transmission (MVT): This upward movement of water vapor is known as Moisture Vapor Transmission (MVT). It's a constant process, and the rate at which it occurs is what building science and flooring manufacturers are most concerned with.

Acceptable Moisture Levels in Building Science

An "acceptable" moisture level in a concrete slab isn't zero; it's a rate of transmission that won't cause damage to the finished flooring or encourage mold growth. The acceptable level depends on the type of flooring to be installed. Hardwood flooring, for example, is far more sensitive to moisture than ceramic tile.

Building scientists and flooring professionals use various tests to measure the moisture level in a slab before a floor is installed. The most common methods are:

1. Calcium Chloride Test: This test measures the MVT rate in pounds of moisture per 1,000 square feet over a 24-hour period. An acceptable rate for most floor finishes is typically 3 to 5 lbs per 1,000 sq. ft. per 24 hours.

2. Relative Humidity (RH) Test: This test uses a probe inserted into a drilled hole in the slab to measure the internal relative humidity. This is often considered a more accurate method, and an acceptable RH level is generally less than 75% for most floor finishes.

If the moisture level is too high, it can lead to a host of problems:

• Flooring Failure: Adhesives for vinyl, carpet, or wood flooring can fail, causing the flooring to lift, bubble, or warp.

• Mold and Mildew: The presence of moisture beneath a non-porous floor finish creates a perfect environment for mold and mildew growth.

• Aesthetics: Even if the floor doesn't fail, high moisture can cause cosmetic issues like staining or efflorescence (a white, powdery substance on the concrete surface).

To prevent these issues, a vapor barrier is essential for any slab-on-grade construction. This is a layer of impermeable plastic sheeting that is installed beneath the concrete. It blocks the capillary action of water from the soil, greatly reducing the slab's MVT rate and ensuring a healthier and more durable floor.