Build a Risk-free Finished Basement
Remodels should still be detailed carefully, though, since dramatic weather-related water events could occur at any time. I have also seen basements compromised because site conditions changed naturally, or more frequently, because neighboring properties were developed.
A basement with a moderate risk level has seasonal challenges. In colder climates, water from the spring thaw can saturate the ground. In warmer climates, water might become a problem during the rainy season. Regardless of the source, a moderate-risk scenario has water issues for up to half of the year. If you’ve lived in the house for a period of time, you certainly know if and when water issues are a problem. If you’re working on a home without historical context, look for evidence of bulk water in the basement in the form of staining on the slab and/or mineral streaks along the foundation walls. Your new neighbors might also be able to provide some insight into existing conditions, but keep in mind that every property is unique. Just because your neighbor’s basement is dry doesn’t mean yours is.
A high-risk basement faces water infiltration for more than six months of the year. Water might even be a daily challenge. High-risk basement remodels can be managed effectively, but demand a system of water management that requires a perpetual solution.
Site and foundation assessment
The initial steps in designing any finished basement are straightforward. First, if any water exists on the site, try to minimize the amount of water near the foundation as much as you can prior to beginning your work. The less water you have to deal with, the smaller the problem will be, and therefore, the smaller and less expensive the management system needs to be.
Once the site is analyzed and a plan for correction or improvement is developed, the foundation itself needs to be assessed. Different foundation types perform differently and can demand different approaches to management. Stone, brick, concrete block, and concrete all have specific characteristics that challenge a finished basement. Stone, brick, and concrete-block foundation walls are all mortared systems. The amount of mortar and the manner in which it was laid (neat and full vs. sloppy and spare) help determine the ability for water to penetrate the foundation wall. Because of their continuous casting, concrete walls offer a more formidable defense to the infiltration of groundwater. Make no mistake, though — if water exists, it will get in eventually.
Managing moisture
Moisture in its vapor form is usually not a cause for concern in a basement. It only becomes a problem when it is given the opportunity to cool and compress to its saturation limit and condense on cool surfaces. For this reason, controlling surface temperatures in the basement is critical. For example, I recommend thermally breaking the framed walls from the concrete slab with a piece of rigid insulation under the bottom plate. This prevents vapor from permeating through the slab and condensing on the cool wood framing, where it could conceivably cause rot and mold growth.
Some builders try to control vapor by applying vapor barriers between the slab and finish flooring or between the foundation walls and the finished space. In some cases, this approach might be successful, but I tend to stay away from barrier systems in favor of management systems. I typically allow the vapor to migrate into the basement and then manage the level of moisture in the basement air with my mechanicals. A heat-pump water heater, for instance, provides partial dehumidification of the air as it heats the domestic water supply. This appliance, though, needs its own drain line for condensate.
When it comes to managing bulk water infiltrating the basement, the strategy is straightforward. The objective is to manage, collect, and discard water from the basement. A drainage plane that’s appropriate to the basement’s level of risk and its actual amount of water infiltration must be employed to manage this water. With a moderate level of risk where water moves through the wall by capillarity or saturation, the drainage plane could be as simple as a layer of rigid insulation applied to the foundation wall and its seams taped. At a higher level of risk, where bulk water comes through the wall, a drainage curtain or drainage mat would be necessary. A drainage curtain needs to be linked physically to a sub slab perimeter-drain system in cases where a footing drain isn’t already in place. This is the “collect” part of the strategy. After the infiltrated water is collected, it is directed to a sump pump for discharge. If the exterior grade allows, however, the perimeter drain can be directed to day light at some point on the property. By virtue of its sub slab position, the perimeter drain handles any ground water challenging the slab from below.
Insulation and air-sealing
In most unfinished basements, moisture issues cause few problems because a leaky above-grade envelope allows the moisture to d ry. In many homes, though, attempts have been made to separate the basement from the conditioned living space above with batt insulation installed in the first-floor joist bays. These attempts usually fail, since air movement between the floors goes unchecked. In addition, as the lowest point in the house, the basement is subject to the high test negative pressure (air infiltration) due to the stack effect. Whether the basement is insulated or not, the mechanical system located there is unavoidably tasked with conditioning the basement. In many cases, adding a proper thermal barrier in the basement allows the mechanical system to work less and still be able to provide the required heating or cooling of the insulated basement.
When it comes to insulating the basement, there are code-minimum R-values that vary based on where the home is located. My approach is to provide an R-value that is proportional to my thermal goals above grade. I typically strive for a basement-wall R-value that is at least half of my above-grade wall-insulation value. My target slab R-value is typically at least half of my basement-wall R-value. At a minimum, these numbers usually work out to an R-10 slab and an R-20 foundation.
Rigid insulation, spray foam, blown insulation, and batt insulation all have their place in certain basements. I typically consider batt or blown insulation to be an additional level of insulation rather than my primary insulating method. I refrain from putting any of them directly against the foundation wall for a couple of reasons. First, they allow air movement between the foundation and the wall assembly, making it more difficult to control the surface temperature of the framing and drywall. Second, water permeating the foundation can easily move through the insulation and damage the framing and drywall.
I like to control the surface temperature of the foundation wall with rigid foam or spray foam. After one of them has been installed, I determine the risk of adding batt or blown insulation. I tend to avoid batt or blown insulation in high-risk basements, and I use it sparingly or as bulk-fill insulation in lower- risk basements.
In terms of air leakage, the concrete walls and slab do a fine job of providing air-barrier continuity along their surfaces. With a stone, brick, or concrete-block foundation, the air barrier becomes more of a challenge. In these cases, I tend to use a drainage curtain, rigid insulation with sealed joints, or spray foam as the primary air barrier linking the slab to the mud sill.
Fit and finish
When it comes to finish materials in the basement, I have heard hundreds of opinions on which ones to use, where to use them, and why. I rely on my initial risk assessment to guide me in material selection with my clients, but I try to accommodate their wishes. In most cases, the installation methods of the selected materials are of prime importance, not the materials themselves. For example, if drywall is to be used in a moderate or high-risk basement, then I will install a tall (8-in. or 10-in.) synthetic baseboard and hold the bottom edge of the drywall just under the top of the baseboard, which I will fasten to blocking. If the basement incurs a flood, the drywall is likely not to be part of the resulting problem. If a client desires carpet in a low-risk basement, I probably won’t have a problem with installing it wall-to wall. In basements of moderate and high risk, a better option is either an engineered-wood floor or a tile floor with large area rugs. Area rugs are easy to remove, clean, and reuse if they are part of a flood. In summary, my approach is pretty simple: As risk increases, materials used in a finished basement should be less permanent or more resistant to moisture and water.
Content by Steve Baczek. Drawings by Christopher Mills.
Source: FineHomeBuilding Magazine, Winter 2017 issue