Understanding air movement, air exchange, humidity, and the proper alignment of building materials is important. Many basements are damp, musty, leak air, and have little natural light but most of this can be overcome with the proper understanding of building science and safety considerations. The bottom line is that it’s senseless to spend time and money finishing a basement if water or moisture will ruin the work or cause mold to grow.
As a lifetime drywall contractor I have hung more than my share of drywall in basements. For years I would just hang and tape some moisture resistant drywall and be on our way. In 2009 I became a building performance analyst and a year later a green home verifier. I now realize that I need to spend more time thinking about solving the moisture/water problem first and incorporate construction techniques that work together to prevent moisture from moving into unwanted areas. By their nature, basements have naturally higher moisture levels than above grade spaces.
Look at the complete picture. Each step affects the next. When dealing with a moisture/water problem first eliminate the source, then the pathways and then install resistant materials. Understanding building science is very important. The practical purpose of building science is to provide predictive capability to optimize building performance and understand or prevent building failures. When creating living space in a basement the failures could involve moisture intrusion, combustion appliance safety, air quality, and energy efficiency.
Moisture intrusion: If a moisture problem is discovered, the moisture problem has to be solved.
- Condensation (Also called sweating): condensation shows up as water droplets, wet spots, or puddles on basement floors and walls. It happens when moist, warm air hits cool foundation walls or un-insulated cold-water pipes, dampening carpets, rusting appliances, and turning the basement clammy. To tell if walls are damp from exterior water or condensation from humid interior air, tape a 2-ft. square sheet of plastic to the masonry below grade. If moisture collects on the front of the plastic, it is condensation.
Solving condensation problems:
a. Insulating: Insulate surfaces where condensation is likely to occur, such as cold water pipes and ductwork. Installing continuous insulation along the foundation wall will help prevent air from cooling and reaching its dew point (see sidebar below). Having a continuous air barrier at the same location will also help.
b. Air sealing: Because air movement can carry potentially moist air into walls and ceilings caulking and foam sealing any gaps in the building envelope is a must
c. Dehumidifier: Install a dehumidifier to remove excess moisture from the air. For continuous dehumidifying connect the unit(s) collection reservoir to a drain.
2)Capillary action, (wicking): If moisture collects on the backside of the plastic after a few days, then water is wicking through the foundation wall from outside. Water can move through solid concrete. The basement should be treated the same as if it were leaky. If there is regular seepage or water puddling after storms (even once every few years), the problem has to be fixed permanently before finishing. The moisture may be present because of runoff or subsurface sources.
SIDEBAR: Understanding Dew Point: Warm air can hold more moisture that cold air before becoming saturated. Danger is when warm air that has a high relative humidity leaks into wall or ceiling cavities toward outside surfaces which are cooler. As the air cools the capacity to hold moisture is reduced. The dew point is the temperature when the moisture turns back into liquid form. When all this happens there is now real water in the assembly that can cause real damage .
3) Runoff: The water most often comes from rainfall and melting snow. Even a small storm can trigger a flood of water because of the square footage of the roof. Once the water accumulates around a foundation, it works its way inside through cracks, joints, and porous material. Rainwater or melted snow that isn’t routed away from the house is a common cause of basement moisture. Runoff percolates through porous topsoil and then stops at the compact soil near the base of the foundation. Accumulated pressure forces the water through gaps or cracks in walls and footings. This can also lead to movement through porous walls by capillary action as mentioned above.
Solving Runoff Problems:
a. Proper grade: If the ground near your foundation slopes toward the house you may need to grade the slope away from it to move as much water as possible away from the foundation. Grade the slope at least one inch vertically for every one foot of horizontal for at least four feet.
b. Gutters: Clean gutters and make sure that they are sloping in the proper direction with no low sections. Install an extension for the downspout to redirect the runoff away from the foundation. A four-foot extension would be ideal if space allows.
4) Subsurface sources: In tougher cases, the problem is rising groundwater, which may even be fed by an underground spring. The water could also come from rainfall or melting snow which may be more seasonal. Groundwater is by far the most difficult water source to deal with. If the wet basement symptoms act like runoff, but occurs all the time it is probably caused by high groundwater.
a. Foundation sealing: If water infiltration is minor, inspect the interior walls and seal all seams and caulk cracks and holes in the foundation with a polyurethane masonry product. For larger cracks or gaps use hydraulic cement. When water infiltration is more extensive exterior waterproofing will be far more effective than interior waterproofing, but is more costly to accomplish so is typically done when problem is severe enough so that outside foundation drains have to be installed. The exterior foundation can be waterproofed and/or an insulation board installed at the same time.
b. Footing drains, sump pumps, drainage planes: For water that can’t be eliminated or diverted, second best is to capture it at the outside perimeter of the foundation just below floor level and drain either to daylight or to sump pump. The best approach from the interior is to collect the moisture in a French drain, connect it to a sump, and pump it out. A French drain is a bed of stone that has replaced the soil around the interior perimeter of the basement just below the floor. A sump pump will at least ensure it gets out of the house quickly and efficiently. I have worked in a lot of basements that have a channel in the actual concrete around the perimeter. Water that attempts to enter from along the perimeter either through the wall or up from below will flow in this channel and enter a sump pit.
Combustion appliances such as furnaces or boilers and hot water heaters often use the air from inside the home for combustion. This air often comes from cracks in the basement walls and gaps and openings along the rim joist areas on the basement. So care must be taken when reducing this available air. The appliance may be struggling for air and the exhaust gases may have trouble exiting through the chimney. Don’t want to hide the appliance in a small closet or seal it in an area that has been sealed for sound control either. A combustion safety test is recommended before the basement remodeling work starts as well as during and after the work is complete. Consider sealed combustion appliance or bring in required level of air with an air exchange system. Safety first is a must when dealing with potentially life threatening gases.
Air movement: When air moves through wall and ceiling cavities it often carries moisture which can cause damage. Air infiltration is the flow of outdoor air into the indoors which is accompanied by an equal amount of exfiltration which is air flow out of the conditioned space. Let the air exchange and ventilation systems control the air movement in the basement. A properly functioning home incorporates air tightness and good ventilation. “Build Tight and Ventilate Right”
Because of the confined space of a basement an air exchange system may be desired. This will vent out stale air and replace it with fresh air. Installing a ventilation system sized for the space is often a better choice because they recover heat from the exhausted air that is used to warm the replacement air. Units can be purchased that also work as a dehumidifier (a real plus for potentially damp basements). An exhaust fan such as a bathroom vent fan can be purchased that can be set to run continuously at a desired CFM. The fan vents the stale air which has to be replaced with fresh air. Ports can be installed away from the exhaust fan that allow outside air to enter. This is a simple air exchange system.
Increasing the air tightness, insulation value, and closing off areas while converting the basement to living space can create a basement moisture problem where there was none before. Changing the flow of air, heat, and moisture can upset the equilibrium of the basement. Sometimes the flow (movement) is what is keeping things dry.
Keep the higher R value insulation to the outside. If using foam panels they should be a continuous surface, from floor to ceiling (sealed on all edges). The rim joist area should also be insulated and connected with the wall insulation. A common practice is to have the best insulation and the air barrier next to the outside concrete wall. This prevents warm air from reaching the cooler wall. The closest the air can get to the concrete wall is the warm insulation surface. If a vapor retarder is used it should be next to the warm side. NOTE: A vapor retarder such as a poly membrane is not necessarily an air barrier. Time must be spent sealing any edges and splices with a good quality flexible caulk. I prefer to make the drywall an air barrier and prime it with a vapor retarding primer when a vapor retarder is required.
When a customer has concerns about water whether it is just high humidity or bulk moisture mold will be high on their list of concerns. Mold need water, oxygen, and nutrients to grow. We have already discussed ideas about elimination the moisture. Air movement within the structure will reduce the amount of oxygen available for mold growth, so air sealing will be helpful. Construction with mold resistant products where possible will help reduce the food source necessary for mold growth.
Materials: I have seen all kinds of materials used to frame out basement wall and soffits. Unless jack posts are being taken out most wall are not structural support walls so light gauge steel is a good choice. The metal is inorganic so it is not a food source for mold and metal does not absorb moisture. I suppose that if it were exposed to moisture often enough the metal would corrode. There is even a vinyl stud that is marketed for basement remodeling because it is mold, moisture, and corrosion resistant. The metal or vinyl studs are also lightweight compared to wood so getting the materials into the basement is easier. If you are going to use wood a treated plate is usually installed along the floor.
SIDEBAR: This photo shows a metal framed wall. No insulation was put n the wall before drywall was hung. The outside of the foundation had 2 inches of extruded foam, the rim joist area was sealed with 3 inches of spray foam. Along the foundation wall there is a channel in the floor that leads to a sum pump pit. The basement had never had any moisture problems. Even with everything in favor not having any moisture problems the customerstill wanted mold/moisture drywall installed.
This Photo on left shows treated wall plate that is caulked in place. The contractor did a lot of caulking and sealing to help prevent any water from entering into the living space. If that much moisture was behind the wall cavity then the moisture problem was never really corrected. But the caulking and sealing was still a good idea because it helped make a more airtight assembly.
It may be necessary to create a barrier on the floor as well. The existing concrete floor should have a vapor barrier under the concrete. But in case moisture was to appear on the floor it is a good idea to have a material in contact with the floor that provides a barrier and space where the moisture can drain away. The 2 ft by 2 ft interlocking wood panels work because they have a plastic bottom that holds the wood away from the concrete and also allows any water to drain away. If an additional layer of concrete is poured over the old, a similar idea to the wood flooring is a plastic membrane that is under the concrete to allow water to drain away in a similar manner.
Use mold and moisture resistant products where practical. Tile, stone, PVC products (such as Trim-Tex Vinyl Cornerbeads), and other inorganic products will help reduce damage should moisture problems occur even after all the preventative work done. All this was done in case there is that freak storm or plumbing break. Basements are the lowest point in a home so even if a second story bathroom pipe burst the water will end up in the basement.
Location: Even if the block or concrete wall appears dry it is not recommended to attach the drywall directly in a basement. Poor adhesion, moisture and mold problems could occur. The same criteria can be used if a plaster finish is desired.
Mold resistance Drywall: Why not use mold resistant drywall? Most if not all drywall manufacturers sell a mold resistant drywall that is also moisture resistant, so a long as it is not exposed to contaminated water it won’t grow mold. If the goal is to not have any mold growth in the assembly then all materials should be mold resistant. Mold need water, oxygen and nutrients to grow. Basements are often damp so moisture resistant drywall always seemed like a no brainer and now there is mold/moisture resistant drywall. There is even paperless drywall, fiberglass mat joint tape, and mold resistant joint compound. All these products are designed to increase the longevity of the assembly which is built in a damp environment.
Decorative ideas: Because of the lack of natural light and lower ceilings the spaces can be easily brightened with light colors and decorative drywall work. Quite often basements have a lot of interesting angles and stepped down and up areas. Trim-Tex decorative corner beads and drywall layers with beads can be an inexpensive way to add some accents. Take advantage of the soffits and boxed in areas to create some decorative accents
As mentioned earlier the prime coat on exterior wall can be a vapor retarding primer. Paint and decorate using light colors to brighten up desired areas. An occasional accent surface can be a paint or decorative surface that has a sheen but to help establish a comfortable inviting feeling I suggest using flat paints and textures.
Recessed lights are common in basements because of lower ceiling heights but they essentially become spot lights. Wall sconces or concealed LED light strips that light across surfaces can offer a nice upgraded look.
Why do we even have basements? In cold areas of the country the house foundation has to be dug down below the frost line so why not go a few more feet and have a full basement. When I was a kid my parents house had a full basement. That was where the huge wood and oil furnace was located, and of course all that ductwork. We had the 60 gallon hot water heater. There was an area where my mother had her summer kitchen where she did all the canning and of course where she kept all the canned vegetables and jams. We had two large storage freezers where more food was stored. And of course every fall we would pack in about 9 full cords of fire wood. And oh yes the washer and dryer was all in the basement. I would say we needed the basement. I have done a lot of energy audits in the past few years and most basements I see are just full of “junk”. These basements are conditioned spaces that can be put to much better use than storage. So move the junk to a storage unit and for much less money than building an addition, use the same footprint as the house itself thus creating some quality living space.
Copyright 2013:Myron R. Ferguson