Finishing basements
Last week we talked about some safety considerations that should be addressed before finishing a basement. This week we’ll move into an issue of particular significance in basements – moisture.
Inspect for moisture – Before starting you should inspect your basement for signs of moisture problems. You do not want to spend thousands of dollars refinishing a basement only to find out you have moisture issues that can lead to mildew or mould and potentially destroy installed finishes.
For example, rot at joist ends or sills can indicate poor ground clearance. If you notice water stains on the upper portion of basement walls, the cause is most likely runoff from your roof or poor surface water drainage. Efflorescence, the presence of white mineral salts, seen on concrete can be a sign of a longterm moisture problem. Rust is another good indicator of flooding or high moisture levels in the basement – check for rusted nails in baseboards or at the base or steel columns or posts.
To correct moisture problems first check the grade of slope around your foundation walls and make sure the drainage system is in good repair. Ensure gutters and downspouts are clean and drain water away from the home. If you’ve had moisture or flooding issues in the past you may want to consult a contractor and take extra steps to waterproof your home before you start finishing the basement. Application of waterproofing products and the proper sealing of any cracks in floors or walls is essential. Recurring basement flooding can be corrected by installing an interior perimeter drain, exterior drainage system, or sump pumps.
Vapour barrier system – It’s good to remember concrete is porous and moisture will eventually find its way through the walls or slab. This means traditional above-floor construction techniques (i.e. wood framing/ fibreglass insulation with a vapour barrier installed between the framing and drywall) does not work in a basement. As concrete releases moisture fibreglass will take on water and become less efficient. Depending on the amount of moisture coming through the concrete, fibreglass insulation could potentially become a growth medium for mould or lead to rot in the framing. Rigid foam board insulation is a much better choice for basements, it is mould and mildew resistant and doesn’t hold moisture should a leak or flood occur.
At minimum, look at applying two inches on walls and one inch on the slab. When applying rigid foam make sure to use an adhesive that is foam rated, if not it could ruin the insulation. Make sure seams are tuck-tapped (bright red tape) and use spray foam to fill in any gaps that occur. Once rigid foam is in place and completely sealed to form a vapour barrier, wood framing can be installed. Some builders like to leave a small gap to allow for air flow around the studs. Wood should never be allowed to come in direct contact with concrete. If you happen to have in-floor heating and are not installing rigid foam insulation on the slab, you can use a piece of composite wood decking below the bottom plate to prevent moisture wicking.
Insulated subfloor panels – After installing rigid foam insulation on the slab you still need to install a subfloor before laying flooring. There are new insulated subfloor products on the market that eliminate the use of rigid foam insulation (e.g. Amdry). These products come as preassembled panels that incorporate vapour barrier, insulation and OBS sheathing. The panels have raised drainage and air circulation channels to prevent the growth of mildew and mould where moisture may be present. You will likely end up spending more on materials but less on labour if you go with this type of product.
It is important to follow supplier installation recommendations when working with this product. As with other building materials it is recommended to let product rest in the space for a week or so before installation. This allows the product to absorb some humidity to avoid expansion issues after installation. Specially designed flex connectors must be purchased with the product to eliminate tongue and groove issues. Also the manufacturer recommends the product is best suited for an entire floor system rather than individual rooms, although it can be installed on a room-by-room basis if special considerations are taken. Lastly, they do offer an alternative product for installation under ceramic or stone tiles. This is a waterproof alternative to the OBS sheathing and is recommended for use in bathrooms or kitchens.
Next week we’ll finish off the basement finishing discussion by looking at some aesthetic topics pertinent to basements such as lighting and flooring.
Remember to look for my column in The News every Tuesday. I will be addressing a wide range of home-related construction and maintenance topics. I welcome readers to submit questions by sending me an email at macgibcontracting@hotmail. com or call me at 902-695-5919.