1. Is it safe?
One of the very first things you want to look at with any basement conversion is safety, and that can take a lot of different forms. Is there convenient and safe access from inside the house in the form of a stairway that meets current building codes, or is there an exterior entrance that works for what you want to use the space for? Is there a sufficient amount of headroom? Would structural supports for the upper floors be in the way, and if so, can they be moved or altered to allow for the necessary space? If you'll be creating a sleeping room, is there safe and legal egress?
2. Is it dry?
Now we get down to what's a big issue in a lot of basements, and that's moisture. Newer homes are often designed with the proper drainage systems and exterior waterproofing to keep the basement areas dry and ready for building, but many older homes had basements -- "cellars" -- that were designed for cool storage and were never really intended to be occupied. If you have a basement with ongoing or seasonal moisture issues, you need to consult with an experienced excavation contractor before you get started on a conversion project. See what it would take to have a drain installed and have the exterior walls properly waterproofed, and perhaps have the exterior grade adjusted to change the flow of water runoff around the exterior of the home. In some cases, you may need to change gutters and downspouts to channel roof water to a different location, or possibly install a sump pump to handle seasonal water issues. No matter what, be sure that your basement moisture issues are handled before you start closing things up.
3. What will the space be used for?
Once you've determined that you have a space that's safe and dry, decide what you want to use the converted basement area for. That will make a big difference in your design, and also in your construction costs. For example, it may be a big game room, in which case you need little more than wall, floor and ceiling finishes. On the other hand, you may be looking at a bedroom or two, perhaps for guests, with a full bathroom and maybe even small kitchen facilities. In that case, you've got plumbing issues to consider, as well as a lot more electrical wiring, and also ventilation. Some basement spaces are even used as rentals in order to generate a little additional income. If that's your intention, you'll need to take into consideration what the access will be for your tenants, so as to provide privacy for them and for yourselves. You'll need to talk with your plumber and electrician about how the electrical and plumbing services will be split and metered to the spaces. And, of course, you'll need to confirm with your local jurisdictions that having renters is a legal use of the space.
4. Finishing the space
Options abound for the actual finishing of the space itself. One of the first considerations is insulating it, so that it's warm and energy efficient, as well as quieter. The preferred insulation method used by many of today's builders and remodelers is extruded polystyrene insulation. Extruded polystyrene is a high-density, water-resistant, closed-cell foam available in sheets, typically blue or pink, as opposed to the expanded polystyrene -- commonly known as beadboard -- which is white and is comprised of small round foam pellets fused together. Expanded polystyrene has a lower R-value, and is not approved for moisture contact applications. You can apply 1 1/2-inch polystyrene (R-value of 7.5) or 2-inch (R-10) directly to the walls with adhesive. The sheets are 2 feet by 8 feet, and the long edges are notched to accept furring strips every 2 feet. You can then install drywall or other wall finish materials directly to the furring or, depending on the level of insulation you want to achieve, you can add a second layer of rigid foam to the furring, then add your wall finish. Conduit can be attached to the walls first for electrical wiring, and the polystyrene is easy to route out as needed over the conduit. Obstacles such as posts or other structural supports can be framed around using two-by-fours. Horizontal duct runs and large horizontal plumbing runs can be framed into overhead soffits. If there's a sufficient amount of headroom, a suspended ceiling allows for the installation of decorative, sound-deadening tiles that are easily removable to allow access to overhead utilities.
Basement conversions can be a very rewarding use of existing space, and they can add a lot of resale value to your home -- but only if they're done right. If you have any questions about moisture control, insulation, structural issues, zoning or anything else, be sure to get them answered by the pros before you get started!
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