First off, let’s take a look at working with existing walls and why you may, or may not, have to make some improvements to them.
If your plans include external walls, you may not have to do too much to them, assuming they are below grade foundation walls. Even then, you might be surprised to see how much heat your foundation can conduct. I have both north and south facing foundation walls in my cellar. Fortunately, the south-facing wall is shielded on the outside by an extensive covered deck.
If it weren’t for that deck I would have had to insulate that wall. While it’s almost all below grade, save the top foot or so, that south-facing wall and the soil abutting it receives unobstructed sunlight virtually all day long. That slow heating builds up after months of sunshine, and while concrete foundation walls serve as huge heat sinks, that efficiency at maintaining a temperature works both ways.
While the cold of winter lingers in the walls well into the spring and early summer, the warmth of summer can stick around well after the first frosts of winter. While no one might mind the first part of this scenario, it’s the second we should be worried about. Without properly insulating our cellar we would be looking at warming walls through the summer and into the winter. OK, so that’s not the end of the world, but it’s not ideal and at least for the purpose of this article let’s work towards an ideal cellar.
So, we’ve got to insulate. There are two ways to insulate an external wall, or any wall for that matter. The first and more traditional would be with fiberglass batting, those rolls of fiberglass we’ve probably all seen in our attics, or at Home Depot. The other way is by applying rigid foam insulation panels. (There is actually a third way -- spray in polyurethane insulation -- but that is both more complex and difficult, particularly if you have any notions of this being a DIY project.)
What are the pros and cons of the two types of insulation, then? Well rigid board insulation is very easy to use. It’s basically just foam boards -- easy to buy, easy to cut, and easy to apply directly to your external walls. In addition, the nature of the insulation -- tons of bubbles in the foam – vastly reduces water penetration, as well as the resultant reduction in insulating efficiency.
Of course the trade off is the simple fact that foam boards lack the insulation value of fiberglass batting. A typical 2-inch thick foam board can have an r-value of 10, as opposed to 20 for normal 6-inch thick fiberglass batting. Now, you can layer foam board on foam board and get an impressive 30 r rating for 6 inches of foam board, but not only is that not cost effective, it’s totally unnecessary.
For most conditions, an r-value of 10 will probably be sufficient; after all, we are just trying to keep the temperature in the basement from rising more than about 10 degrees over the course of the year, instead of perhaps 20. It’s a bit different from trying to prevent your whole house from rising 40 degrees in the summer.
Once you’re got the foam board up on your exterior walls you can focus on your interior walls. These will be much more important than your external walls since the temperatures typically encountered in a basement can run much hotter than the ambient temperatures of the soil around your house. Keep the heat from your house out of your cellar and you’ve got it made!
In order to frame out your cellar space you’ll have to build walls. Since these walls are not load bearing, you might be tempted to run 24 inch on center 2x4’s as your framers but don’t do it! While that works perfectly in most cases these walls are really just there to support your insulation so you should build them with that in mind.
You’ll want to maximize your insulation potential here so the best plan would be to use standard 16 inch on center for spacing and to go with 2x6’s for your framing. That will let you use standard 15-inch wide 6 inch thick batting with an r-value of 19. Now I’ve heard people say they’ve had no problem packing that in 4-inch spaces, which may be true, but the fact is that fiberglass batting is designed to be gently laid between the studs. Forcing the insulation to fit just compresses the insulation, forcing the trapped air (that’s really doing the insulating!) out from between the fiberglass strands and lowering the r-value of that insulation!
Two points to remember when building walls directly on the concrete walls of your basement: Lay down a layer of vapor barrier over the floor and under the sill of the wall to prevent water wicking into your sill, and leave about a ½ inch gap between the floor and the bottom of your sheetrock, to prevent water wicking into the rock.
Chapter 2: The Importance of Vapor Barriers
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