How should I insulate my cold cellar?
We live in a 2 year old house with a cold cellar under the front porch. In the cold cellar there is a sheet of plywood under the concrete slab of the porch with 2X4's holding the sheet of plywood up - I'm assuming that this material was used as a form until the concrete cured. I would like to know if I should remove the plywood and the 2X4's and then insulate or can I insulate right on top of the material that is there? How difficult is it to remove the plywood, would it cause any structural problems and is it even necessary to remove the plywood and the 2X4's studs. Also when insulating what type of insulation and R value should I use? I would also like to know what needs to be done as far as a vapour barrier is concerned. Should the plastic be placed before installing the insulation or after? What is the order for the material to be used?
Expert
Maestro
It is very common to find the form boards left in place below the concrete slab of the front porch, as you are describing. Please understand that without looking at it, I can't guarantee that the plywood is performing no structural function in your specific home, but generally, the 2X4s and plywood are waste material and can be removed easily, andwith no ill effect. Eventually, the wood will rot anyway, unless it has been protected somehow. The most common type of insulation in this application is polystyrene. It's easy to work with, and it's rigid enough to remain in place on the underside of a horizontal surface without sagging. Most polystyrene insulations act as a vapor retarder to some degree, and an additional vapor barrier is usually not installed when using this material. Plastic insulations are considered combustible, though, and should be covered with drywall in living spaces. This is often neglected in cold storage rooms. There are no requirements for insulation in this area, but R-8 to R-12 is common. Other types of insulation that could be used are fiberglass or mineral wool (rock wool). These are also available in rigid boards. These insulations don't have very good vapor-retarding properties. A vapor retarder or vapor barrier is not a priority in a cold storage room. The most important thing is to make sure that it's not put in the wrong place. Assuming that the below-grade portions are left as they are (see the attached link), there will be moisture exchange between the exterior and the interior anyway. Depending on the temperature and humidity of the cold storage room, there will be the slight tendency for vapor to move from the inside to the outside, but the vapor drive is pretty low. If one wanted to reduce this, the vapor barrier should be installed on the inside (bottom) of the insulation, but it's not really that harmful, assuming that the wood has been removed. The summer is another matter, though. Moisture in vapor form will try to get into the cold storage room through the slab. This vapor drive is much stronger than the winter vapor drive, especially on a sun-drenched porch after a rainfall. If there is a vapor barrier below the insulation, all of that vapor will be trapped on the cold side (inside) of the insulation, and will collect as water. This is not desirable. In short, the addition of a vapor barrier is not needed. If polystyrene is used, it will have some of the right properties anyway.
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