Insulation, Radiant and Vapor Barriers - steel stud insulation

have basement walls covered with poly then steel studs how do i insulate?

I disagree with the poly against the basement wall with the exception if it is mandated by the State or city. In most cases the poly is going to act not only as a moisture barrier but also a vapor applied to the wrong side of insulation. The probability of condensation occurring between the insulation and moisture barrier (poly) is high, especially if the basement is heated furing the winter.

Basements differ from the rest of the structure because they are or part of the basement is below grade. This feature gives basements a more stable and constant temperature than the rest of the structure. This is true in the summer and winter. Humidity plays an important role in basements. One key factor is Equilibrium Relative Humidity (ErH%). What this is basically is, if the air has 23 grains of humidity in it, everything it comes in contact with will eventually equalize in humidity. For example, if the air had 23 grains of humidity and the basement wall had 19 grains, eventually they would equalize. Since the rule is absorption, the basement wall might increase to 21 grains and the air decrease to 21 grains, depending upon volume. This is in essence how materials in the home dry out. The other factors are relative humidity, heat condenses against cooler surfaces and sources of moisture.

There is really no easy way to explain relative humidity. Water vapor or grains of moisture in air remains constant as the temperature of the air increases or decreases. This is because the air reacts quickly to temperature and water vapor does not. Another way of saying it is air expands when heated and contracts when cooled and the water vapor in that same air does not, it remains the same. What does change is the percentage of water vapor or humidity in the air as the temperature changes in the air because of the expansion and contraction. Let us say air at 35 degrees Fahrenheit has 23 grains of water vapor and has a Relative Humidity of 70%. The temperature of this air is raised to 70 degrees Fahrenheit. This air still contains 23 grains of water vapor but because of the expansion of the heated air, it is capable to hold more water vapor, it Relative Humidity drops to 17%. In other words, the rise in temperature causes the air molecules to separate, giving it the ability to hold more water vapor. The reverse is true when the temperature drops, the air molecules come closer together and it can hold less water vapor. Dew Point, Absolute Humidity and 100% Humidity is the temperature of air with grains of moisture or water vapor, where the air can no longer absorb humidity or drop temperature without forming condensation. This is why heat condenses on cooler surfaces. It also explains why a cold can of soda sweats and a hot cup of coffee does not. Temperature of air dropping precedes air causing condensation.

Sources of moisture in basements are the humidity in the air, appliances like dryers, washers, water heaters, heating systems, people and even moisture through the walls and floor. These sources can increase the humidity in a basement and some can decrease it. For example, let us say the basement walls are dry because of good drainage around the house. This usually results in the walls having a low percentage of humidity in them. This gives the walls the ability to absorb humidity in the basement, thereby reducing the amount of humidity in the basement. On the other hand, let us say the walls are seeping water through them. This will increase the humidity levels in the basement. Equilibrium Relative Humidity applies to both situations. In the former, I would not recommend water proofing the walls because it contributes to the reduction of humidity in the basement. In the latter, I would recommend water proofing the walls because it is contributing to the humidity levels in the basement.

This by itself will not solve all the problems with moisture in basements. In fact, when determining an appropriate action to deal with a moisture problem, everything in that basement should be considered. For example, a standard heating system will consume air in the basement for combustion. This will create a vacuum and draw air from outside into the basement. Since the outside air is colder and contains less humidity, when it enters the basement it will be heated and the humidity percentage of the cold air will decrease. This will give that air the ability to absorb humidity in the basement and it is contributing to drying out process in the basement. A clothes dryer with a properly seal exhaust can do the same. On the other hand let us say the exhaust is not sealed and moist air from the dryer is leaking into the basement. It is contributing humidity to the basement.

When we give solutions for applications in basements, like water proofing, we sometimes look only at that application. The problem with this is different applications interact with each other and may affect the drying out process in the basement adversely. Like water proofing a dry basement wall, prior to water proofing the wall was contributing to the drying out process, afterwards, it would probably increase the humidity level in the basement. Just imagine adding insulation to that same wall without even considering all of the above. Site conditions, including the occupants, should determine the appropriate action.