I'm only finishing a part of my basement. My question is: do I need a vapor barrier on an insulated partition wall between the finished and unfinished adjacent rooms?? Or is it a no-no?

Also, I'm framing with 2X4's with a 1.5 inch gap at the concrete walls. Where I stop framing, do I need to close that 1.5 gap up? It makes sense to me to leave it open to allow fresh air to travel there, but that also means moist and cold air from the unfinished section will get in...

What do you guys think? Any help is really appreciated.

Mark

 

Throw some thoughts around, this would really help me

 

You close the gap because the 1.5 inch gap is a drainage plane. It is there to make up the difference between the different rates of absorption and expulsion towards moisture the different materials have. For example, fiberglass has a high expulsion rate towards moisture and wood and/or masonry has a slower rate of expulsion towards moisture. This works the best when the air in this dead air space is still because its function concerns diffusion.

If you leave the gap open, you defeat the purpose of it because convection will literally nullify the effect. In other words, the still air in the gap creates the time delay between the moisture absorption and expulsion of the different materials and air movement in this gap will accelerate the moisture transfer between the materials. Best advice, seal the gap and create a dead air space between the two walls.

 

Nice to get this type of in-depth advice here.

Now what do you think about a vapor barrier on a partition wall between finished and unfinished rooms? Need one?

Thanks again

Mark

 

Install the vapor barrier on the finished portion or conditioned room. This is because all heat has moisture in it and insulation slows down heat flow but does not slow down moisture flow in the heat. Vapor barriers slows down the moisture flow in the heat. There are two reasons for this, Equilibrium and high pressure areas are attracted to low pressure areas and not vice-versa. What that means is the heat in the conditioned room is attracted to the lower temperature in the unconditioned or unfinished room. So the heat flow is from the conditioned side of the wall to the unconditioned side of the wall. If you do not slow down the moisture flow by using a vapor barrier, as the temperature drops as it approaches the unconditioned side of the wall, condensation will form. Or if you prefer, 100% Relative Humidity (RH%). This means the percentage of humidity a specific air mass at a specific temperature can hold before condensation occurs. The RH% increases as the temperature of this air mass drops but the amount of humidity in the air remains the same.

For example a specific air mass has 100 grains of moisture in it with a RH% of 40% at 70 degrees Fahrenheit. If you drop the temperature of the air to 60 degrees Fahrenheit, the 100 grains of moisture remains the same but the RH% increases to 60%. Another way of saying this is the Dew Point Temperature for this air mass is 45 degres Fahrenheit means that the above stated air mass at 70 degrees will start forming condensation (100%RH) when the temperature drops to 45 degrees Fahrenheit. Another way of putting this is warm air expands and allows humidity to be held inbetween the molecules. Cool air contracts allows less humidity to be held inbetween the molecules than warm air.

 

Thank you for this.

I see now that even though there is no concrete wall directly behind the framing, I could still have condensation on the cold side due to moisture travelling through the wall and hitting the cold air.

I appreciate your help, resercon.

 

In my opinion, I don't think you need a vapor barrier between the finished and unfinished spaces for three reasons: (1) The temperature difference won't be that great, (2) There is probably a doorway between the finished and unfinished areas which will equalize the humidity levels on both sides anyway, and (3) Any moisture that enters the insulation will be able to dissipate on the unfinished side anyway since it isn't a trapped space.

Having said that, I recognize that others here (including resercon) have much more expertise in this area, so I'd be interested if they think my reasons are invalid.

 

John,

You got it, I seen no reason for it either.

You never know, they might finish that area at some time in the future.