I am drywalling the ceiling of a hallway on the second floor of my house (after putting in potlights).
-the ceiling is insulated, as the house has a flat roof.
-i am drywalling over existing drywall, as i cut out major holes and it has a plaster design that i want to get rid of
-do i need to put up a vapour barrier between the insulation and the drywall?
-if i need to, does that mean i need to take down all the old drywall, or can I put up the vapour barrier on top of it (after making some ventilation holes)?
thanks all,
ryan.
Replies
Hey Ryan,
The vapor barrier normally goes toward the conditioned (living) area.
I guess you are asking, can I sandwich the V.B.between the old existing plaster and the new dry wall?
I guess you could, but.......if the V.B. works the plaster would act as a sponge and retain any moisture present. Flat roofs are not the best with ventilation to begin with .
If you don,t want to go thru all the work of tearing down the plaster, then I would probably not put anything up. Technically , some may even consider the paint a V.B. .
What type of insulation/V.B. is existing?
Any problems?
Fill out PROFILE, big difference between N. Dakota and Florida, between heating and cooling.
DAVE
Assuming a "heating" vs "cooling" climate:No reason why the old plaster would act any more like a "sponge" with VB than without. In theory the humidity would be on the inside of the VB, and the old plaster should be drier with VB than without.You wouldn't want a new VB if there were a decent VB behind the old plaster, however. Better to patch the old VB.
If ignorance is bliss why aren't more people
happy?
Hey Dan ,
I was thinking in a cold climate during heating, frost/moister forms within the shell. Without any V.B. it dissipates in any direction it can. Taking the path of least resistance.With a V.B. the escape is directional, and it must wait until conditions are right.
So the moisture would be trying to go to heat ,but end up getting stored in the plaster until conditions are right to go the other way.
Probably a moot point and as long as the moisture is not excessive, not a problem.
DAVE
What causes the frost to form? With less vapor barrier, MORE moisture will migrate into the space.
If ignorance is bliss why aren't more people
happy?
Dan,
I am no expert on the subject and my thinking may be wrong,but here are my thoughts.
There is always moisture in the air.
Temperature is a factor. If the inside of the house is 70* and it is 32* outside, frost may form on the out side of the house. As it gets colder outside, The frost moves to inside the wall. The moisture gets carried in as a gas and settles out as it gets close to 32*. If it gets cold enough or no air movement on inside walls(especially near the floor) they sweat,next would be frost.
The other part of it is pressure. As weather fronts come and go pressure changes. Even though its only a couple of tenths of psi,it seeks equalization.
Think of a insulated glass unit, all the trouble they go thru to seal them up and they still fail. You cannot fight mother nature and win. You can only make allowances.
With less barriers vapor will move easier. A barrier directs movement not stops it.
Think of the IGU again,the air got in, but then not all the moisture could get out.
Just my thoughts.
DAVE
In the winter, the air outside the house is dryer than the air inside -- frost doesn't "move" to the inside."Sweating" occurs when warm, moist inside air cools to below its dew point. This is why you want the vapor barrier -- to keep the moisture inside the house.The only reason you might not want a vapor barrier is if you're expecting a leaky roof (admittedly not unlikely with a flat roof).
If ignorance is bliss why aren't more people
happy?
DAN,
Less or more matters not . There is moisture in the wall and at the point in the wall where water freezes, frost forms.
When a IGU fails does it matter where the moisture came from.
A barrier doesn't stop it directs. there is moisture on both sides of the barrier.
Compare the extremes, no barrier vs. two. Which causes the most undesirable effect?
DAVE
What you haven't explained is where the moisture is coming from. If the inside VB is perfect (admittedly impossible) then the absolute moisture content of the air in the "attic" will be the same as the moisture content of the outside air. But the air in the attic will be warmer and hence no condensation can occur.
If ignorance is bliss why aren't more people
happy?
Morning Dan,
The moisture comes from the air, humidity, water in it's gas state.
And yes, conditions have to be just right for problems to occur, but there is a lot of water in the air, and an endless supply of air.
In the posters scenario(flat roof), There is a potential for problems.Poor ventilation, but still an exchange of air.
The frost (humidity/moisture) may form on the outside on the roof, on the inside on the sheeting,in mid air, on top of the insulation, two inches into the insulation, or on the vapor barrier depending on the temperature.
The air exchange allows for an endless supply of moisture, lack of ventilation allows for excessive evaporation time. Mold, mildew, rot yech!
I'm sure you have seen houses having all the ply wood and even rafters replaced during a re-roof Because of mold, mildew and rot.
DAVE
I'm sure you've seen those situations, but I'll bet they were due either to lack of a vapor barrier inside or a leaky roof.A flat roof needs a vapor barrier MORE, due to the poorer ventillation.
If ignorance is bliss why aren't more people
happy?
Hey dan,
I guess we will just have to agree to disagree about the vapor barrier.
The real problem is ventilation, lack of ventilation.
Good discussion,
DAVE
True about the ventillation. That's the reason that flat roof structures rot (when they aren't leaking). My point is that if there's ANY ventillation at all to the outside, then you won't have wintertime condensation problems unless moisture is getting into the space from inside.
If ignorance is bliss why aren't more people
happy?
I live in Boston. Every house in my neighborhood has a flat roof and is 150 years old. No vapor barrier on the inside. I don't see these roof's rotting away from moisture problems.
Probably because there is halfway decent ventillation, and Boston isn't an especially bad climate with regard to attic condensation. Also, five coats of oil paint on the ceiling is a very good vapor barrier (if the plaster isn't too badly cracked and there aren't too many bypasses).
If ignorance is bliss why aren't more people
happy?
thanks all for the tips. i decided not to put any VB up. paint as vapour barrier is a good point, and the idea of not trapping moisture in the roof also made sense.
thanks again.
ryan.
what are "potlights"?
are you growing something illegal?
I'm assuming you want to put a plastic vapor barrier up.
I would never use a plastic vapor barrier on a ceiling under a roof. If (or when) the roof leaks, you want to know it. You don't want to have a lake up there breading all kinds of stuff without you knowing it.
This is especially important with a flat roof that is not ventilated. The rubber or tar and gravel is a near perfect vapor barrier. You do not want to have another barrier at the ceiling level. Any moisture that gets in there will have no place to go.