Should I Install a vapor barrier on the ceiling?

I designed and built my 3-story 3-car #garage by myself from foundation to siding in only a few months. I took videos and timelapse of the entire project from start to finish.

Videos shot with the:

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[srahm]

Im doing a ceiling in my house and my papa, a retired master builder, told me not to put a vapour barrier on the ceiling. Vapour will condense on the barrier and damage the ceiling. It needs to flow and if you've got good airflow through the loft it shouldn't condense. Houses need to breath too

[DennisMathias]

No, I think this guy is right. I've analyzed like this guy. If you consider condensing surfaces and high humidity such as I have here in the summer outside air would condense out on a vapor barrier. So you have moisture captured between the VB and insulation. That's where mold and fungus grow.


In the winter you try like crazy to maintain 40% RH. That heads for the VB and condenses out on the other side (inside).


Either way, the idea is to mitigate the moisture retention. Keep it moving. Vent the attic above the ceiling joists (or rafters with an air gap) from soffit to roof vent and move it out.


Try to use insulation that is not affected by moisture like rockwool.


Think of it another way. When you put a coat in in the dead of winter you sure don't want a vapor barrier on either side. Put a raincoat on inside the coat and you'll be wet. You are the heat and humidity source. Put a raincoat on the outside of your coat and your coat will get wet.

[profkrumdieck]

Dew Point! In the summer you do not get condensation in your attic unless your house temperature is below the dew point which would be a bit mad.

[BorisYeltivinshki]

Vapor barrier is a 2 for 1 product. The main purpose of a vapor barrier is to restrict air flow, but it is also designed with vapor retarding properties that allow a small amount of vapor to pass through. The vapor is in the air itself.

You will likely lose a fair amount of heated air through your ceiling.

You can combat mold growth in insulation with mineral fiber insulation. Mold will not grow on mineral based insulation.

[mikemarcu]

I agree totally with you Mr.Garage- generally the vapor barrier on ceiling can be a head ache -because you cannot make it perfect and  hence you have condensation  under it. Better to have  about 16 inches of insulation bats, no vapor barrier, then all possible gaps properly covered-like the ones around  the access trap to attic  then proper venting from soffit to vents- vents to be 1:150 /and this is it.Indeed vapor barrier is good provided is installed properly  when the house is built  to have 100% cover seal  which is almost impossible to be done at an old house.Remember that in the winter time you have in house  at 20 Celsius cca 50% humidity that is 10 g/cubic meter of air. Immediatly  above the drywall you'll have  cca 15 C and at the top of insulation you have cca  -5 C when outside is  -20 .At that moment, the vapors had no time to condense and migrates to the vent- I did this experiment  this  winter  near Ottawa . I've seen this faint steam going up the vent like at  the tail pipe of the car.In my case, I  had  water coming in the  room from under the insulation because of  the gaps at the hatch and massive condensation- I simply took the drywall out, I took the  vaport barrier out I put the drywall and I  redone the insulation-so that to be breathable.(16 inches thick bats)-all OK now.

[karenmiller]

You are correct about controlling the flow of moisture migrating through a wall or ceiling from the inside (heated) air to the outside (cold) by installing a vapor barrier. Without a vapor barrier, in the winter (sub zero temps), water vapor from the heated interior will condense inside the wall or ceiling at the temperature point within the wall where the "dew point" is reached.

However, you are completely wrong about the reverse situation. During the summer, the "dew point" will never be reached inside the wall or ceiling. Typically, the difference between outside temps and inside air conditioned temps might be (100 - 110F vs 70F). If you took an object at 70F outside in the summer, you would never experience condensation on the object. That is simple proof that you will never have vapor condensation inside a wall in the summer time.

You will have condensation in the winter time with inside temps of 70F vs sub zero temps outside. As simple proof of this, ask anyone who wears glasses about condensation on their lenses when they come inside from the outside in the winter time.

[BourneAccident]

You need a complete moisture impervious barrier protecting your non-living space (typically attic, not necessarily basement).  Without a vapor barrier, warm moist air rises into your attic, condenses and causes moisture at first and then MOLD.  I did an experiment.  I sealed a plastic bag with duct tape over an unused hvac ceiling return vent.  Then one winter day my plaster ceiling was all wet.  I found the bag 1/4 full of water and leaking!  I pulled the bag, the vent,  installed plywood, a vapor barrier, 13" of unfaced insulation, then I boarded and plastered inside on the ceiling.  Experiment over.  With no vapor barrier, moisture builds up in your attic and causes big problems.  Just my .02 cents.

[steveboerner]

In your case, if this is a garage with no liveable space above, then I can agree with your approach. However, I only agree in that case. In almost every other case, even in the Northeast (where I live), it is absolutely necessary to install an air/vapor barrier in the attic/ceiling. At the same time, you're structure will need proper ventilation in the attic in order to ensure the entire building system works properly. 

[chevy]

One side needs a solid barrier from moisture. That side is the side with the insulation and air cap that is insulating your dwelling. Otherwise you will draw in moisture from outside of your dwelling, It does not matter if it is hotter or colder outside.  Someone correct me if I am wrong.

[RobertWewer]

You have a metal roof.  Have you ever seen moisture bead up and produce frost on the underside of metal roofs?  You say just let it go through the ceiling.  Wow, what a concept. If you heat that garage and produce any moisture, it will rise up into the attic, especially after the wood has seasoned, if you are building a new structure.  I am a little confused as you seem to advocate a paint vapor retarder.  So do I.  I am in the Northeast and after thousands of roof removals and wall openings, I have never found that the pressures of the Summer (from the outside) rival that of the Winter (from the inside).  Key point is to test your slab with the "doormat test" which is a doormat or plastic laid out on the slab for a day or two.  If you have moisture on the backside of it, your slab is bringing up moisture into the building because it does not have a vapor barrier.  Take hygrometer readings.  If it is a garage, maybe no worries. 

[tyronesunde]

Does a vapor barrier make venting in the attic unnecessary?

[Brandon-novc]

What about vapor barrior for the walls then

[awesome]

Would have been better to probably have sheathing with taped seams on roof with 30# felt (equivalent is only what store stocks these days but it works) and air tight dry wall

Do you have any venting (well do you have enough i guess?)

[Tonykayemusic]

House wrap for outside entry of moisture.

[sardoggy]

Thanks very much.  We have some contractors doing some work here in our home in florida and I didn't see them install any vapor barriers so I was wondering if it was a good idea or not.

[josemorrasge]

Vapor barrier keeps the warm moisture leaking into your insulation

[brendarobinson]

i do have a question i have cardboard square type ceiling, i had a central air unit put in  they had to put the vents in my ceiling cuz no crawl space under my house anyway it has caused condensation where my vents are and they came and put insulation around the ducts, well i painted the ceilings and so now its doing it again, my question is what needs to be done i dont want my ceiling falling around me it needs to be done right and with it causing condensation isnt the insulation getting wet and could cause mod? please i need answers

[brockbakerable]

You absolutely need that vapor barrier. I haven't read the prior comments so this may have been answered but your thinking is a little off. The main reason for that vapor barrier is to prevent condensation in the attic or space above the ceiling. I personally went into my attic to find the brand new roof dripping water from underneath due to condensation. I don't have enough ventilation to remove the vapor that builds up. Mind you, if left unchecked this will rot the whole underside of my roof. I took out the old insulation and placed a vapor barrier using ridged insulation and even without the ventilation the problem was fixed. You always want to prevent moisture vapor from entering your attic. By fixing my moisture issue I also fixed a second issue which is airflow. You might think it's not there but I promise you it is. You have a lot of airflow into the space above based on the dirty dark spotted insulation. That's caused by airflow. In my case, I noticed my heat and ac didn't run as much and I seem a 9% reduction in costs. I didn't up the r value at all. Just stopped the airflow and moisture migration. Just my two centsa

[RealCanadianable]

I don't disagree with anything said in this video regarding vapour barriers. I think it is all basically common sense. But the installation of an actual vapour barrier is institutionalized in the building codes of virtually every jurisdiction in North America. Just how is one supposed to get around that?!

[wade]

My personal experience has taught me NOT to put a plastic vapor barrier on the ceiling. More than once have seen excessive condensation on the plastic and water damaged sheet rock. If attic ventilation is correct you won't need a vapor barrier.