vapor barrier question

Just curious..... In the section about ceilings, I see no mention of a vapor barrier in Rod's book. If, say, I am doing the standard 'room within a room' thing, does the outer leaf ceiling need a vapor barrier? My outer leaf ceiling (which is really the attic floor of my garage) consists of 1 layer of 3/8" drywall, 2 layers of 1/2" drywall and a final layer of 3/8" drywall on top of that. I plan on putting pink insulation in the joist bays. Do I need to put a vapor barrier over that? I would think that if my inner room is airtight enough and vented properly then this would not be necessary. Please advise. Thanks.

Not sure about the vapor barrier, but I can tell you that your plan for the drywall is a really bad idea. 3/8" and 1/2" drywall are too thin for that. The acoustic properties, in terms of stiffness, resonance and mass, are not what you want. It would be far better to go with just two layers of 5/8, or even three layers of 5/8 if you really need that extra isolation. Thin light-weight panels aren't much use in isolation, even if you stack lots of them together: they still act individually, to a certain extent, as well as together, and their individual characteristics are not good. - Stuart -

Thanks, Stuart. It might not matter but I'm just realizing that I mistyped.... Starting from the inner most layer, it's 1 layer of 3/8" ply, 2 layers of 1/2" drywall, followed by 1 layer of 3/8" plywood on top of that. In my inner leaf, I plan on doing 3 layers of 1/2" drywall. Would you suggest I go with 5/8"?

Is your build going like you drew here: viewtopic.php?f=2&t=13842&p=98316#p98316 It reads like you are installing the materials on the top side of the existing joist and the bottom side of the same existing joist. Which one is correct ;)?

That is a piss-poor picture, xspace. The lack of Sketchup support in Linux really blows.....hence these "works of art".

It reads like you are installing the materials on the top side of the existing joist and the bottom side of the same existing joist.

And although that drawing is bad I'm not sure how you came to this conclusion. (Not being surly here......and I appreciate the response/feedback.) On top of the outer most joist of that of that drawing (faintly labeled 'joist RM 1'), is in fact (starting from the bottom of the stack) 1 layer 3/8" ply, 2 layers of 1/2" drywall, 1 layer of 3/8" ply. This is essentially the floor of the attic in my garage. I plan on building another ceiling underneath that one which, from what Stuart says, will need to be extra beefy since my material choices for this attic floor weren't as substantial as "should be". Thoughts? Thanks!

It was simply a measure to make sure I understood what you are doing, it is that important.

Pure L wrote:

If, say, I am doing the standard 'room within a room' thing, does the outer leaf ceiling need a vapor barrier? My outer leaf ceiling (which is really the attic floor of my garage) consists of 1 layer of 3/8" drywall, 2 layers of 1/2" drywall and a final layer of 3/8" drywall on top of that. I plan on putting pink insulation in the joist bays. Do I need to put a vapor barrier over that? I would think that if my inner room is airtight enough and vented properly then this would not be necessary.

A vapor barrier, in general if EVEN required, is always placed on the warmer side of the room. Warm air having better ability to hold moisture and cold air having the lesser ability to hold moisture, when the vapor tries to equalize by moving from the warm side (high pressure) to the colder side (lower pressure) it will condense on the barrier, not to be confused with a vapor retarder. There is much debate over this issue, mostly by those that have "always done it this way", but the facts still persist. I have never seen a vapor barrier placed in an over head area, I have seen a vapor retarder [kraft paper facing] which slows the vapor but still allows it to get to the outside environment, but never a barrier which stops the vapor and allows it to become moisture. I might add that the wall assembly has as much to do with if a vapor barrier is required as does the geographical location, and I had to work to find out the construction of your build, so your as much at fault for the perceived misunderstanding as I might be. The main reason I wouldn't use a vapor barrier in an overhead area, the above not with-standing, is gravity, and the potential for the polarities of warm and cold to flip specifically in an attic area. The first one, gravity, remembering that vapor moves from high pressure to low pressure or warm side to the colder side, once it hits a barrier it [vapor] will condense and become moisture. Now that moisture has no where to go but back into the hard boundary, the sheetrock and plywood you installed. It does not turn back into vapor but is now water and has ability to feed mold growth and deteriorate the materials over time you have installed. Secondly, in an attic area, the potential for the warm side in warmer periods of the year, to become the >outside part of the attic< now creates the situation where, when the vapor attempts to equalize [warmer to the colder side] the vapor will stop at the barrier, become water over time, ruin the insulation, set in the rot of your framing and still feed mold. Neither one a pretty site, nor wanted from a financial fully insured point of view. I would not use a plastic vapor barrier but I would use a high quality primer paint, two coats, and a high quality finish coat paint, two coats as well on the interior of my room. This will allow the well vented attic to move it [vapor] out of the attic when needed and allow the HVAC to extract it from the interior of the room as needed in the other event. The above assumes a well vented attic area and a properly spec'ed HVAC unit.

Well said, amigo. Many thanks.

xSpace wrote:

The main reason I wouldn't use a vapor barrier in an overhead area, the above not with-standing, is gravity, and the potential for the polarities of warm and cold to flip specifically in an attic area.

I would agree that placing a vapor barrier (or retarder) on a ceiling does not make sense - however I would agree for different reasons. With the exception of ceilings that are insulated on the exterior of the roof deck - attic spaces are required to have ventilation from the soffit and either ridge vent or gable louvers........ the requirement is specific as to the minimum area and the reason is that the heat gain (within the attic or cavity) will create a condition where natural convection takes place- which will always draw cooler air into the attic or rafter cavity from the soffit vents which will (in turn) disperse the moisture ridden air into the atmosphere....... This makes perfect sense as a means to rid the building of moisture.......... now - a comment in that regard - ridge vent would be preferable to gable louvers - this due to the fact that there will be a certain level of stratification that takes place above the hight of the louver while the ridge vent will always vent air. Both are not allowed.......

The first one, gravity, remembering that vapor moves from high pressure to low pressure or warm side to the colder side, once it hits a barrier it [vapor] will condense and become moisture. Now that moisture has no where to go but back into the hard boundary, the sheetrock and plywood you installed. It does not turn back into vapor but is now water and has ability to feed mold growth and deteriorate the materials over time you have installed.

This is (however) not correct - the only thing that turns the vapor back into water would be if it were to reach dew point - which is directly related to temperature and nothing else. There are 2 causes for the formation of mold - one is a physical leak through the structure - the other is caused by relative humidity high enough to sustain mold generation. When there is no leak - but there is mold - that means that the relative humidity within the space is too high - not that water is forming within the drywall or the frame cavity. this is why proper exhaust in a bathroom is so critical........ and why it is critical to design the HVAC systems properly - so that they can properly dehumidify the space (especially in the summer months when the decrease in air temperature raises the relative humidity tremendously.) In a properly insulated wall cavity a vapor retarder (polyethylene sheeting is NOT a vapor barrier) placed directly behind the drywall on the building interior (in cold climes) or directly behind the building sheathing (in hot climes) will never cause (or allow) a condition where vapor will condense in the sheet covering it. that sheet will be one with the space it is within - ans the relative humidity will be constant throughout (small variations in stratification that exist notwithstanding. The same is true with a vapor barrier behind the ceiling. However in this case the problem becomes the inability of the building to "breathe" in order to expel the vapor - it is actually a requirement of FHA

Secondly, in an attic area, the potential for the warm side in warmer periods of the year, to become the >outside part of the attic< now creates the situation where, when the vapor attempts to equalize [warmer to the colder side] the vapor will stop at the barrier, become water over time, ruin the insulation, set in the rot of your framing and still feed mold.

Although I disagree with the theory being applied here (as relates to a properly vented and insulated attic) due to a number of issues involved (not the least of which is that if this were true the condensation would take place at the ceiling in an air conditioned home regardless of the vapor barrier) it is not worth going into in any real sort of depth because the ventilation is what makes it all work. And we both agree that this is not the right place to put a barrier regardless.

Neither one a pretty site, nor wanted from a financial fully insured point of view.

If done improperly I would agree.

I would not use a plastic vapor barrier but I would use a high quality primer paint, two coats, and a high quality finish coat paint, two coats as well on the interior of my room. This will allow the well vented attic to move it [vapor] out of the attic when needed and allow the HVAC to extract it from the interior of the room as needed in the other event. In a typical studio construction - unless the ceiling is painted (which I would not typically design) then I recommend no paint on the ceiling - in the summertime you are increasing the relative humidity inside of the house when you are cooling the air - and thus the mo9se humidity you allow to escape the house via the vented attic the less strain you place on the cooling system - in the winter time you are taking in (via fresh air sources) air that may well have very high humidity levels - but as you heat it up you lower the relative humidity - and often times have to add humidification to achieve a reasonable RH level - again if you have a lower RH level the passage of humidity through the ceiling would be a plus not a negative the only issue you face that could possibly cause a problem would be an attic space (or rafter bay in the case of a cathedral ceiling) where the roof decking would receive damage (due to the vapor condensing on that cold surface).

The above assumes a well vented attic area and a properly spec'ed HVAC unit.

That is always the case.... Sincerely, Rod

Soundman2020 wrote:

...and their individual characteristics are not good. - Stuart -

I understand, maybe incorrectly, that sheetrock in thicknesses less than 5/8 are not as consistent and that they have occasional voids in them - making them poor choices for soundproofing and also keep them from having the same fire rating. Is that what you are referring to and is my understanding correct? Scott

It's not just that, Scott: 1/2" drywall has much less mass, is more flexible, and has a higher resonant frequency than 5/8". None of that is good, acoustically. And stacking two layers of 1/2" together doesn't help either: the individual layers still act individually, as well as together. The overall effect is that you don't get good isolation. I'd stick with 5/8" for everything. The plywood on the studs is fine, and is useful for a "all-wall" nailing surface (as well as greatly improving structural integrity of the wall), but that could be thicker too. I would just use the plywood directly on the studs, then one or two layers of 5/8" drywall. That should be plenty. - Stuart -

I was just curious. I've used 5/8" in mine with 3/4" OSB next to the studs. Just getting the OSB finished up in the biggest room. It'll be next summer before I can get the sheetrock up. By the time it's caulked I'll be heading into my busy season, so I'm out of time this year. Thanks for the information, Scott

"polyethylene sheeting is NOT a vapor barrier" Works under concrete, seems like it would be the same effect in a wall...a vapor barrier and an air barrier...right? Not that we have many calls to use plastic on walls where I am, my home has it, but it was there when I got here. We all place polyethylene sheeting directly under the concrete pour as a barrier to water and in some places radon gas, isn't that what is being attempted?

xSpace wrote:

"polyethylene sheeting is NOT a vapor barrier" Works under concrete, seems like it would be the same effect in a wall...a vapor barrier and an air barrier...right? Not that we have many calls to use plastic on walls where I am, my home has it, but it was there when I got here. We all place polyethylene sheeting directly under the concrete pour as a barrier to water and in some places radon gas, isn't that what is being attempted?

There is 2 worlds of difference between a water barrier and a vapor barrier............ water molecules are huge compared to vapor molecules. In order for a product to be a vapor barrier it would have to have a perm rating (the permeability of the material to the passage of vapor) of zero...... poly has perm ratings of from .06 to .16, (.006" to .002" sheeting respectively). A vapor retarder had a perm rating of 1.0 or less. Therefor they are (by definition) vapor retarders not vapor barriers - they do not stop the passage of all vapor - they simply retard it. An "Air Barrier" is a totally different animal - Dupont Tyvek Homewrap is (for example) both a water and air barrier - but it has a perm rating of 48 and allows the free passage of vapor through the material. I have dealt in a lot of properties where this was an issue - and although poly sheeting may be part of the design that is used - it has to be coupled with venting the gas from beneath the building - it is not - cannot be - never will be, a radon barrier. It is defined (by the EPA) as being a radon retarder - not a barrier. This from the EPA publication "Model Standards and Techniques for Control of Radon in New Residential Buildings":

6.1 Basement and Slab-on-Grade Foundations. The model building standards and techniques for radon control in new residential buildings constructed on basement and slab-ongrade foundations include a layer of permeable sub-slab material, the sealing of joints, cracks, and other penetrations of slabs, floor assemblies, and foundation walls below or in contact with the ground surface, providing a soil-gas-retarder under floors and installing either an active or passive sub-slab depressurization system (SSD). Additional radon reduction techniques are prescribed to reduce radon entry caused by the heat induced "stack effect." These include the closing of air passages (also called thermal by-passes), providing adequate makeup air for combustion and exhaust devices, and installing energy conservation features that reduce non-required airflow out of the building superstructure.

You can read it yourself here: http://www.epa.gov/radon/pubs/newconst.html Sincerely, Rod

So I'm re-reading this thread and realizing that I'm still somewhat confused..... I'm doing a room within a room that starts (from the outside layer) with a typical beef-up job (i.e. 2 layers of 5/8 between the studs. I will then leave about 8 inches between this first, outer layer where I will then frame in a new room that has 3 layers of drywall (this inner room will have its own ceiling as well. Is it recommended to put a vapor barrier between these 2 rooms? If so, where? Thanks, amigos.

Brien is the "vapor barrier guy", but from what I've read in his comments, the vapor barrier always goes on the warmest side of the wall, never the coldest side. But that's all I remember! I hope he's around to help you with that! - Stuart -

Particularly in cold climate...the VB will be attached to the interior wall framing on the interior side of the room, then your mass will be installed over that.

Thanks for the response guys. Brien, if I understand you correctly (and since I'm in Seattle which would be considered a "cold" climate) my vapor barrier should be placed like this, correct? Another question (while I'm at it)..... If I choose to use pink insulation between my leaves, what's the best method to keep it in place? I plan on having 8" between my drywall leaves. Is there a certain R-value I'm shooting for knowing that distance? Something that would be more "stuffable" than something that's too thin? Thanks.

I plan on having 8" between my drywall leaves.

If you are planning on 8" between your leaves, then based on what you show in that sketch your frames are going to be maybe 2" apart. It shouldn't be too hard to get it it to stay upright like that. The insulation and studs on either side of the gap should keep it in place fine. - Stuart -

Also, I can't tell or not but is it recommended to put the Vapor barrier on the ceiling too? Mucho confused (as if you couldn't tell).

No barrier on the ceiling. As best I can tell, you guys up there build with a 2 X6 wall assembly, depending on several things, but still looks right. So if you have a 5-1/2 wall and the thermal insulation works in your environment, with your build you get 7 inches minimum...so that is a good thermal protector. Things like fire blocking will help to maintain a good thermal value, keeps the cold from moving out of the wall assembly and into the upper cavity that winds up being the upstairs floor. And is jut a good idea in general in this type of build (basement).