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Where to put vapor barrier?

mikku's picture

I have worked on many homes and the rule for vapor barrier was "nearest to heated space"!  This is the rule for cold climate construction.  It is supposed to stop migration of moisture from the heated space into the insulated space.  Right?  If insulation becomes saturated with water, it looses its' efficiency.  Also, it will cause the framing members to rot!

My problem is:  I work in an area that is cold in the winters and very humid in the summer months.  The reverse occurs during summer months when there is very high humidity.  The vapor barrier is to the inside, the homes are air conditioned so a condition exists where the humidity within the houses is low but the humidity outside is very high.  The hot humid air penetrates the house wrap and migrates inward and condenses on the outer surface of the vapor barrier.  It is acting the same as a house with an improperly installed vapor barrier (many leaks)!  Now you have moisture in your wall cavity and it is short circuiting the insulation and causing rotting conditions or at the very least (mold conditions)! 

Dupont distributes a product in Japan that is used as the vapor barrier.  It is similar to Tyvek or other house wraps where it allows water vapor to pass but not water as a liquid.  They claim that this vapor barrier will allow a certain amount of water to pass to the conditioned space and the air conditioning will remove it by dehumidification.  In that way preventing moisture from accumulating within the walls.

In the United States, I have only worked in very cold climates.  For the past 10 years, I have worked in Japan and this problem keeps coming up.  Many companies have different solutions for the problem and get government certification for their processes.  How are vapor barriers used in Mississippi, Alabama, Florida, Georgia (ie the southern states) where the winters occasionally have temperatures near 0 degrees F.  but summer temperatures and humidities that are extremely high?  I build American Style homes in Japan and I want to give the best product to my customer.  

Does anyone know where is the proper placement for a vapor barrier in hot humid climates?


(post #175921, reply #1 of 10)

The short answer to your question is: in hot, humid climates, put the vapor barrier towards the outside surface (or as I was taught, the warm side of the wall, although that varies as you noted, from season to season). In a climate though, where summer cooling is of more concern than winter heating, the VB should be towards the outside--keeping the hot, humid air out of the wall cavity.

There's a website called (I think) that talks about such things in more detail. Whatever you do, do not put a vapor barrier on both sides of the wall (that would realy trap moisture inside the wall and no matter what you do, moisture will get into the wall). Some folks (like those who frequent the Breaktime forum sponsored by Taunton, as this one is) argue not to use a vapor barrier at all--just to use a water barrier like Tyvek. That way moisture can migrate to the dry side, whichever side that may be at any given time, and not stay in the wall.

(post #175921, reply #2 of 10)

Hey Danno,

That is what I am finding out at Building Sciences link.  I posed the same question at "breaktime" and was referred by BillHartman to investigate the site.  There is a lot of reading to get the entire picture and then you still have to make your own decisions.  At least I can draw from a pool of other builders experiences.

Thanks again


(post #175921, reply #3 of 10)

You are welcome. It does get confusing and there are no real clear black and white answers. I told someone who asked a similar question at Breaktime that I'd heard that cellulose insulation was good because it would absorb moisture and then release it to the drier side without damage to it and someone said they found that cellulose would matt down and get moldy and lose its insulating value, but fiberglass would not (and I had heard exactly the opposite!), so.... "You pays your money and you takes your chances" I guess. One thing I heard that makes lots of sense is that stuffing the cavities tightly will prevent convection currents which are bad and also prevents moist air from entering the cavities.

One more thing to throw into the mix are SIP's--Structural Insulated Panels--a sandwich of OSB filled with foam. The foam is like one big thick vapor barrier, so it should work in any climate (at least that's what I think--may be wrong and I make no claims or warranties!).  ;-)

(post #175921, reply #4 of 10)

I also live in a climate with hot humid summers as well as cold winters. I have a couple more questions and would appreciate any suggestions. I'm starting a remodel project on an old farmhouse which originally had wood siding and little if any insulation. Over the years, cellulose has been poured into most of the walls and vinyl siding has been added on top of the wood siding. I haven't yet found out if any insulation was added between the wood and vinyl but don't think so. Most of the interior plaster walls will be removed during the remodel at which time I will reinsulate probably with blown in cellulose or fiberglass.

My questions are: Should I use a polyethylene vapor barrier between the cellulose and the drywall? I'm leaning towards no.

What about vinyl siding, could it act like a vapor barrier on the exterior?

What about the old wood siding in the wall cavity, won't this act like a giant sponge during hot, humid weather preventing the wall cavity from drying?

(post #175921, reply #5 of 10)

Hard questions to answer. I see by your profile that you live in southern Illinois. That's sort of "on the cusp"--not just worries about hot, humid summers (like if you lived in Florida), but you also get cold winters. (BTW, I see your birthday is three days before mine, though you are about 20 years younger.)

Again, I would recommend the building sciences web site and the Breaktime forum, as I just don't know for sure any good answers. I'll give you my thoughts though, for what they're worth.

I think in southern Illinois I would be more concerned with the summer humidity--especially if you use air conditioning (actually, the HVAC experts call it "refrigeration"). I think vinyl siding has enough holes and air infiltration so it really doesn;t act as a vapor barrier. (But even if it did, with your humid summers and the spongy wood siding you described, that may not be a bad thing.) There is vinyl siding that is backed with Styrofoam, and that may be tight enough to where the Styrofoam would act as a VB, but I'm not positive. I tend to agree with you about not putting in a VB under the new drywall.

As for insulation, lots of guys on the Breaktime forum like dense-packed cellulose, but others still swear by fiberglass. (Some say fiberglass won't rot or pack down, others say it contributes to the studs rotting and doesn't stop convection currents in the joist bays.) If I could afford it, I would go for blown in foam, but it is expensive.

The wood siding may absorb moisture, but should release it again--and that's another reason for no VB inside. (Although releasing it to the inside would not be as good as to the outside. If you ever remove the vinyl, you might consider a VB and foam insulation under any new siding you put up--VB on the outside of the insulation.)

Hope this helps. It gets complicated!

(post #175921, reply #6 of 10)

Thanks for the advice.  I've been reading through parts of the Buiding Science website.  Lots of stuff to sort through and most of it regards new construction rather than remodel.  (Maybe I should have just built a new house after all!)

I'll leave off the interior poly vapor barrier as this seems to be the advice from BS website.  This will also allow me to add a VB on the exterior like you suggest if I ever take down the vinyl. 

I used dense pack fiberglass in an addition to current house and like it really well.  Insulated it during the winter and couldn't feel any air infiltration anywhere on the wall even around electric receptacles on windy days.  I'd like to try cellulose just cause I really hate the itchyness of glass.  What about foam?  You say it costs more, how much more?  Is it like "Great Stuf" which seems to break down over time?  I assume you spray it on. 

One thing I've learned is that there doesn't seem to be any one answer that's right.(except maybe 2 vapor barriers=bad)

(post #175921, reply #7 of 10)

I know what you mean about remodeling--every time I've opened up something in the old houses I've owned it's been a surprise (never a pleasant one)!

I think you meant to say the BT web site, but they do give some BS at times too! ;-)

I have not had experience with foam myself (other than Great stuff--didn't know that broke down). I'm pretty sure the stuff I've seen is not a do-it-yourself deal; professionals in moon suits and respirators spray it on. (It's not toxic, but the stuff I saw on "This Old House" expanded 12 times or so, so a drop in your lungs would not be good!) If you've had good luck with dense pack fiberglass I see no reason not to use it again. One thing I've found that helps quite a bit (besides long sleeved shirts, goggles and mask) is to rub skin with talcum powder before you work with fiberglass--the talc sort of plugs the pores of your skin and keeps the glass fibers out. Good luck.

(post #175921, reply #8 of 10)

BT...BS...whatever!  (Maybe that was a Freudian slip?) 

Thanks again for the help.

No direct answer ... some (post #175921, reply #9 of 10)

No direct answer ... some food for thought, though. Tyvek is NOT A VAPOR BARRIER. It is an AIR barrier as is anything with a similar design ... including the material you described. So don't call it a vapor barrier. Second, we don't call them a vapor barrier any more ... vapor retarder is the term commonly used ... although Joe Lstibrek (sp) says it is a 'vapor control layer', I believe.

You might consider in your situation placing the vapor retarder on the outside for the summer and then in the winter ensure you ventilate mechanically (i.e. fans) on a reguar basis to maintain a negative pressure. This may make the most sense since in the winter you are most concerned w/ air quality.

Could you describe your climate conditions a little more? You are in Japan, right? I'd think the winters are cool, but generally mild and don't really picture the summers as hot and humid (like Texas/Louisiana). If you don't get too much snow, then you really don't get too cold (I know this isn't literally true, but I'm just making a point, here). I suppose, though the climate may vary a lot in Japan w/ the mountains and all. I assume the influence of the warm ocean currents keeps the winters pretty mild.

I'm NOT an expert in vapor retarder applications in hot humid climates, so take what I say w/ a grain ... someone else may provide better food for thought.

Also ... you may have better response posting this under the 'energy' category. Most similar questions ar posted there.

There ain't NO free lunch. Not no how, not no where!

12/08/2005 (post #175921, reply #10 of 10)


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