Last year we had cellulose insulation blown into our 1920s house. Now I am working on the bathroom on the 2nd floor and have taken down lots of original drywall for various reasons in different spots. The insulation in the exterior wall stays nicely packed into its stud bays without falling out! I started reinstalling greenboard on this exterior wall yesterday but then heard about something called a “vapor barrier.” If there was no vapor barrier before, do I have to rip out what I already did and install this underneath? I have heard it is necessary, but I’ve also heard no, that moisture gets trapped between the plastic and the greenboard and then the greenboard rots…
Other details if necessary: The greenboard will be primed/sealed with Kilz Premium and will have beadboard on the lower half. I’m also installing an exhaust fan. The wall is only about 8’x 5′. Behind the insulation there is wood (original clapboard maybe?) and we’ve got aluminum siding.
Thanks, everyone!
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What are you located?
Every house needs someway of controllying moisture.
But there are many different things that can controll moisture. And depending on the climate the details of the whole installation some of the things called vapor barriers might make things worse.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
It is generally wise to have a VB on the interior face in heating climates, esp in wet locations like a bathroom. Even more so if this is on a north wall.
BUt if you have already taped, it is a 50/50 call whether worth tearing back off again.
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We're in central NJ, and it's an east-facing wall.
Now I'm also hearing that I need one behind the ceiling drywall because there is an attic up above....???Some say it's a good idea if I rip out what I did in the exterior wall and put in a vapor barrier, but what are the consequences of not doing it? Why it was okay that there was never one there before? Insulation is blown in all the time to walls without vapor barriers. What about every other exterior wall in the house that does not have one?Thanks, everyone.
If moisture gets into the wall cavity, it can either migrate out thru the exterior wall, or dry again to the interior, or get locked in place and cause mold, all depending on many of the variables.By adding a VB you reduce the number of variables by not allowing the moisture to gain access to the wall cavities. By not adding it, you take a gamble.The reason I said 'especially' with regard to bathrooms, is that a bath is a wet and warm environment. Warmth drives that moisture out so there is a higher odds of driving it into the wall.So you are left wondering if it will migrate to the exterior for drying the wall... Often, in older homes, things are loose enough that iot will.But I have often found these older places where there was suddenly a preponderance of moisture related problems beginning at the moment in their history when cellulose was added. They had been built with no insulation and methods that worked fine when the walls could 'breathe', but then when moisture transmition slowed from choking that convection current, the trapped moisture encouraged mold and rot, blistering paint, mildew, wood movement, etc.so - You have 50/50 odds you will be OK without the VB, but if it won't kill you to add it, it is better to have it and keep that water out of the wall in the first place.
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This is a complex subject. Here is a link to a document produced by Building Science Corp. that is a starting point for learning. Look around the the rest of the web site as well. It's a wealth of information.http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-designSteve
Generally, you don't put vb on the ceilings. This lets the moisture migrate into the attic where the venting takes it away. If you put in a vb (or kilz), you will have to have a way to get the moisture out of the room or it will be damp from body moisture.You get out of life what you put into it......minus taxes.
Marv
Generally, you put a bath fan in a bathroom. And you DO, in a mostly heating climate, put VB on the ceilings where they adjoin the attic if the attic is unconditioned vented space. Attic venting is there for the holes in the vapour barrier you don't know about, not to eliminate the necessity for the VB in the first place.
Old homes were so leaky that they dried out regardless. But they did so at the cost of LOTS of heating fuel.
mmoogie, that document was really helpful. According to my particular wall construction profile (figures 2a and 3a in the booklet), a vapor barrier is not necessary in mild climates like Seattle or even moderately cold places like Chicago. A significantly colder place like my old stomping ground of Minneapolis, though, would require a vapor barrier. If I'm reading the booklet correctly, following this logic, it seems that a vb would also not be necessary in a place like NJ for my particular wall construction.
That's what I said, not necessary, but definitely better.
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View ImageSherwin Williams Prep Rite SF-1
Seals smoke stains, knots and tannins
Seals out odors
Provides an interior vapor barrier (perm rating <1)
Ideal for drywall, wood and wood paneling, block, masonry, stucco, cured plaster, ceiling tiles and previously painted surfaces
Now, what are you doing for the beaded board wainscot? That might be the harder construction to 'get right' -
Jeff
Wow, this looks great! Thanks to everyone for their time and suggestions.What do you mean what am I doing about the beadboard? What type of product am I using, or what am I doing for moisture?I want to minimize the spaces available for expansion and contraction, so I thought I would use the half-sheets of pine beadboard sold at Lowe's. They're sort of like a plywood construction and a little thicker than the other products they sell (11/32) so less vulnerable to warping, and they already have built into them a rabbeted edge so that you can do sort of a lapped joint type of thing. I haven't seen that anywhere else in the sheet form. I plan on priming all sides and edges.... HD sells a vinyl tongue and groove plank, which might be nice for a bathroom, but they are really pricey compared to all the other materials (pine t&g planks, pine sheets, MDF planks, etc.)What else do you want to know?
That sounds reasonable ... I've just seen lots of beadboard in bathrooms look awful after a year - people don't prime t & g edges, backprime, etc.
Jeff
?????Maybe you don't, but in my thinking, there is no place more important. The roof venting ios for what accidentally ends up there, but you want to keep as much out as possible to begin with. Letting it get in, means more condensation in some climatic conditions.The fart fan is there for removing the moisture.
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What about moisture generated by a family in the house as a whole. How can it get outside... thru the fart fan? It should me allowed to migrate thru the roof (in my opinion).
I use a travel trailer to do work for Habitat for Humanity. In cool weather, in the morning, you can see condensation on the windows from two people breathing thru the night. This moisture accumulates and must be allowed to escape.You get out of life what you put into it......minus taxes.
Marv
>>How can it get outside... <<Build tight, ventilate right. You want to get the moisture out where it will do the least damage. Windows, vent fans, doors, HRV systems. You don't want it migrating willie-nillie through the framing, condensing god-knows-where, rotting things out, creating mold and mildew, etc. As Andy says, air-sealing is the key.Steve
I guess I like a house that "breathes" a little bit. You guys don't.You get out of life what you put into it......minus taxes.
Marv
How much is a "breath a little".How do you control how much?What about the heat that often goes with the "breathing"..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Mmoogie is right, "Build tight, ventilate right." Letting a house "breathe a little" means you're losing heat too, and you have no control over how much it breathes.
When the air outside is coldest, your house will be "breathing" the most. When the outside air is close to the same temperature as inside, it won't be "breathing" much at all. You should have just the opposite, if you care at all about fuel costs or moisture control.
That was fine in another century, but wasting all that heat energy is unconscionable now.
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Agreed! And that's why an ERV makes so much sense.
Jeff
By "breath" I meant to allow moisture to migrate thru roof to attic and moisture to migrate in and out of basement walls. A house can be built tight and still let moisture migrate thru the drywall.
Uh oh! another hot topic!
You get out of life what you put into it......minus taxes.
Marv
Edited 9/17/2008 9:12 am by Marv
Is your brother named frenchy by any chance?
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You are right that it needs to vent. That is what windows and ventilation systems are for. Forcing the roof vents to handle what was only designed for incidental vapor is dead wrong.
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Check the Kilz can. It may be a vb. Many paints are.
More important than the vb is air sealing. Air leaks generally have the potential to move a lot more moisture than vapor transmission through materials. You should have no major penetrations leading from the room into the walls or ceiling. If you do, seal them with caulk, expanding foam, gaskets, etc.
Central Jersey is a mixed climate, and it's difficult to say for sure that you need a vb, and if you do, where it should go. VBs go on the warm side. In the winter, you'd want it on the inside. In the summer, if you have AC running, you'd want it to be on the outside. Until someone invents the migrating vb, there's no simple answer.
All houses are compromises.
Andy
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