I have a question about vapor barriers. We are building a new house, with a full basement. The downstairs walls will be studded with 2×4’s, and insulated. We intend to install a 4 or 6 mil vapor barrier on the inside. The question is, should we install plastic on the inside of the block wall? THis would be on the outside of the stud wall.
I’ve heard and read so many different ideas about vapor barriers, so I thought I would see what the opinions here might be.
By the way. we are in mountains of North Carolina, so we seldom experience extreems of temperature and/or humidity.
Replies
It's not temperature but humidity that you need to worry about. Go to http://www.buildingscience.com. Skip the vapor barrier, duhimdify, and be dry.
Andy
Never try to teach a pig to sing. It wastes your time and annoys the pig.
I'm very aware that humidity is the problem, but differences in temperature is the cause of condensation.
In our state (NC), the code requires a vapor barrier on the interior of walls.
There is no good reason to install a vapor barrier where you are, and no correct place to put it. And there is a boatoad of reasons not to install a vapor barrier. I'd install it for the inspector if I had to, and tear it out when his tail lights dropped out of sight. They'll just rot your walls in a mixed climate.
Foam against the masonry walls, followed up with a stud wall inside the foam, no insulation between the studs, and for God's sake, no vapor barrier is how I'd go.Andy
Never try to teach a pig to sing. It wastes your time and annoys the pig.
Does that code specify above or below grade finished walls?
Here in MN they have to insulate the basements on new construction. All the builders throw the fiberglass against the block/concrete wall and cover with a poly vapor barrier. The vapor barrier is to protect the owners from the un-faced fiberglass batts, due to to cancer concerns. This is however the wrong approach for mold growth which can be just as harmful.
I just finished my own basement. Since my daughters room is down there, and she is allergic to Mold, you can be sure I researched it and did the right thing. NO VAPOR barrier. I used polystyrene. The brand name is Owens Corning. The product I used was Inso-Pink. They are glued right to the block and the framing 2' on center is recessed into the insulation. R~7.5 and no vaper barrier. I due however have above grade exposed walls. These have R~22 FG insulation and yes a vapor barrier (6 mil) between the stud and DW, but only on the above grade.
Did you check the back issues on FHB? In the last year their was an excellent article on insulating the Mold free basement.
Good luck.
Hi. I recently built a home in Nova Scotia. I insulated the outside of the foundation walls, thats right the outside, with 2" R-10 polystyrene (I think it was Owen Corning), prior to backfill. The slabs were insulated with the same product. No Vapour Barrier is required as the foam board acts as the vapour barrier. It is -8 dec C (17 deg F) right now and the basement is warm and dry with no other insulation. You can just put up your stud walls and dry wall away. Cost a little more upfront but you cannot beat the comfort. The reason for insulating outside is to prevent the moisture in the soil, that is always there, from coming in contact with the concrete at all. A vapour barrier inside the foundation walls only stops the moisture from impregnating the insulation but does trap the moisture between it and the concrete leaving the concrete wet all the time. If you are already backfilled do not use any vapour barrier on the foundation walls. Use a Vanee HRV or other good quality dehumidifier to dispose of the moisture. Hope this helps.
Losh
Using the system you described, with insulated, framed walls inside your concrete foundation, you should run the vapour barrier from top to bottom on the front side (interior), down around the bottom of the frame wall, and back up the backside of the frame wall (between the framed wall and the concrete) - only on the back side, leave about 10-15" at the top. On the interior side, seal the top, bottom, and seams with acoustical caulking before putting on the drywall. This will prevent most moisture getting into the wall in the first place, protecting your insulation (especially from contact with the concrete wall), and the 10" gap in the back will allow any trapped moisture to escape (you rarely if ever seal a wall completely).
Another option would be, instead of batt insulation, use rigid insulation glued directly to the concrete: 1.5-2" would give you an ~R10-R12, which is plenty for a basement. In this system, the rigid insulation becomes the vapour barrier and the walls are left empty. You can then frame using 2x2 lumber and save some costs there. You need to use drywall for this system becasue you always need a fire barrier when using rigid insulation.
With either system, make sure you insulate and seal your headers as well - air leakage in the area where the house meets the foundation can be one of the biggest heat losses in a home.
oomingmuk <!----><!---->
oomingmuk, based on our situation, your first solution sure sounds like the most logical. In this area, we have cold winters (low humidity), but hot summers (high humidity). Sort of the worst of both worlds.
I particularly like the idea of the open vapor barrier on the back side. That makes a lot of sense, especially since that area will be above ground, and any trapped moisture can perk out the outside of the foundation.
Do you have experience with this type of installation? If so, has it been all good? Anything that I need to look out for, other than what you mentioned? Thanks again!
I do have experience with it, and other than it's a bit finicky getting the poly up on the backside of the wall - and of course that acoustical caulking is super messy - I've had no problems with it. That said, it's best used in a relatively dry basement. i suppose if you are building a new house, you won't know one way or the other. Rigid insulation (or spray foam) on the walls is a superior system in some ways - especially where moisture is a problem. Here's a link that explains both systems I mentioned a bit more... note the plastic on the back side is just to protect your studs and insulation from contact with the concrete - you can use building paper to do the same thing ... the front side is your vapour barrier. The better you seal the front, the better the system works.
http://oee.nrcan.gc.ca/keep_heat_in/chapter_5/chapter_5_2.cfm?PrintView=N&Text=N
ooming:
Several questions:
1. In the framing example, if you run the plastic up thee wall (between the concrete and studs) why would you also run it up the front side (directly under teh dry wall)?
Are you protecting the wall from moisture from teh house and moisture coming through the concrete wall? I would think the biggest problem would be from moisture coming through the concrete and your suggestion does not alow the wall to dry to teh inside. ????
2. Why can you get away with 2 x 2 in the framing if you us the ridgid insulation route?
ooming:
Several questions:
1. In the framing example, if you run the plastic up thee wall (between the concrete and studs) why would you also run it up the front side (directly under teh dry wall)?
Are you protecting the wall from moisture from teh house and moisture coming through the concrete wall? I would think the biggest problem would be from moisture coming through the concrete and your suggestion does not alow the wall to dry to teh inside. ????
The vapour barrier is situated in relation to the insulation, and always to the warm side of the wall. Warm air has more moisture in it than cold air. The poly on the back side of the wall only serves to prevent contact between the concrete and the insulation/studs. Moisture coming through your foundation wall is a different bag of worms and needs to be addressed before you install any wall system.
2. Why can you get away with 2 x 2 in the framing if you us the ridgid insulation route?
The rigid insulation is fastened directly to the concrete wall, and the 2x2 wall is framed in front of this. The framed wall is not structural - as long as you are not planning to install bookshelves or the like, 2x2 framing is plenty - you could even do 24" centres. In fact, you could get away with 1x2 strapping every 4 ft if you didn't want to hang shelving or many pictures - you really only need it to provide a nailing surface for the drywall. Some rigid insualtion panels are shaped for this exact purpose. Which system you choose all depends on the homeowners end use.
ooming:
Great.
You posted: . Moisture coming through your foundation wall is a different bag of worms and needs to be addressed before you install any wall system.
I would say that by definition a basement concrete wall is always damp to one degree or another. So don't you need to address this somehow in all cases?
Are you not addressing this to some degree when you run your plastic up both sides of the wall as you described it?
I would say that by definition a basement concrete wall is always damp to one degree or another. So don't you need to address this somehow in all cases?
Are you not addressing this to some degree when you run your plastic up both sides of the wall as you described it?
Not by definition, and not always - unless you are considering completely dry as a one degree of dampness. But in general maybe - either way, addressing moisture in the basement is the first step. The builder is the judge as to what degree the moisture will be a problem, and which steps to take to address it. Using poly or building paper to back the framed wall is suitable in relatively dry basements - wood should never come into direct contact with concrete because the dry wood will absorb moisture and rot over time. If the wall is actually damp or moisture is visible, then I might consider using a different system (i.e. one that is not as susceptible to moisture damage).
And yes - that is exactly what this wall system is addressing, but it is not a continuous vapour barrier on the back (nor is it effective as a vapour barrier) because you leave 10+ inches at the top exposed. Vapour is gaseous water and carried in the air - it only becomes moisture when it condenses - as it will when warm air comes into contact with a cold surface (like on your windows). The vapour barrier is trying to stop the flow of air into your wall - which is why the better you seal it, the better it works.
Thanks again.
I guess, what I meant by "by definition" is as you defined it, "some dampness".
A concrete wall is poreice (tiny wholes) (spelling?) and dirt is damp, so teh dampness of the dirt by definition moves through the concrete. I would think that trapping any of this moisture behind a finished basement wall would be bad. I suspect this is why several in this thread recommend STRONGLY against any Vapor Barrier on basement walls.
I am guessing that you believe that this minimal dampness as I ahve defined it is not a problem when trapped inside the system you suggested.
Dude, didn't you read Andy's post?
"It's not temperature but humidity that you need to worry about. Go to http://www.buildingscience.com. Skip the vapor barrier, duhimdify, and be dry."
NO vapour barrier...
The men may be the head of the house, but the women are the neck and they can turn the head anyway they want.
OK, people, for the 957th time: ONLY ONE VAPOR BARRIER MAXIMUM IN THE WALL. For a basement, that vapor barrier should be the waterproof membrane or coating on the OUTSIDE of the concrete wall.
It's not rocket science: no VB is perfect. If you have more than one barrier in the thickness of the wall, you will trap moisture in the middle. Ever look at a double-pane window that no longer has a perfect seal?
DG/Builder
Here's my exact situation, as it stands right now:
The basement has about 6 feet of backfill in the front of the house, and none in the back. The back is at grade level (steep lot). The grade on each side is slanted. This leaves a little over half the foundation above grade and half below. All backfilling is complete.
The foundation is fully waterproofed with a layer of bituminous sheeting. Outside thate, we have 1/2 inch blueboard, sealed with glue. We then have a solid waterproof drainboard (can't remember the manufacturer). Of course, we have drain tile around the footings, draining to outside. I doubt if there will be any water infiltration.
Inside, we have 2x4 stud walls, 1/2 inch from the block wall. These stud walls are already installed. They are to be filled with batt insulation. Our code requires an interior vapor barrier on all interior walls.
The house is in the mountains, and will be closed up for periods of time, sometimes several months. This area seldom needs air conditioning, however the humidity can get a little high. Winters are a little cool, but nothing like up north. Lowest temperaturs usually run in high teens, and that's only a few days each year.
So, that's where we are right now. Given this situation, what would be your recommendations. We can't install blueboard against the block wall... too late. And we can't insulate the outside of the foundation.
If ALL of the foundation (both below grade and exposed) is covered with the waterproofing materials described, no further VB is necessary or advisable. You are good to go.
Code normally requires VB on the inside of ABOVE GROUND structures. That is the normal Kraft-backed fiberglass between studs with the Kraft paper towerd the inside. Even if your code neglects to recognize the difference between above and below-ground, you should be able to reason with an intelligent and knowledgeable inspector and point out why you should not have 2 VBs in the same wall. If you can't reason with him/her, I can't make a recommendation... :)
DG/Builder