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Old 10-26-2009, 02:25 PM   #1
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Vapor Barrier Ceiling


Hello All,

I am just about to insulate the attic of my new garage. Looking for tips to installing the vapor barrier on the underside joist face before sheeting the ceiling.

It is a 10' ceiling so it may be a bit of a nightmare.

Any ideas would be a big help.

PS Do the drywall lifts from HD reach up to 10' ceilings? I am installing OSB which should be comparable in terms of weight to drywall (at a small fraction of the price!) Thought a lift might make that part a lot easier.

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Old 10-26-2009, 03:26 PM   #2
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Vapor Barrier Ceiling


For a vapor retarder, you can use 6 mil poly sheeting. Then use unfaced fiberglass or blown in insulation. Most panel lifts will go to 11 feet. I'm not sure why OSB should be a fraction of the price of drywall. Sure isn't around here.

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Old 10-26-2009, 03:28 PM   #3
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Vapor Barrier Ceiling


I am going to use 6 mil poly but the question is more in terms of the methodology for installing poly upside down, 10' in the air without it looking terrible and full of wrinkles.

For insulation we are using 2x6 batt made for 24" spacing but that should be pretty easy since we will have something to stand on. I was planning to fire all the insulation up in the attic before we do the vapor barrier so we don't have to struggle to get them up there.
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Old 10-26-2009, 04:24 PM   #4
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Vapor Barrier Ceiling


As far as the building code goes the inspector doesn't really seem to care what I do inside the building because it is a detached garage! But I just figure if you are going to insulate you create a temperature gradient => moisture => vapor barrier.
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Old 10-26-2009, 04:41 PM   #5
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Vapor Barrier Ceiling


The insulation goes on the warm side of the insulation, so in your case it would be between the ceiling board and the insulation, so the the insulation cannot be spread until the vapor barrier and ceiling board are in place. - Vapor barrier and board before insulation. That is why many people use blown cellulose which has a better insulation properties. Wrinkles in the poly are not a problem unless they cause any tears in it.

Check with your code if this is an attached garage since many places want a fire-resistant board instead of wood.

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Old 10-26-2009, 05:48 PM   #6
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Vapor Barrier Ceiling


I figured some of you guys would have a good tip for how to install the vapour barrier. I know where it goes, I know what it does, I know what kind to use.

I'm just not sure how to tack it up nice and flat.

Does anyone use the vapour barrier sealants like this stuff?

http://www.lepageproducts.com/produc...d=189&plid=260
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Old 10-26-2009, 07:25 PM   #7
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Vapor Barrier Ceiling


Staple an edge up, pull it tight from the center, staple, continue to other edge.

With your research you know that 4 mill. poly has a perm. rating of .08, 6 mill. - .03, both Class 1 vapor barriers. Did you know OSB, 3/8" has a perm. of .75, a Class 2? So Your moisture in the space when heated will rise and slowly go through the OSB and stop at the poly continuing the keep the OSB wet to mold and rot unless forced ventilation dries it?

This is interesting about OSB and plywood: " The inner and outer layers comprising the OSB panels do not work well together when moving excess moisture to the exterior surface, resulting in a product that is more susceptible than plywood to the deleterious effects of cyclic moisture exposure." --- from: http://www.rci-online.org/interface/...ton-murphy.pdf
http://www.panhandleinsulation.com/b...materials.html

If used as vehicle parking, all the water on the hot car will rise to be deposited on/in the OSB.
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Old 10-27-2009, 05:25 AM   #8
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Vapor Barrier Ceiling


Regardless of the perm rating of the OSB, there would need to be a temperature imbalance to cause condensation on the inside of the poly in order for the OSB to get wet. If this were the case, every drywall ceiling installed over a vapor retarder would be wet and moldy, since drywall has an even lower perm rating than OSB. The surface of the poly would need to drop below dew point, for moisture to condense. The insulation will prevent that from occuring.
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Old 10-27-2009, 06:43 AM   #9
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Vapor Barrier Ceiling


Yeah, the vapor barrier is meant to STOP the movement of moisture to keep the insul dry in the winter and retain it's insulating properties. And because of the insulation the temperature right at the vapour barrier should not have a gradient, right? The vapour barrier should be exactly the same temp as the board as the bottom of the insul.

I assume the opposite would happen in the summer in an AC space.
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Old 10-27-2009, 11:02 AM   #10
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I stand corrected. But wouldn't the hot air rise and deposit the water on the OSB from a warm vehicle? Wouldn't a vapor retarder paint on the OSB surface be the best place for it? It would be similar to a bathroom without a fan, the hot moist air rises and collects on the surface.
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Old 10-27-2009, 11:09 AM   #11
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Vapor Barrier Ceiling


It is true that there would be quite a bit of moisture when you park in there. But I think that would be pretty initial, definitely not as mush as with a shower/no fan scenario for sure. Just picture it in your mind if you park a car in a garage you don't see water vapour like in a poorly ventilated bathroom, for sure slightly damper air but not visible water vapour. Plus this is a 31x24' garage, prolly only ever to have the wife's station wagon in it, the truck will live outside.

I was thinking that I would paint it eventually, maybe an exterior opaque stain? That must have water repellent qualities, right?

Also I don't plan on keeping the garage heated except for when I am in there working. So for the most part the inside of the garage will be as cold as outside. I am only insulating it for the times when I am working in there and it's colder than a witches you know what outside and I have a heater going.
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Old 10-27-2009, 12:46 PM   #12
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Vapor Barrier Ceiling


True, the warm air will rise, but as long as the indoor air temperature remains above dew point, no moisture will condense. The relative humidity level in the ASB will match the indoor RH level. If you leave an area of poly uninsulated, and cold air can impinge on it, then you may have a condensation issue, because the plastic will reach dew point. The idea is that warm air absorbs and will hold greater amounts of moisture than cold air will. Keeping the warm moist air contained inside means that it will not penetrate into the insulation and condense someplace within the layer where dew point is reached. Also the moisture that has absorbed heat energy will not travel away from the conditioned space to lose it's energy someplace outside of the living space. Where there is no temperature differential, insulation does not function. Vapor retarders do not function either in that case. For example, if you taped a piece of plastic sheet over the open window of a warm house in winter, it would "sweat" on the inside, because the plastic surface was below dew point. If you turned off the heat, and allowed the house to reach outdoor temperatures, the plastic would not "sweat" because there would be no differential to cause condensation. The absolute ideal place for a vapor retarder is as close to the inside warm air as possible. A low permeability coating applied to the underside of a ceiling is the most effective vapor retarder you could get. It's a shame that we get hung up on vapor retarders, when a good low perm coating applied to the inside building surfaces would be way cheaper and easier to install.
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Old 10-27-2009, 01:14 PM   #13
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Vapor Barrier Ceiling


I guess the issue with an external vapour stop is that as the paint or whatever wears and loses it's vapour stop property you could get moisture into the insulation. Then you have a problem. Vapour barrier under a durable product like drywall or in my case the sheeting it is protected and can live a long happy life stopping air & moisture from moving through the wall or ceiling or whatever assembly.
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Old 10-27-2009, 05:47 PM   #14
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When you install a vapor barrier in any building, that is only one component of the system. The moisture will build like it does in a bathroom or kitchen area. An air exchanger is the other component. To have one, necessitates having the other. In older houses there were neither. They were drafty and leaky and this helped move the moisture before it built up. You're not going to get condensation if the dew point isn't reached but it may be a bit moist and if there are windows they are going to drip a lot, especially if you are going to keep the temp at 68 degrees during the winter and it is below freezing outside.

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