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Old 10-13-2011, 11:25 AM   #1
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General Insulation Questions


Hello, I have an attached garage with an unfinished ceiling. The walls are insulated, vapor barrier'd (unknown r value), and sheet rocked. The garage door is an insulated one. The 1 shared wall with the house is also insulated vapor barrier'd and sheet rocked.

I don't plan on conditioning the garage, but it would be nice to keep it above freezing in the winter (Calgary, Alberta, Canada), so I am thinking about insulating, vapor barriering, and eventually sheetrocking the ceiling. The joists are comprised of 2x4's on 24" centers.

Questions:
When shopping for insulation, how do I know if it has kraft paper on it? I don't see that listed anywhere on the packages (typically looking at owens corning). And is it very important to have the paper if you are putting up a vapor barrior anyways?

If you are putting up multiple layers, the 2nd layer shouldn't have kraft paper on it, so can you just tear it off, or do you need to buy specific paperless kinds?

Since the rafters are 2x4's, do I have to buy a low r value insulation (about 3-4" thick), and then crosshatch higher r value insulation over that, or can you put 6"+ thick insulation between the rafters and be just as effective? (I would imagine this wouldn't cover as well, but does it baloon out over the top of the rafters?

This isn't as easy to answer, but any suggested r values for this would be appreciated.

Some insulations are listed as "Cathedral ceilings", "Attic", "Basement", etc... Does it matter which one you use, as it's all made of the same material (I think).

Thanks for any input!

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Old 10-13-2011, 02:20 PM   #2
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You know, it's been decades since I've seen the dark-paper-faced insulation you're referring to and up here in Canada am not too sure it is available anymore - unless you really look for it. That's hardly a scientific survey, just my impressions, but I also tend to think that our climate either takes a separate vapour barrier - or not - depending on the app.

On this bb, all people ever talk about is the paper-faced insulation (and then have questions about double vapour barrier layers) but that's because they're mostly from a lesser climate that what you have. Even a quick hunt through the Owens-Corning website from Canada didn't make any mention at all of the paper-faced variety...What you'll find in most big box stores is the straight, pink stuff that has no 'sides', top or bottom.

So, if you're putting up a plastic vapour barrier, you don't need it anyway. Of course, that goes for any layer you eventually put up. just lay the pink stuff perpendicular to the first layer. Aim for R55 or so...Start with a first layer of 3.5" deep to come up to the top of the 2x4's, then another lyer perpendicular to that one, That'll cover the 2x4's completely, cutting down any thermal bridging from the wood. You don't really want to puit, say 10" of pink right in between the 2x4s, as it'll be all floppy-wobbly and not do a good job.

The (taped) vb goes on the inside of the 2x4s, facing the garage floor, then drywall. Plan on some sort of ventilation, like a fan, in there for when it gets humid with the snow-melt. You only need that from time to time, but it is a necessary step to control (reduce) the amount of moisture now that you've gone and sealed it up. Helps reduce rusting too.

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Old 10-13-2011, 03:19 PM   #3
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Ah thanks. That clears up a lot and explains why I don't see any mention of paper on the insulation bags. The garage roof looks to currently have 1 vent (pasive vent only, no fan).
2 layers of insulation... not as nice to install but thanks for clearing that up. Bats alone with this method would be aprox $600+tax for r52

I called Home Depot about renting a blower to price out that method, as that's currently what's in the house attic. But I think I have to go in to talk to someone to get a clear estimate on the loose insulation costs.

It's all driving weather I should vapor+drywall before insulating.

One more question, I see articals all over about recessed lighting being IC or not. I have standard octaganol light gang boxes with standard bulbs (not recessed). They don't appear to have an IC rating, but that might be because they aren't recessed. Can you cover these light fixture boxes with bat or blow in insulation, or is that a fire hazard?

Last edited by Drizzt; 10-13-2011 at 03:35 PM.
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Old 10-13-2011, 05:14 PM   #4
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Not conditioned (i.e. heated or cooled) = no need for a real vapor barrier.
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Old 10-14-2011, 07:10 AM   #5
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No, you cannot cover any light fixture - such as the ones you have - with insulation. I see no real need to have recessed lighting in a garage, so I would just put the fixtures onto the drywall once that is installed; something like a fluorescent fixture, or just keep the ones you have and put a CFL bulb in it - but again over the drywall. That eliminates the need to build a box-like structure to surround the electrical box up in the roof portion.

Bear in mind that the installation of more insulation won't keep it appreciably warmer than without; yes, some - but since it won't be heated or cooled directly, then what you've done is really reduce the size of the area you're concerned with, and kept out the draft from the attic. This might appear to be 'warmer', so you might have made some progress there. But since you've already gone ahead and insulated + put up a vapour barrier, there is always the possibility of putting in a baseboard heater of some size later without the need to put up a vapour barrier before doing that.

But as I said, ventilation is the key here. You say you have ventilation in the roof portion, albeit passive, but you should make sure it works (like clearing the soffits) and that's fine - but I am also talking of ventilation in the garage part where the car sits. The added insulation + vb will increase the relative humidity in there and there's nothing worse than having a car sit in that for months of the year, so keep the air moving. Here again a baseboard heater might add some movement of the air...

I don't think - in retrospect - that you'll need to go quite as high in R-value as R55 or so... I mean that'd be nice - but you could get away with 10" total fibreglass whatever the R-value of that works out to; it is meant to be a cooler zone than the house in winter anyway, so I wouldn't go throwing a ton of money into it.
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Old 10-14-2011, 10:51 AM   #6
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Yes...you can cover the electrical boxes. The only time you cannot cover electrical items is recessed "can" lighting. That's because the heat from the bulb being recessed within the fixture creates heat that needs to be dissipated. In your case, the light bulb is within the garage space and dissipates the heat to the garage.

I also agree within Windows, that unless you plan to condition the space, you don't need a vapor barrier.

I wouldn't worry about ventilating the garage space, you'll take care of air exchange when you open that large thing they call "the garage door"...
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Old 10-14-2011, 11:47 AM   #7
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Ah-ha...well there you have it: two views on something, somewhat opposite, I guess. Take whichever one suits you best for your corner of the globe and good luck.
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Old 10-15-2011, 02:24 PM   #8
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I have to side with Cc. The vehicles park in the garage with snow melting off them, a hot engine and brakes/wheels, warmed interior for passengers, the water has to go somewhere. Either vapor barrier the ceiling with poly or add some vapor barrier paint (though not as effective) because you don't want the moisture wetting the fiberglass to degrade its R-value up to 70%: http://www.nrc-cnrc.gc.ca/eng/ibp/ir...ling-heat.html

Cc's post #5 pretty much covered it for your location. I agree on the money throwing part...... how much do you gain after so much; http://www.enersavesystems.com/pdf/E...Insulation.pdf

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Old 10-16-2011, 10:13 PM   #9
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good comments in here. so essentially if the space is conditioned it should be vapor barriered
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Old 10-17-2011, 06:45 AM   #10
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Well, yes - for where the OP is calling from and that's in a really cold climate...not for everyone so you can't really make a blanket statement like: "where space is conditioned, you need a vapour barrier". The OP does because drying of the air takes places towards the exterior and you don't want moisture in your walls.

Now in places where the drying of the air doesn't go to the outside, then not, or not where the OP puts it...so it depends.
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Old 10-17-2011, 02:25 PM   #11
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The real purpose of a vapor barrier is to keep water away from the "dew point". The dew point is where warm moist air meets cold air and the water condenses into liquid form. In the case of unconditioned spaces, the air inside is realitively similar to air outside, so the condensation factor is practically eliminated. In such a case, the mere presence of a painted surface will retard the small amount of vapor that is present in the "cold" garage. If the space is conditioned, then you change the whole dynamic of the wall assembly.
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Old 10-17-2011, 11:41 PM   #12
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Quote:
Originally Posted by ccarlisle View Post
Well, yes - for where the OP is calling from and that's in a really cold climate...not for everyone so you can't really make a blanket statement like: "where space is conditioned, you need a vapour barrier". The OP does because drying of the air takes places towards the exterior and you don't want moisture in your walls.

Now in places where the drying of the air doesn't go to the outside, then not, or not where the OP puts it...so it depends.
Quote:
Originally Posted by AGWhitehouse View Post
The real purpose of a vapor barrier is to keep water away from the "dew point". The dew point is where warm moist air meets cold air and the water condenses into liquid form. In the case of unconditioned spaces, the air inside is realitively similar to air outside, so the condensation factor is practically eliminated. In such a case, the mere presence of a painted surface will retard the small amount of vapor that is present in the "cold" garage. If the space is conditioned, then you change the whole dynamic of the wall assembly.
thanks for clarifying. this makes sense. so for a conditioned space vapor barrier is usually needed. will painted drywall be enough for a room in the mid atlantic region?

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Old 10-18-2011, 08:32 AM   #13
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A couple layers of paint on well sealed drywall is sufficient for this region. We don't really need one in this area.
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Old 10-18-2011, 08:39 AM   #14
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The newest Residential Codes say that - where you are, in DC, which is IMO in area #4 of the climate zone maps of the US - for any wall construction type, you need a Class III vapour retarder installed on the inside of the wall. A 'Class III' vapour retarder is a medium permeability material met by a number of products out there, one of which is a layer of latex paint...

So to condense a mountain of info into a sentence, there you have it; this says nothing about the other part of the insulation equation, notably air flow. It is more important where there is constant and pressurized air flow in constantly cold winter months, say in Alberta, than where you are, but the principles of tight wall construction are equally applicable. The point is you lose as much heat and cause the problems we see, through neglected air flow as you do by the lack of the amount of insulation, so it makes sense to air-seal joints and openings - as well as insulate the walls - wherever you are.
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Old 10-18-2011, 09:48 PM   #15
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thanks for the info. yea i know i am kind of asking for the short answer, which is tough to say because of all the various factors. instead of latex paint i can use a more glossy paint right? that should be even more of a retarder than latex i imagine

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