Still confused about vapor barrier

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  #1  
Old 11-13-14, 08:20 PM
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Still confused about vapor barrier

All,

Been browsing long enough, so if figured I should post.

I am renovating an old home, built in 1922. The attic presently has a bit of blown in insulation, but it is nasty and patchy, so I'm removing it. I'd like to replace with batts or rolls.

Here's where I'm confused. I can't tell if I should used faced or unfaced batts or rolls. I've heard faced, as there's no vapor barrier in place, but also heard unfaced because my location is hot and humid in the summer (Memphis).

The ceiling in my house is plaster and lathing, though the kitchen area will just be lathing and drywall.

So do I need faced or unfaced. I'd prefer unfaced rolls, as I can do the entire house for about half of what faced batts would be. $13 for unfaced rolls, $53 for faced batts. Need 33 rolls vs. 19 packages of batts. Rolls also seam like they'd be quicker.

However, I want to do this the right way. So, which way is that? Also, both are r30. Should I run unfaced rolls across the top, perpendicular, to make the attic r60? And what are your thoughts on that aluminum reflective foil you staple to the joists? Be nice to keep the attic cooler and drop my ac costs in summer.

Thanks!
 
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  #2  
Old 11-14-14, 12:05 AM
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Air sealing the ceiling/attic floor is important, especially with fiberglass insulation, related link below.
Fill the joist cavities and then go perpendicular with more insulation. More is better.
The thinking on vapor barriers has changed over time and currently they are requiring a VB only in the far north cold regions. They are also recognizing that even a good coat of paint will provide a degree of vapor resistance. They now refer to VB as vapor diffusion retarders. Following this thinking you should be fine with unfaced insulation.

As for the foil radiant barrier, with lots of insulation and some good air sealing its benefits are greatly diminished. Deep south and hot summers is where it works best and you are not quite there. If you have your ac equipment in the attic it might help. If no ducts or ac units I would stick with more insulation.

The one drawback is while that area is open, adding the RB to the bottom of the rafters would be easy. Going back after you add all of the insulation is always a mess.

Be sure to not block your soffit vents and use a baffle if possible to protect the ends of the insulation in each joist cavity.

Bud


http://www.efficiencyvermont.com/ste...ide_062507.pdf
 
  #3  
Old 11-14-14, 06:24 AM
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Radiant barriers need to face an open space, so if you have R60, you would clearly not be able to "staple it to the joists", it would need to be on top of the insulation. I'm not a big believer in the benefits of radiant barriers unless they have been designed into the roof system.

As stated in this source, a well insulated home doesn't need a radiant barrier.

Radiant Barriers: A Solution in Search of a Problem | GreenBuildingAdvisor.com
 
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Old 11-14-14, 09:32 AM
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XSleeper,

Sorry, I mistyped. I meant stapled to the rafters, not joists. So yes, they will be stapled to the rafters with the insulation below, between the joists.

Also, R60 is definitely the way to go, or is it overkill?

Bud, we're gonna have to disagree about Memphis not having hot, humid summers. In the dog days of it, temps are 90-100 degrees, with high humidity. My HVAC and ducting will be in the attic, and my attic is only 4' tall at the highest.

My attic has no baffles or even soffits. The only opening to the outside is at the front, a 2'x3' opening that has a grate over it to prevent animals from entering. Should I add something at the rear to open it up more? If so, what? The rear slopes flat, so I'm thinking a fan, or one of those spinning vents. Thoughts?
 
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Old 11-14-14, 10:24 AM
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While we are considering other thoughts, have you considered a "hot roof" application, an unvented attic with all of the insulation in the rafters. The one primary application for this approach is when the ac and related ducts are in the attic. Where you do not have soffits for low vents and only a 4' attic space passive venting would be poor. The turbine vent, aka Whirlybirds, are powered by the exhausted air which needs an equal amount of intake air. Powered fans tie you to a life of added electrical use and still need to be run in cold season to exhaust moisture.

I'll wait to see if this interests you, but lots of information available on hot roofs.

As for Memphis qualifying as hot, no argument especially with limited ventilation.

The primary drawback to powered ventilation is the low attic pressure which pulls conditioned air through the many leaks you cannot find as well as directly from the ac unit and ducts. The energy lost can make the powered vent a net loss.

Back to work while we still have a few hours of above freezing.

Bud
 
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Old 11-14-14, 10:49 AM
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Bud,

Roofing and insulation are definitely among my weaker areas, so I'll entertain all options. I had intended to go with rolls in between joists and possibly again perpendicular, but if there's something else I should consider, please let me know. I have no doubt I can do the work, but getting the right setup is not my forte
 
  #7  
Old 11-14-14, 11:19 AM
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Cost is a big factor and now that you remind me, a hot roof is significantly more expensive. The trade off is bringing the hvac equipment inside the conditioned space is good.

I'm installing a window at present so have to get back to it. Here is some related reading.
Bud

Creating a Conditioned Attic | GreenBuildingAdvisor.com

Fans in the Attic: Do They Help or Do They Hurt? | GreenBuildingAdvisor.com

Creating a Conditioned Attic | GreenBuildingAdvisor.com
 
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Old 11-14-14, 11:25 AM
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Thanks for the reading, Bud. I'll try to read up on it later today.

Of course, I do always have the option of putting the A/C in a closet in the house. There is no duct work or current unit in place, so I have an empty chalkboard. And I haven't pulled the trigger on a unit, so I can always flex and put one inside the home.

I was aiming for attic as my home has 1,100 sq ft, so space is definitely a premium
 
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