Air Barrier vs. Insulation

[David Meiland]

Re: Air Barrier vs. Insulation

Allan, "air barrier" is applied in a couple of different ways, and they aren't consistent.

In a wall, the insulation is usually enclosed and in contact with an air barrier on all six sides. It's fairly easy to detail walls so that the insulation is exposed to a minimum of wind-washing.

In a crawl space or attic, the insulation is almost always completely exposed to wind-washing on one side. The kraft face is typically against the drywall or the underside of the floor. It does seem to me like both situations could be improved by covering the insulation on the open side. As far as I can tell, this is not usually done because (a) it's hard work, and (b) no one cares that much. I have seen some crawls where Tyvek or another sheet material was installed under the joists, but there are usually a lot of obstructions. If you are building on slab you are avoiding this issue nicely. I know that my crawl is drafty be design, and when the wind's blowing through there I'm sure it degrades the insulation performance, probably by a lot.

[ThingOfBeauty]

Re: Air Barrier vs. Insulation

I heard this from someone who would know.
ENERGY STAR (and programs that require ES, like LEED for Homes or NGBS) require all vertical insulation to be installed in contact with an air barrier on six sides, because fibrous insulation (ie not spray foam insulation) loses a lot of its insulating value in a vertical install, unless enclosed on the back side, like Mr. Meiland mentioned.
Attic insulation loses some value, not being enclosed by an air barrier on the top, but there is less convection compared to vertical, and more insulation, and "for now [ENERGY STAR] is not addressing that because it would be a pretty big change".
You can read the articles about how blown fiberglass does permit convective loops to form, especially at very high delta T situations. Cellulose is more dense and less succeptible, but overall it's not a huge effect supposedly.

[Allan Edwards]

Re: Air Barrier vs. Insulation

Why not plywood the attic floor (2nd floor ceiling) and spray foam the underside of it.

[Bill Robinson]

Re: Air Barrier vs. Insulation

It is possible Allan however for the cost it makes more sense to design an air barrier at the drywall or plaster line.

Spray foam should be used at the rafters if at all.

[dgbldr]

Re: Air Barrier vs. Insulation

Why not plywood the attic floor (2nd floor ceiling) and spray foam the underside of it.

If you're building a vented attic, I absolutely WOULD do exactly that. In your price range, it makes perfect sense.

[davenorthup]

Re: Air Barrier vs. Insulation

If you're building a vented attic, I absolutely WOULD do exactly that. In your price range, it makes perfect sense.

My only problem with that is here we put a lot of insulation in the ceiling / attic. You could not simply ply over the attic floor b/c that would loose insulation space. Unless you are saying add the ply and then continue insulating over that?

[Martin Holladay]

Re: Air Barrier vs. Insulation

Allan,
Considering the amount of time you spend at GBA and here at JLC, I'm surprised you would even ask the question. Really, now, Allan -- why would you even consider building a ceiling without an air barrier? Once you understand the basics of air leakage, why would you ignore the issue?

Every part of your thermal envelope needs an air barrier. Skipping the air barrier makes no sense -- assuming you are a builder who wants to avoid callbacks.

Your question -- "what's more important, an air barrier or insulation?" -- is kind of like the question, "What's more important, a foundation or a roof?" It reminds me of the old game kids play in the 3rd grade: "What's worse, getting your foot cut off with an ax or being boiled alive in a big pot?"

[John B]

Re: Air Barrier vs. Insulation

Why not plywood the attic floor (2nd floor ceiling) and spray foam the underside of it.

Allan,
I think you are trying to combine your Insulation and Air Barrier all with the same product (Sprayfoam)
Why not Create an Air Barrier by other means as Bill R suggested
You could detail the ceiling with Airtight Drywall Approach.....or....

I will offer you another "suggestion"
Since You are already using ZIP system for your Wall sheathing.

Why not "ZIP system" the attic floor (above the ceiling Joists)?
Use ZIP tape to "connect" the wall sheathing to the attic floor deck.

You would also need a gasket to "connect" the wall sheathing to the bottom plate
and a gasket to connect the bottom plate to the Slab.

Assuming that you "connect" all the windows & doors to the Wall Sheathing....
You should have a pretty airtight "package"

You could then build a vented attic above "the deck" and pile on the not-so-expensive Cellulose.

Edit to say
I think plywood would a better choice for sheathing

[Ted S.]

Re: Air Barrier vs. Insulation

If you look at these weatherization programs, they have really done a good job understanding the air barrier, ventilation, and insulation.

The standards first require air sealing be performed which is blower door assisted. The CFM reduction is monitored but also they tap across the attic plane and measure the connection of the attic space to the house, they keep air sealing until the have made a good air barrier at the attic.

Once they are successful with the air barrier, they bring the attic ventilation up to code. This is also very well thought out with the calculations and such.

Once the attic ventilation is corrected and the air barrier has been established, only the do they increase the thermal insulation. All machines are checked for air pressure/calibration, etc.

It's quite impressive seeing the whole operation working as it does. It's a far cry from "blow and go" from years ago.

The best part is before you can do any of the work, you have to be trained. All these weatherization techs have been through the basic building science coursework.

Like I said, I am surprised at some of the questions on this thread, the basic necessity of an air barrier at the attic ceiling can't be over emphasized. As noted above, the lack of the air barrier could lead to considerable condensation issues within the attic.

The best part about doing down and dirty weatherization is that you can see how effective knowledge can be. You don't need $10k in CCSPF, just a little bit of training, some basic materials, canned foam, and blown cellulose and you can do some great work.

[Ted S.]

Re: Air Barrier vs. Insulation

I heard this from someone who would know.

I'm sorry I lead the thread on a tangent with this. If you look through the new Guidebook that I posted a link too you will see some stark differences between the older (and better IMO) Guidebook that Bill posted.

Every time I call the RESNET folks or my Provider I get conflicting information. We are becoming a Provider this year so hopefully that will take some irons out of the fire. But anyways, these resources are invaluable in my mind, and what is more important is that in some regard they lead code changes.

IECC 2012 is due out in 12 days and supposedly these insulation and air barrier checklists are modeled after the Energy Star Thermal Barrier Checklist (or possibly the newer Thermal Enclosure Checklist) and the Guidebooks go into great detail the items of the Checklists. So it's like looking into an hourglass of what is to come.

[Allan Edwards]

Re: Air Barrier vs. Insulation

John B and others:

What I am asking is what are the various methods and materials used to create an air barrier from a 2nd floor to a non-conditioned, vented attic. Something that is reasonably easy, affordable, and works on large houses with varying plate heights, groin ceilings, semi-vaulted ceings. Don't think of a 30' x 80' rectangular building, that's easy. Think of a 10,000 sq ft home with all kind of offsets, raises, bumps, vaults, 2 story ceilings, etc.

[Dancing Dan]

Re: Air Barrier vs. Insulation

Seems like you might want to figure out your ventilation strategy first - that may drive some of the other decisions.

[Allan Edwards]

Re: Air Barrier vs. Insulation

Seems like you might want to figure out your ventilation strategy first - that may drive some of the other decisions.

What would be your ventilation strategy.

[Dancing Dan]

Re: Air Barrier vs. Insulation

That would be telling. I dunno, Allan - if it's all cut up it might be easier to not ventilate, to go for a hot roof and treat all that space as condition. Are you thinking generically or do you have a specific house in mind? Post a roof framing plan if you have one.

[Ted S.]

Re: Air Barrier vs. Insulation

All the details are in those Guidebooks, Allan.

The knee walls, partition tops, penetrations, etc. are all the same details whether it a 20x40 single story ranch or 10,000 square foot home. You just have to find the right sub, Ontility should be able to refer you to someone. Knowledge is power with this stuff, or you can pay for SPF.

It the end of the day it comes down to 2 things:

Air leakage
R value

How you get there is your own choice, pay the $$$ or learn the lower cost techniques.

SPF is over-rated and knowledge is under-rated in my opinion.