Airtightness Sans Sprayfoam

First, I'd like to get rid of Dr. Joe's ACL terminology - both because that abbreviation is already taken and because it seems to suggest a single layer, like a membrane. I prefer the language of the Air Barrier Association of America (ABAA), which differentiates between air barrier:

 materials 0.02 l/sec-m² @75 Pa (0.004 cfm/sf @ 1.57 psf) – 25 mph equivalent
 assemblies 0.20 l/sec-m² @75 Pa (0.04 cfm/sf @ 1.57 psf) – 25 mph equivalent
 enclosures 2.00 l/sec-m² @75 Pa (0.4 cfm/sf @ 1.57 psf)– 25 mph equivalent

This is a much more complete picture of the complexity of creating a continuous air barrier in a building, with different standards for each level. And it allows the possibility that the air barrier may be exterior on one plane and interior on another, as long as they are connected.

Given that, I also think that an air barrier, much like a vapor barrier/retarder, is best placed on the warm, humid side because the goal is not simply to prevent air and moisture movement through the envelope but also to prevent movement into and within the envelope.

I want to get back to Robert's comment about "Goo as you Go"
because I still see a weakness in the ADA instructions and Ilustrations.
http://www.buildingscience.com/documents/information-sheets/4-air-barrie...
If I were to give the subcontactors the above document......
I see a weakness (a path) in the areas hilighted in blue.

Robert,
Is there anything wrong with the term Air Control Layer(s)?

John, I may be entering the world of the heretic for saying so, but I think you are right that the BSC's drawings in this information sheet are... misleading.
Maybe you should make you own instructional drawings using the "Goo as you go" approach.

Why not "Goo as you go" behind this partion wall?

Why not "Goo as you go" under the bottom plate of that exterior wall - or go with the gasket?

...and because it seems to suggest a single layer, like a membrane.

This is one reason I like the term "ACL". Not because it suggests "membrane", but because it suggests continuity... something that can be traced without lifting your pencil off the paper.

Martin,
I think Lucas can "see" it

Martin, I think what John is trying to show is that these illustrations suggest that the "goo" in certain areas be applied after framing.
If this is the case, then you cannot continuously seal the bottom of that rim joist to it's plate where the joists are connected - unless you trace each entire joist connection with "goo".

Martin,
Your article includes this instruction

When installing the rim joist, don’t forget to install a gasket or caulk between the bottom of the rim joist and the mudsill.

translation "Goo as you Go"
I do not see the same instruction on the BSC page
maybe I just missed it?

John, et al:

I thought I already explained the confusion. The original ADA is a goo-as-you-go approach, which was later modified into a caulk after installation approach for those who were too lazy to do it right and wanted an easier but minimally OK alternative. The BSC drawings and instructions seem to be a hybrid of the two. I think that makes it even harder and more problematic. (as noted, it requires some head scratching).

It's much easier to maintain continuity if you GOO AS YOU GO (or gasket as you go).

And I continue to think that ACL is misleading because an air barrier is neither a layer nor layers but an assembly of multiple materials and components that, properly combined, connected and integrated, form a continuous barrier to the movement of air.

We are fortunate to have the expertise and standard-setting of the Air Barrier Association of America, and even Dr. Joe uses their terminology (such as in his latest screed "Don't be Dense"), including air barrier material, air barrier assembly, and air barrier enclosure - each of which have different air leakage requirements.

If Dr. Joe has let go of his idiosyncratic term, it's best we all do.

Here is my question For North Texas...
Mixed/Hot Humid Climate...
Where is best place for the Air Control Layer?
A. Drywall
B. Exterior Sheathing
C. Drywall and Ext.Sheathing

And Why?

John, when I answered "warm-side" I was being cold-centric.
Maybe the correct answer for mixed/humid is C.
What exterior sheathing can you use that will dry to the outside?
Robert's emphasis on "moisture buffering" within the wall assembly seems especially valuable in walls that dry in two directions.

For North Texas...Mixed/Hot Humid Climate...Where is best place for the Air Control Layer?

On the Gulf Coast. Stop the moist air at its source ;-)

Even in cold country, I use a belt-and-suspenders approach. I believe the entire building envelope should be considered the air barrier assembly (a much better term, as it gets us out of single layer thinking).

I consider my air barrier to include the siding and trim, the doors and windows, the housewrap, the insulation and the drywall. With at least four "layers" to help stop air movement, there's a much better chance of success.

John, when I answered "warm-side" I was being cold-centric.
Maybe the correct answer for mixed/humid is C.
What exterior sheathing can you use that will dry to the outside?
Robert's emphasis on "moisture buffering" within the wall assembly seems especially valuable in walls that dry in two directions.

Answered by Lucas Durand
Posted Sun, 12/26/2010 - 15:45

Lucas,
I am thinking Air Tightness at the Drywall AND the Sheathing
In North Texas it is almost standard procedure to fully sheath with OSB
I would suggest changing that to Plywood and perhaps Liquid Applied coating at the joints.

It should be possible to achieve Good Airtighness (between 1 and 2 ACH50)
If we would only make the EFFORT to apply an Enhanced Airtight Drywall Approach.
I think it could be done by trained friends, family and volunteers.

see photo of trained Habitat for Humanity volunteers installing siding in Dallas

Getting back to Robert's suggestion at comment #12
And also connecting the ceiling drywall to the exterior sheathing thru the top plate.

link to higher res PDF
http://dl.dropbox.com/u/16030874/pd.pdf

If you focus on air tightness with sheathing and insulation in the wall, sealing penetrations, etc, do you really need to worry about your drywall?

John, what are the "mystery materials"?
Looks like you could get better than "good" air-tightness with this system.
How would you detail partition intersections?

Allan,
The primary Intention of my detail is to "connect" the "ceiling drywall" to the "wall plywood".
With just a little more effort (and gaskets) a secondary air barrier could be created at the "wall drywall" too. (which should be ideal for a mixed climate)

Lucas,
the black stuff is mineral wool
the blue stuff is XPS

Robert's early comments suggested a partition "connection" using a 1x6 to tie the drywall together between rooms.

The reason for the XPS in the detail is because I hope to suggest some new details to the local Habitat for Humanity Chapter.
They are currently getting the DOW XPS for FREE

attached is a photo of the current H for H sprayfoam strategy.
Believe it or not....
The drywall goes on without any additional "gooey stuff"!!!

FREE is definately affordable.
I was just at the local HFH "re-store" looking for reclaimed rigid foam and copper pipe. This chapter doesn't get that kind of stuff.
I'm surprised they use sprayfoam - the installation seems a bit specialized for a volunteer organization.
Your detail seems a lot more "do-able" by volunteers - not to mention "tighter".
Especially if the drywall is "airtight" along with the sheathing.

John,
I think you are right that an "enhanced" ADA in the fashion of "goo-as-you-go" would make for a "good", simple and AFFORDABLE air barrier.
In the HFH context, the less obvious pieces (ie: 1x6 partition ties) that need to be gooed before installation could be marked with an red "X" so that the voulunteers would know that some "gooing" is required before fastening.
"Goo-as-you-go" reference material could be stapled to the plans.

John

I think it is pretty common these days to foam or caulk cracks and penetrations visible on the interior. This, coupled with creating a tight sheathing assembly and either cellulose or foam insulation in the walls, creates fairly air tight buildings. Or not? I guess my question is what are the tangible benefits of trying to create air tightness with drywall, given the cost and effort. Is it worth it?

Lucas,
The foam is by Demilec...
Not sure ..but I think they are paying retail price(no discount) for the foam

You would think that the foam contractor would provide or at least suggest taking care of the "GAPS"

Allan,
Are you asking me about the advantage of airtight drywall at the ceiling?
...or
the advantage of Airtight drywall at the exterior wall in addition to the airtight sheathing?

The advantage I see for airtight drywall at the ceiling...
It works with a (vented attic)... the lowest cost, highest performance Roof System
A System with a John Straube rating of 24

A Cathedralized Attic only scores from 11 to 15

The advantage of 2 air barriers at the wall would be more of a durability enhancement than energy performance....especially in a mixed climate

I guess what I’m asking John is for someone like myself who is regular building new homes, and recognizes the importance of air tightness, but also has limited money to spend on energy efficiency. I’ve never sealed drywall or used gaskets, I just want to know if I focus on air tightness with sheathing and wall insulation (in my case spray foam), is there a payback to seal the drywall? I have read Martin’s articles and links. There will be a labor and material cost component to do this, is it worth it?

Allan,
For your climate (Houston)
If you continue to use spray foam and caulk /seal the "other gaps" and use airtight sheathing on your exterior wall... I do not think you need to add airtight drywall.

What I am proposing/exploring here is an alternate method to achieve good airtightness and thus avoid the need for Expensive and Not-So-Green Sprayfoam.

Something with equal or better perfomance at equal or less cost.

The next step would be to add more insulation at a lower price than spray foam.