Comments on: Where Should the Air Barrier Be Located?

By: Cindi

Cindi — Sun, 26 Sep 2021 15:14:41 +0000

In reply to M Jolie. A single great air barrier is better than two good air barriers. And once you have one great one you don't need a second. I'd do it on the outside.

By: Richard Howell

Richard Howell — Mon, 06 Sep 2021 18:15:56 +0000

Looking to build a house on a lake in Central Alabama soon. I’m also interested in an answer. to the question M Jolie asked above.

By: Brian Butler

Brian Butler — Fri, 03 Sep 2021 08:40:39 +0000

In reply to Bryan Uhler. Wow that's an interesting twist on the base code! Levels the playing field in some respects...

By: Bryan Uhler

Bryan Uhler — Wed, 01 Sep 2021 20:54:48 +0000

In reply to Brian Butler.

This particular home is vested in the 2015 WSEC (a stricter version of the IECC). The way the Washington code works is that there is a menu of energy credits that have to be selected which causes the home to be more energy efficient than following the code only. The base code requires 5 ach50, but the choice can be made to select energy credits based off of a tighter envelope and ventilation solutions/efficiency.

Where the 2018 code gets challenging is the decision to go with 8.5 feet x square footage as the cubic volume. No more does one get an easier blower door by a large double height wall or similar volume input.

In fact, I’ve been tracking this metric (the code cubic volume vs. actual cubic volume), and the 5 results so far are below. If the code credit decision of 3.0 ach50 was used, it would pass via the 2015 code but fail via the 2018 code.

Actual CV Code CV Difference
ACH50 ACH50
11.7 9.34 2.36
11 8.78 2.22
10.8 8.63 2.17
9.7 7.75 1.95
3.16 2.5 0.66

By: M Jolie

M Jolie — Wed, 01 Sep 2021 19:37:14 +0000

I have a cabin in Tennessee. I don’t have drywall interior walls by rather wood planking, with fiberglass insulation. Would I be better off with an air barrier inside and outside, with water barrier on the outside?

By: Brian Butler

Brian Butler — Wed, 01 Sep 2021 10:15:54 +0000

In reply to Bryan Uhler. Hi Bryan, just curious what IECC version your locality is on…? Aren’t those blower door ACH numbers going to fail code requirement?

By: Deniz Erkan

Deniz Erkan — Wed, 01 Sep 2021 04:17:48 +0000

Great post. Speaking as a dry climate homeowner doing retrofit, only so much can be done to improve the outside air barrier after the fact, mostly in the attic and around doors/windows. This makes me think that there is no harm in then also trying to seal the interior as much as practical. Why not seal all electrical penetrations/outlets/switches, openings behind cabinets, medicine cabinets, around baseboards, canlight trims, pipes, and any other place where the gypsum was compromised? The house itself could have a stack effect in its interior or when the various forced air units/fans run could pull air around the rooms/floors including via interstitial gaps.

I have noticed another area seemingly glossed over are the various fan exhaust dampers. Those can add up too.

By: Robert

Robert — Wed, 01 Sep 2021 02:11:42 +0000

I think placing the air barrier at the exterior makes more sense. You can align the air and water barriers. When the air barrier is at the outsurface you also keep bugs out.

The air barrier will be better protected. It doesnt take many small leaks to have a proportionally big impact on a tight structure. An easily comprised barrier could turn into swiss cheese in a couple of decades. We should think long term durability.

Matt Risinger has a good video on blower door testing of a house that was well air sealed but they wanted to take it to the next level.

By: Kent Randall

Kent Randall — Wed, 01 Sep 2021 00:08:55 +0000

I am interested in reader opinions on the best air barrier design for the lower portion of an exterior wall of the second story of a house that extends below the roof (the upper portion is lap siding over house wrap). In my case, the attic is vented/un-conditioned.

For example, would you use sealed OSB, drywall over house wrap or a sheathing like Georgia Pacific DensGlass?

By: Allison Bailes

Allison Bailes — Tue, 31 Aug 2021 22:56:57 +0000

In reply to Bryan Uhler. Good point, Bryan. There's definitely resistance to air flow from one stud bay to another, but in the case of air permeable insulation, it's not hard to find lateral wiring penetrations and gaps around studs. I know you're using spray foam, at least on some of your houses, so that resistance is much higher there. If you haven't seen it, here's an article I wrote about a bathroom remodel I did five years ago. The sheathing was Celotex, which created some interesting air flow pathways. Air Flow Pathways in a Leaky Bathroom Wall