Air Barriers in House Design

Air Barriers in house design sounds downright boring, doesn’t it?  Just another technical issue to be ignored, right?  Well, not if you want to comply with Energy Code and save yourself tens of thousands of dollars in saved energy costs over the lifetime of your house.

Read on, to learn more about this subject.

First of all, what is an Air Barrier? 
An Air Barrier is a continuous membrane enclosure that separates the inside of your house from the outside.  No one thought about things like this until recently.  As during the last 5 to 10 years or so.  And it’s only addressed in some of the latest building codes for residential projects.  In the 2015 IECC (International Energy Conservation Code) section R402.4 AIR LEAKAGE (Mandatory) “The thermal envelope shall be constructed to limit air leakage in accordance with the requirements of  Sections R402.4.1 through R402.4.4. 

And WHY do we need Air Barriers?  Answer: because too much conditioned air in our houses is escaping to the outside world, wasting our energy dollars and increasing our energy consumption and ultimately, increasing greenhouse gas due to more fumes from energy utility plants.  So: to save more money in your wallet and to do the environment a big favor, you need to have more effective Air Barriers in new house construction, to make your house tighter and conserve energy. 

The various referenced Code sections limit the number of Air Changes per Hour in new houses to 5 ACH in Climate Zones 1 & 2, and 3 ACH in Climate Zones 3 through 8, when tested in accordance with ASTM E779 or ASTM E1827 at a pressure of 0.2 inch w.g. (50 Pascals).  This is commonly referred to as a Blower Door Test. 

Climate Zones 1&2 refer to the hottest and most southern portions of the USA: Florida, southern Texas, SW Arizona and a very small portion of SE California.  All the other Climate Zones are north of these.  The reason that the more northern zones are allowed fewer ACH is that it costs you more money (and the Country more energy) to heat houses in those cooler areas.

There is also a large chart on IECC Table R402.4.1.1 AIR BARRIER AND INSULATION INSTALLATION that reviews locations in a house where insulation and air barriers must be installed to comply with the Energy Code.

Now then: while the Code tells Architects, Contractors and others that you need an Air Barrier, and it defines Air Barriers in Section R202 as: “Material(s) assembled and joined together to provide a barrier to air leakage through the building envelope.  An air barrier may be a single material or a combination of materials.”  However, the residential section of the IECC doesn’t mention materials.

However, in the Commercial section of the IECC, under section C402.5.1.2.1 MATERIALS are listed for Air Barriers.  Among those materials “deemed to comply” are: plywood, OSB, polystyrene, foil-backed polyisocyanurate, closed cell spray foam, open cell spray foam, gypsum board (1/2″+), cement board (1/2″+), built-up roofing, modified bitumen roof membrane, single ply roof membrane, plaster of certain types, cast in place & precast concrete, grouted CMU, sheet steel or aluminum, solid or hollow masonry of clay or shale units. 

However, per the commercial IECC, materials/assemblies meeting the commercial requirement of air leakage not greater than 0.04CFM/SF under a pressure of  0.3 inch water gauge (75Pa) tested in accordance with  ASTM E2357, ASTM E1677 or ASTM E283 will comply with the Commercial Air Barrier section. 

Now then, the Residential portion of the IECC is separate and distinct from the Commercial section for good reason: when you build a house you are not building a hospital or airport or other commercial structure.  So, it becomes highly questionable to apply the commercial requirements on residential projects. 

In other words, as long as the materials and systems used in residential construction meet the residential ACH testing required for residences, then that should be acceptable.  There does not appear to be specific systems or materials enumerated in the residential section of the IECC that require specific materials for the air barriers used in residential construction.  Therefore, a little creativity between Licensed Architects and Licensed Contractors might be able to suggest some possibilities for Air Barrier systems that might be a good value for houses.

What would not be a good value?  Having to use commercial air barrier materials and systems when some less expensive (but durable) alternative might work to satisfy the ACH requirement. 

So: where are you going to need an Air Barrier in a house to comply with the ACH requirement?  Take a look at the diagram near the top of this article: see that dashed red line?  That’s a pretty good indication of where a good thermal and air barrier should be located: the interior framing line of the conditioned space.  In other words: where the thermal insulation interior vapor barrier is.  Think of it as a large red balloon holding the interior air inside.  And what is that thermal insulation vapor barrier in 95% of houses built today? 

That would be asphaltic coated kraft paper on the back (interior) side of the fiberglass insulation.  And at that same location is another component that the Commercial IECC recognizes as an effective Air Barrier: 1/2″ standard gypsum board (what many people call “drywall”).  Therefore, wherever you have standard gypsum board or cement board at ceilings and walls in a house, THAT can be your Air Barrier, which should comply with both the Residential and Commercial sections of the IECC. That’s a huge step forward in effective & economical Air Barriers: using the material that coats the interior of more houses as an interior finish than anything else! 

However, what about the many wood plank ceilings, walls & floors that are in many houses today, where used over framing that is thermally insulated?  What if the homeowner and Architect and Contractor do not want to squander the homeowner’s money on first coating those surfaces with gypsum board before coating them with the preferred visible finish material of wood planks?  Good question! 

Rand Soellner, ArCH, NCARB, Senior Architect of HOME ARCHITECTS ® had an in-depth conversation with Kenny Guffey, General Contractor, of Kenny Guffey Construction, Sevierville, TN, about Air Barriers, Codes and costs.  Mr. Guffey was very concerned that building codes are getting more involved every year, making housing more and more expensive.  He was worried that if more expensive options for items like Air Barriers are used, in some cases doubling material and labor costs involved with those, that could raise the cost of affordable housing beyond the level of many Americans.  Kenny and Rand share these concerns, so they put their minds and decades of experience together to see if they could suggest an Air Barrier system that was economical and worked behind wood plank ceilings, behind wood plank walls, and under wood plank flooring.

 Let’s think about this.  Let’s come back to what is already there, before the finish material is applied.  Kenny and Rand noticed that the material that is on the wall and ceiling framing before installing a wood plank finish is: the kraft paper backing of the fiberglass insulation.  This paper is covered with a flexible asphaltic coating on the side facing the fiberglass.  While no testing has been accomplished of which either Kenny or Rand’s companies are aware, it stands to reason that air itself, under the minimal pressure during an ACH test would be unlikely to be able to penetrate the kraft paper with the asphaltic coating.  However, the joints between the asphaltic coated paper provide opportunities for air to escape.  What to do?  Some insulation manufacturers have acrylic tape that they offer for sale, engineered by their companies to work with general construction and insulation products, like the kraft paper on the back of fiberglass insulation.  If the insulation flanges are unfolded and overlapped over the face of the framing members at both ceilings and walls, then stapled over each other (and the staples hammered flat), and if the end butts are overlapped by perhaps an inch or so, then a wide soft bristle brush was used to wipe away any dust or interfering particles, it would seem logical that the construction tape made by the insulation manufacturer could be used to bond the kraft paper pieces together, creating a single, monolithic Air Barrier out of the kraft paper and tape on the ceilings and walls.  Keep in mind the image of the “BIG RED BALLOON” as the air barrier: it all needs to create a single, cohesive membrane,otherwise there could be gaps and misalignment that could allow air to escape and violate the entire purpose of the Air Barrier concept.  Think of it as a total system rather than as separate treatments in different places.  It all needs to connect together.

Furthermore, Kenny noted that if the little bit of additional taping work was accomplished by the same insulation installers while they had their scaffolding and ladders in place, as they installed the insulation, that this taping would be accomplished fairly reasonably both in terms of labor & material costs, as well as constructability, and not impact the time schedule or other trades.  This is starting to make sense: using materials already in place and simply taping them together.

But why, you may ask is it necessary to tape the kraft paper together?  Isn’t the wood planking to be installed over the insulation worth anything in terms of an air barrier?  The answer: yes and no.  Kenny observed that while air isn’t going to pass directly through solid pieces of wood, air could in fact move, under pressure, through the joints between the wood and the end joints.  Therefore, Rand suggested that the wood used on both walls and ceilings be wider: such as 1x12s, with t&g (Tongue & Groove) joints between them.   Using wider planks could cut the joints down by as much as 1/3 to 2/3 compared with narrower boards, so that helps.  However, air can still get around the boards and through those joints.  Therefore, while using wider boards helps, taping the kraft paper together is the last line of defense to hopefully secure the air barrier tight enough to resist the specified ACH.  Okay: now we have the walls and ceilings taken care of in terms of an economical residential air barrier approach.

What about wood flooring?  There are joints between the plywood or OSB (Oriented Strand Board) subflooring upon which the finish plank flooring is installed.  And if this floor is over a crawlspace (or worse: the outside world), the insulation under that floor will be installed with many gaps between the inaccessible kraft paper, because Code will require that to be installed toward the warm side in winter, so the kraft paper side will be jammed up under the floor joists/trusses and there will be, typically, a 1-1/2″ wide space where the floor structural members are, which will not have the kraft paper running over them.  That = gaps in both the vapor barrier and any theoretical air barrier performance there.  What to do?

It so happens that most good Contractors will install a layer of good old economical 15# asphaltic felt over the subflooring before installing the wood plank floor. Why: as a final vapor barrier under the finish wood to help avoid warping due to moisture penetration from outside air and to separate the wood from the subflooring.  Hmmm.  Kenny and Rand thought about this and realized that there are tapes manufactured (such as from WR Grace) that are specifically engineered to bond to asphaltic surface like asphaltic felt!  If you want a whole, total comprehensive Air Barrier, you can’t forget that floor.  There’s no gypsum board there to act an air barrier.  Dr. Joseph Lstiburek (Building Science Corporation) previously answered a question of Rand’s years ago that yes, 15# asphaltic felt CAN become an air barrier, if it is taped together with tape manufactured to bond to it.  Therefore, if you tape the seams of the felt on the floor (that good builders are going to place there anyway) and also use that tape to seal the joint between the surrounding wall kraft paper of adjoining walls (before installing drywall on those wall or wood planks), then you will have sealed the floor under the wood plank finish fairly well, and also sealed it to the wall portion of the Air Barrier (big red balloon). However, many Architects call for the gypsum board on the surrounding walls to be raised up 1/8″ to allow injection of sealant (like Lexel) into that joint, to seal the bottom of the drywall to the floor, for improved acoustics and to eliminate air leaks there.  Therefore: if a flat strip of asphaltic felt were run on the floor, along the wall edges of perhaps 4″ in width or so, before the gypsum board was installed, then the sealant at the bottom of the drywall was injected and bonded the bottom of the drywall to the asphaltic felt, then all that would to be done later when laying down the felt for the finish wood plankt flooring would be to tape the perimeter of the new felt to that 4″ perimeter strip around the walls.  That would merge the floor felt and retain the Big Red Balloon concept.  In other words, the Contractor would not need to tape the joint between the wall and the floor, if the Contractor was using sealant under the drywall bottom edge and had a strip of asphaltic felt there.  With a little bit of care, this can be done with not a lot of extra materials or extra work.  It’s all in the sequencing.

Of course, the wood plank flooring should be t&g and receive a durable coating that seals the wood, which in part may also contribute to sealing the end joints and side t&g joints, however, the real air barrier will be the taped felt under the finish flooring.

What do we now have?  A completely sealed interior house: (big red balloon) ceilings, walls and floors, whether finished with gypsum board or wood planks.  And we’ve used economical materials that are going to be there anyway.  The other additional material is some construction tape, engineered to properly bond to those surfaces.  And, you don’t need to tape the kraft paper behind any drywall, as the gypsum board itself will be the air barrier where it is used.

Problem solved: meeting the new Air Barrier requirements, using existing economical materials, with only some taping to induce them to perform as air barriers.  Now some field testing needs to be performed to discover if these system actually do have the the proper ACH resistances.  It’s certainly worth a try, given the economies that should be achieved, not having to use commercial assemblies. 

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