Membranes…Interior Air-Control Strategies That Can Work

This article first appeared on the Green Building Advisor Website.

I’m from Minnesota, one of the few areas in the lower 48 that still regularly uses polyethylene sheeting as a vapor retarder, we’ve become very good at detailing these interior membranes for use as an air barrier.  (The practice is also very common in Canada and Alaska.)  The average blower door test I conduct for new homes in my market has now moved below 2 ACH50, most achieving these scores only using an interior air control strategy.  I don’t recommend using poly in most cases, instead I suggest moving to a class II vapor retarder, (smart, variable, and responsive vapor retarders), these products can also be used for air control.  Installing the two products is similar, so, what are the tricks to an effective interior air barrier installation?

No matter what product is used as an air barrier, or whether it’s located outside, inside, or in the middle of a wall, continuity is the most important part for it to be effective.  You may remember the old song Dem Bones from your childhood.  The foot bone is connected to the-ankle bone, the ankle bone is connected to the-leg bone…this is exactly how an effective air barrier works, by connecting the foundation to the wall and the wall to the ceiling.

Thinking through the process at the planning stage.

The most difficult part of interior air sealing is dealing with interior partition walls.  Interior walls create lots of breaks in continuity both at the ceiling and where any interior wall connects to the exterior wall.  Do all those walls need to be constructed before the roof framing is placed?  The homes I blower door test with the lowest numbers are designed so that the roof is supported only (or mostly) by the exterior walls.  This leaves one big open area and simplifies any membrane installation.

If this isn’t possible, can a piece of the membrane be attached to the top plate of the partition wall before trusses are set or roof framing begins?  Later, when the rest of the ceiling membrane is installed, the two are sealed together, creating continuity.

This same strategy can be used for the continuity of the air barrier on the exterior walls.  A membrane can be installed before the interior wall is constructed and placed.  Another option would be to leave a small space between the interior and exterior wall to slide the membrane past, or simply leave the end stud that attaches to the exterior wall out until after the membrane is placed.

Attaching a membrane to framing.

There are a few ways to secure a membrane to the framing.  The choice and quantity of fasteners are dependent on the type of membrane used and whether the membrane will be needed to support a dense pack or heavy attic insulation.  The most common attachment method is by using a slap stapler.  They are easy to use, fast, the fasteners are cheap, very portable, and can be effective if the membrane is durable.  The downside is the staple can pull through if the membrane is fragile, you are putting small holes in the membrane by using a staple, and if you miss a framing member while swinging the stapler, you probably poked a hole in your air control than now needs to be fixed.

Pneumatic and electric staplers offer more control, but they still create a couple small holes that can become larger ones as the membrane is pulled and adjusted.  Occasionally I’ll use my pneumatic cap stapler for interior membranes.  They provide the best hold while requiring fewer fasteners.  The downside is you have a hose to drag around and listen to a compressor with the pneumatic options.

Double sided tapes designed to be used with interior membranes are great.  I’ve had the opportunity to use Siga’s Twinet a couple times.  The biggest advantage is there are no holes that are produced by fastener.  Installing the tape is easy, stick the tape to the framing and when ready to install the membrane, peel away the release facer.  Installing the membrane to the tape has a bit of a learning curve.  Once the membrane is attached, the hold is very impressive, creating a problem if you need to reposition the membrane.  The one drawback, the tape is much more expensive than the mechanically attached methods.

I have used spray adhesives when attaching a membrane to a metal framing member.  This was years ago, it worked, but now that I know about the double-sided tapes designed for this purpose, I would use the tape instead.

Creating continuity from the floor to wall and wall to ceiling.

There are a few options for creating continuity between a foundation or floor and wall.  The insulators in my market (who also install the interior air control membrane) prefer to use an acoustical caulking at the joint of the bottom plate to floor connection.  They then drape the interior membrane over the connection, allowing a few inches to hang past the wall.  This connection can work, but I’ve seen on several occasions where a flooring contractor cuts the membrane above the acoustical caulk, using the bottom of the drywall as a straight edge for their utility knife and removing the draped membrane.  Someone on the team must educate other subcontractors as to the dos and don’ts of interior air control strategies.

My preference for the bottom of wall connection is to use tape.  The type of tape chosen will depend on the surface of the floor.  If it’s concrete, the best tape I’ve found is Siga’s Fentrim.  If it is a wood subfloor, I usually use the same seam tape as used for taping seams and other penetrations in the membrane.  Whichever tape is used, I typically order a wider roll for use on the wall to floor connection (and sometimes the wall to ceiling) with a split release liner.

Air sealing between the first and second floor is problematic when using an interior only air control strategy.  Traditional platform framing makes it nearly impossible to keep continuity with a membrane.  You will probably need to change strategies for air sealing between the floors.  Closed cell spray foam installed at the rim joist is very common in my market.  A cut and cobble method using rigid foams and a sealant or canned foam can also work.  (These strategies are also used for basement rim joists.)  One method that worked well for a couple of my projects was moving from platform framing to balloon framing.  A membrane can be installed before the floor framing, maintaining the continuity of the air barrier.

Continuity between the ceiling and wall is usually simple.  Drape the ceiling air control slightly onto the wall so that the two seams are easy to connect using a tape.  If the connection needs to happen at the change in plane between the ceiling and wall, use a wide split liner tape.  Remove half the liner at a time and tuck into the corner.  I like to use a plastic wallpaper smoothing tool for this technique.  The trick is to get the tape into the corner as tight as possible.  A tape joint with a radius (or a radius in an inside corner when installing the membrane) will frustrate the drywall or other finishing crew, they may cut the tape creating a possible air leak.

Sealing seams, breaks and penetrations. 

Straight walls with no interruptions are the easiest surfaces to install interior membranes, but they are rare.  Windows and doors, plumbing and HVAC penetrations, electrical boxes and wires and seams where two or more pieces of a membrane come together are all things that require sealing.

Let’s start with the biggest interruption, windows and doors.  Most code-built homes in my market will have an acoustical sealant applied around the perimeter of the window or door rough opening.  This is the most cost-effective option for creating continuity in these areas.  A couple drawbacks, a standard acoustical caulk doesn’t produce a good enough bond between the membrane and framing to be used by itself, it’s going to require at least a few staples to keep the membrane in place.  (I have used Contega HF from 475 Supply in the past, this sealant will hold a connection).  Another drawback, acoustical caulking is messy, I’ve found that nearly every time I use the stuff, it ends up on my clothes and tools (and occasionally, my hair).  Most of the trades have the same complaint.  Personally, I prefer to use a tape to make this connection, but the cost of both the labor and product increases substantially.  Whichever method is used, the seal will need to be protected from damage.  I’ve seen where rotary tools used for cutting drywall have damaged the continuity of the air barrier, you might want to think about how, or whether they are used.

When using an interior membrane as an air barrier, any electrical boxes integrated into the membrane should have a flange to seal the membrane to the box.  There are a few boxes with an integrated foam gasket that helps create the seal, personally I back that gasket up with a bead of acoustical sealant or tape.

Wires are also sometimes just passed through a wall or ceiling cavity and into the living space, penetrating the membrane.  (An example is shown in the photos with all the plumbing pipes.) The easiest way to address these is simply using two pieces of tape or some sort of gasket designed for electrical wires.  I’ve written a couple articles on the topic of sealing electrical boxes and other electrical penetrations.

Airtight Electrical Boxes 

Air-Sealing Electrical Penetrations

Sometimes plumbing and HVAC penetrations require sealing.  Personally, I prefer using a gasket whenever possible.  This will allow some movement of a pipe, duct, or wire independent from the membrane.  Tapes can also work but take some time to place.

Sealing seams in an interior membrane is usually accomplished by using a tape.  Tapes work best when you can press firmly against the surface receiving the tape, rollers are commonly used.  A flexible membrane doesn’t always allow for good pressure if a seam in the membrane lands between two framing members or travels parallel to framing.  I’ve found it best to use better, often more expensive interior tapes.  The red tape in the photo is commonly used in my market to seal seams and penetrations in polyethylene sheeting.  I’ve been in countless homes where the tape is separating from the surfaces it’s trying to seal.  For this reason, I prefer to use Siga’s Rissan.  I’m sure there are other interior tapes that will also be an improvement over the red tape, I suggest trying several to see what works best for you.

Interior only air sealing strategies can work, my recent barndominium project is proof.  That 160,000 cubic foot structure hit 997 CFM50, 0.38 ACH50, 0.038 CFM/ft² of surface area.  That being said, one of the drawbacks to using an interior membrane is how easily it is damaged, either by accident or by a trade modifying the structure or a system.  Working as an electrician in the late 1990’s, we would drill holes to fish wires through exterior walls without ever thinking about air or vapor control.

My preferred location for an air barrier is where it is protected, is easy to install, is durable, and is easy to change if an addition or renovation occurs in the future.  To me, the outside is the preferred location.  In some instances, that just isn’t possible.  With careful planning and execution, interior air control is an option.