Construction Design-Interior Air Barrier? Exterior Air Barrier? Or Both!

When designing the concrete-less slab on grade home, I gave serious thought to the location of the air barrier.  I have used water resistive barrier (WRB) or house wraps for years without a good understanding how they work as an air barrier.  In my climate, most homes use polyethylene sheeting as an interior (and main) air barrier.  As it turns out, there are better choices.  Not saying you can’t get a product like Tyvek or polyethylene (in my climate) to perform as an air barrier, but today we have so many better options.

I knew I wanted to make the main air barrier on the exterior of the home.  I wanted a product that first would effectively shed water, but also stop air movement.  A durable product that was easy to work with.  I chose Zip Systems Zip sheeting.

The first step in making an exterior sheeting product an air barrier is to seal all areas where air can leak.  In the case of where the sheeting meets the insulated concrete form (ICF) foundation, we used a fluid applied product (Prosoco’s Fast Flash) to connect the concrete foundation to the Zip.  This seal also makes a water tight connection.  Other leakage points (again both air and water leaks), are the seams and corners where the Zip panels meet.

We used a 4 inch Zip tape designed to work with the Zip panels to seal each seam.  Zip recommend rolling the tape with a J-roller, an important step to assure proper adhesion.

Another important part of an air control layer is to make it continuous.  I needed to connect the Zip sheeting to a ceiling air barrier.  I did not want the entire exterior roof to act as the air barrier, instead I chose the inside ceiling of the home.  Connecting the two required some research.

What I decided was to extend the Zip with a piece of wide tape over the top plate.  After the roof trusses were set and the material I was planning on using as the ceiling air control layer (more on that in a bit), was installed to the roof trusses, there would be a continuation of the air barrier.

Connecting the Vycor Plus tape to the interior air control layer.

The tape that was extended over the top plate was adhered to the hanging ceiling air control layer in the picture above, making a continuous connection with the outside Zip sheeting.

I wanted the interior air control layer to stop air movement but still allow moisture to move through the product if need be.  It could not be vapor open because of the climate zone I build in, zone 7 (class I or II vapor control layer is required on the warm in winter side of the assembly,  a class III vapor control layer is allowed when exterior insulation is at least R-15).  I chose a smart vapor retarder called Intello, which is a class II vapor control layer and is also an effective air barrier when installed correctly.

Intello under negative pressure during a blower door test.

The air sealing using the Zip and Intello needed to be tested before all windows and doors were cut out and utilities were started.  The first blower door test on the assembly resulted in 1ACH@50Pa negative pressure test and .4ACH@50Pa positive pressure test.  Read more about blower door testing here.  I believe the difference between the two tests was the staples used to attach the Intello at the ceiling and house to garage wall.  The photo above shows how hard the Intello was being pulled against the staples, which was creating a bunch of tiny air leaks.  During the positive pressure test, the Intello was pushed into the framing, eliminating the pull on the staple.

We got the first blower door test result I was hoping for.  The utilities were next to be installed in the home, which creates holes in the air barrier.  Ventilation and electrical penetrations from inside to outside the home needed to be addressed.

All pipes and wires extended outside the Zip sheeting (main air barrier) were taped on the exterior and sealed with spray foam from the inside.

I used an air sealing product called Contega HF to seal the Intello to the electrical box flanges and also the Intello to the window rough framing.  Acoustical caulks are the traditional material used for sealing these areas, but I found this product creates a very durable seal.

A couple final details, we stripped the ceilings using 2 x 4’s so that all the ceiling electrical wires and boxes were kept inside the building envelope.  I also designed the house to have an plenum truss.  This truss has a notch for ductwork and other utilities so that they remain within the building envelope.

So, should the air barrier be located on the exterior, interior, or both?  If I were building in a warm climate I would vote for the exterior.  Because my climate requires vapor control, I prefer both interior and exterior.

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