Above Grade Walls

Why air tight exterior wall construction is necessary

Cold air entering our homes during a Manitoba winter through poorly sealed electrical receptacles, under baseboards or around window frames can account for up to 20% of heating costs and make the home uncomfortable and less healthy to live in.

The residential home building industry has recognized the importance of limiting air flow through exterior walls for some time now and for the most part in Manitoba, understands the techniques needed to build a reasonably air tight exterior wall. But how air tight is "air tight"? Don't walls have to breathe? Won't it be unhealthy for us to live in an airtight home?

Once you understand the benefits provided by air tight exterior wall construction such as comfort, energy savings and durable walls and agree that it forms a major component of a homes construction, then it follows that the more air tight the better!

Walls don't have to "breathe". Houses have to "breathe" and do so through a properly designed, installed and commissioned ventilation system such as are mandatory in every R-2000 Home. Combined with a mechanical ventilation system, properly built exterior walls that do not permit airflow allow you, not Mother Nature, to control the interior air quality in your home. Airtight exterior walls are a necessary component in a building system to provide your family with the healthiest homes available in the marketplace.

At R-2000, we believe that if you measure anything, you can compare and improve its quality. Not even experts in building science can visually determine the air tightness of an exterior wall. That is why all R-2000 Homes have their exterior walls tested by a blower door test. Licensed R-2000 Builders homes either pass the test or deficiencies are identified for correction. Without testing, there is no way of determining the level of airtightness of the walls. It costs very little in additional materials to maximize an exterior walls resistance to air flow but does involve additional time and labor to get things right.

It's all in the details

In Manitoba, the home building industry typically uses a 6 mil. polyethylene plastic sheet (air/vapour barrier) installed over the interior of the batt insulation to prevent the movement of air and water vapour into and through the wall. To be properly installed for maximum benefits, the air/vapour barrier must have all penetrations and connections properly detailed.

Electrical receptacles

Electrical receptacles must have the polyethylene (poly) air/vapour barrier wrapped continuously around the junction box. This is typically done with a preformed poly box or 6 mil. poly wrap behind the box. The box must be totally sealed to the interior poly on the wall and all wiring penetrations into the junction box sealed airtight. Airtight junction boxes, made out of plastic, are available and work quite well in place of the standard metal junction box wrapped with poly. Experience under blower door depressurization conditions has shown that the following typical installation deficiencies require additional attention to ensure they are done correctly:

• all wiring penetrations into the receptacle need to be sealed to the air/vapour barrier with acoustical caulking or expanding polyurethane foam
• Proper sealing of the air/vapour barrier junction box wrap to the wall air/vapour barrier on the side farthest from the wall stud is difficult. Without wood backing, the preformed poly box is flexible and the physical connection between the two pieces of poly takes more care to insure a complete airtight joint seal.

Interior partitions

Any interior room partition that joins into the exterior wall must have the air/vapour barrier run continuously behind the intersection of the walls. All wiring penetrations must be sealed with acoustical caulking or expanding foam insulation to prevent airflow around the wire.

Windows

If we install one of the better new windows available in Manitoba, we don't have to worry about airflow through the window but we do have to be concerned with airflow around the window frame. An airtight physical connection must be made between the window frame and the wall air/vapour barrier. We defeat the advantage of installing a high performance window if we allow air to flow in or out around the perimeter. Low expansion foam, acoustical caulking and tape can be successfully used to seal the window to the wall poly.

Details at the top and bottom wall plates

A good building envelope maintains a continuous thermal and air/vapour barrier completely over all exterior surfaces. That said, it is important to properly maintain the continuity of the air/vapour at the base and top of all walls. The wall poly air/vapour barrier must be lapped down onto the poly under the bottom plate from the floor below and sealed continuously as is the top of the wall to the ceiling. Failure to pay attention to this detail will allow significant air to enter your home under baseboards causing increase energy consumption, drafts and discolouration of carpet along their perimeter.