Airtight – how do we put that into a perspective that we can wrap our minds around? Anyone who has spent a night or two on an air mattress know that they soon need pumping up again. Anyone with kids knows that the balloons eventually lose their lift, and no matter how expensive those food storage containers with the air tight seal, food will rot in all of them if left long enough.
As a good Southern gal I couldn’t resist the idea of equating air tightness with the saying “tighter than a tick on a hound dog”, as a lady I won’t repeat some of the other “tighter than…” sayings I came across ;-) Fortunately we have a good benchmark to judge our Passive House projects by so we can just let sleeping dogs lie. Spend any time around a PH consultant and you will hear the airtight requirement as less than or equal to .6 air changes per hour at 50 pascals (.6 ACH @ 50 pascals). Simply put all the air in the building can not be swapped out for new outside air faster than a rate of 60% of the volume of the building per hour under a pressure of 50 pascals (negative or positive pressure).
The reason for this requirement is to prevent drastic temperature changes within the building that would drive energy costs higher because you would be forever heating or cooling new air inside the building as the old air escaped and took the heat with it. So if you are building a construction that is air tight so that you can keep the conditioned air at a steady state you have solved one problem while creating another – if the building is too tight you won’t get the fresh air needed for a healthful environment or could risk trapping too much moist air within the structure that could allow for mold growth. Those issues are addressed by the installation of either a Heat Recovery Ventilator which can exchange old air for new air while keeping the warmth inside efficiently. If humidity is an issue, a similar unit called an Energy Recovery Ventilator does the heat recovery AND addresses the issue of humidity.
So now we know there is a need for air tightness, but well planned air tightness so that we can have a healthy environment. How do we get our constructions that tight? Buildings have LOTS of openings in them! While it may seem a huge task, ask yourself – do you insulate EVERY exterior wall? Do you caulk EVERY window? Do you shingle the ENTIRE roof? Of course you do. When looking at air tightness it is no less a task than what you are already doing – it is just a matter of looking at the envelope as a whole rather than looking at how many holes. You’re going to have the electrician wire EVERY outlet on an exterior wall. You are going to have the drywall guy make access for each of those outlet boxes so that the painter can paint around each and every one and then put covers on them. You are already doing a lot of something with each of those holes – plan to do a step that makes them air tight. For outlet boxes than can be as simple as using one designed to be airtight from the start. If that isn’t in your budget, figuring out another way to seal those penetrations such as foam, gaskets, or other caulking material before that phase of the project is built will give you the opportunity to incorporate air tightness into the construction without a hassle.
There are actually two kinds of systems that the Department of Energy has developed for making buildings more air tight. The Airtight Drywall Approach (ADA) takes quite a bit more planning, but is a more inclusive system than the other, called Simple Caulk and Seal (SCS). Both seek to create an airtight, or fairly airtight construction. According to the DOE “The main difference between the ADA and SCS methods is that with the SCS, seams and gaps are sealed after the exterior sheathing and drywall have been installed and finished. With the ADA, sealing is carried out during the entire construction process.” 
The ADA system involves caulking and sealing almost all aspects of a wall, floor, or roof assembly as you go along. Here’s the rundown from the DOE “The typical procedure for ADA is to seal any seams and joints where the foundation, sill plate, floor joist header, and sub-floor meet. The spaces between floors, the sub-floor, rim joist, and plates are also sealed. The wall-framing plates are sealed to the lower sub-floor and the upper rim joist. Gaskets are often used at the top and bottom wall plates (between the drywall and framing) and between ceiling drywall and attic joists. Airtight electrical boxes (or standard electrical boxes sealed with caulk) complete the air barrier. Holes where pipes and cables pass through also need to be sealed before the wall and ceiling finishes are applied. After all this has been done and the perimeter drywall seams have been finished, the room is effectively sealed from expensive and uncomfortable drafts.” 
Sounds like a very thorough plan for keeping the construction airtight. The other system however would be faster and cheaper to implement. Basically you are still sealing and caulking, but not until the GWB & sheathing is up. You can skip all the stuff inside the walls under this system, but sometimes that is where the difference between building a Passive House and a house that is NOT a Passive House lies. While the ACH numbers the government touts are close to PH (.67-1.8 ACH @ 50 pascal) they are still not at the Passive House requirement. The difference could be a house wrap, a type of window, a kind of caulk or seal that is even better than what they used for their trial. It is not difficult to make the air tightness if you plan for it and build with that in mind. We have seen a good number of PH consultants achieve the .6 and better.
If you would like to see a great graphic that demonstrates all the areas that would need to be addressed by the ADA system, Martin Holloday wrote a great article with a wonderful graphic for Fine Homebuilding.  If you have that kind of detail in your plans then you are already thinking of the final outcome – a Passive House that is sealed tighter than a tick on a hound dog!