Leaders in building performance circles have been talking about and promoting tight building construction for over 40 years; yet in 2019, many in the construction industry are still skeptical of (and sometimes downright hostile to) the idea of building airtight buildings.
For one, these antagonists assume that all buildings will get wet so they need to be constructed in a way that they can dry out quickly. Secondly, they believe that if the building is too tight, people will get sick due to improper ventilation. And lastly, they argue that it costs significantly more to build an airtight building which is a non-starter in a cost-conscious world. All of these arguments fall way short.
We could use the same logic in the first statement to say that we should build large holes in the bottom of our walls so that when snakes get in, they have a way to get out. Air migration through the building envelope is the largest source of moisture in buildings and must be controlled. Besides, without a tight building envelope, we will significantly overspend to try and control the indoor environment with our mechanical systems.
The answer to the second objection is properly designed and balanced ventilation systems. The answer is not to let air come into a building anywhere it can, but to “build tight and ventilate right.” Ventilation should be designed into a building from the beginning, taking into account the building design, envelope performance and all sources of ventilation, including make-up air ventilation, bath and kitchen vents, and dryers. Ideally, the active ventilation systems would be designed to balance outflow and inflow so that the building is not positively or negatively pressurized.
The cost argument is always the toughest one to address, at least in the beginning. This argument assumes there are no economies of scale or technological efficiencies that make costs decrease as improved technologies come to market and we learn how to build better. This argument also fails to address the huge long-term, consequential cost of low-performing buildings discussed above. Besides, getting to a moderate level of airtightness isn’t as hard as people make it out to be.
To put things in perspective, the national average of existing commercial buildings is 1.55 cubic feet per minute per square foot of building envelope at 75 pascals of pressure (CFM75). Here are the infiltration rates of several other current standards in use today:
0.40 CFM752015 International Energy Conservation Code (IECC)
0.25 CFM75 United States Army Corps of Engineers (USACE)
0.15 CFM75 Canadian Building Code
0.08 CFM75 Passive House US (PHIUS)
At the conference, we heard about an architect in Austin who is building his own home to the Passive House standard and who achieved a 0.02 CFM75 air infiltration rate! That is 20 times less air infiltration than the current energy code in Texas and 77.5 times less than the average existing building in this country. Meeting current building code is not difficult. Going 50% better is also quite achievable. Here’s how Joe Lstiburek put it to the audience:
To get to 0.4 CFM, just get rid of all the big holes in the envelope.
To get to 0.25 CFM, get rid of all the small holes.
To get to 0.6 CFM, get German (Passive House).
This argument that it is too expensive to build airtight buildings might have held a bit of truth in the 1980s, but it is certainly not true today. Besides, building codes now require more airtight construction. These building code requirements will soon become the “new normal,” at least until they change the codes again.
The construction of the Ironsight Office Building in San Marcos was a great example of airtight construction that achieved a 0.25 CFM blower door score simply using Bautex Blocks and Bautex Air and Moisture Barrier.
Are you measuring the airtightness of your buildings? Are you testing new technologies and methods to see what it will take to continue to drive down air infiltration in all of the buildings you design and build?