Building Science-“Breathe”

In this blog, I’m going to discuss building tightness and the code dealing with air leakage.  The 2012 International Residential Code for One and Two-Family Dwellings is the current code in force for the state of Minnesota at the time of this blog.  Chapter 11 deals with energy conservation, what most in the building industry call the energy code.  The code on building air leakage states:

” The building thermal envelope shall be constructed to limit air leakage…the building or dwelling unit shall be tested and verified as having an air leakage rate of not exceeding…3 air changes per hour in zones 3 through 8. Testing shall be conducted with a blower door at at pressure of 0.2 inches w.g. (50 Pascals).”

The code refers to zones 3 through 8, zones 5 and 6 are considered a cold climate, zones 7 and 8 are a very cold climate.  The northern half of Minnesota and North Dakota, and the northern one-third of Wisconsin are in zone 7. Below is an image of The Department of Energy’s climate map.

I’ve mentioned in previous posts that 10% to 40% of heating and cooling costs are the result of air leakage in the building envelope.  Uncontrolled air leaks can also have an effect on building durability and indoor air quality.  If you talk to some builders and home owners, they will tell you we are building structures too tight, they need to “breathe”.  The question comes down to what do they mean by needing to “breathe”?

The “breathe” argument may be referring to a buildings ability to dry. Older homes had the ability to dry because air and moisture moved easily through the building assemblies.  Insulation and air sealing techniques required by current codes along with modern building materials can make it harder for structures to dry.  Building science has identified the areas of concern and have created ways to balance building durability and energy efficiency.  Managing moisture is the number one concern in building science.  The sources of the moisture need to be understood.  Bulk water concerns on the exterior and air leaks in the interior of structures are the two largest sources of moisture in building envelopes, vapor diffusion is a distant third.  Understanding moisture drive both inward and outward is the way to assure a structure will dry.  Dr. Joseph Lstiburek has a great paper on this subject called “Inward Drive-Outward Drying”, here is the link.

“Breathe” can also mean fresh air for the home.  All new homes are required to have ventilation, and in the case of Minnesota, balanced ventilation. A balanced ventilation system will bring an equal amount of fresh air into the home and exhaust the same amount of stale air out of the house, usually accomplished using a heat recovery ventilator (HRV) or energy recovery ventilator (ERV). Mechanical ventilation not only affects indoor air quality, these systems are also designed to control indoor humidity levels during the heating season. Humidity levels in older homes used to be dictated by the amount of natural air leakage in the building envelope. There was no control and often times the humidity became so low, the occupants needed humidification to be comfortable. The fresh air supplied to the home was being drawn through the building assemblies, and sometimes through the adhesives, insulations, and other man-made products containing volatile organic compounds, or VOC’s. Yuck!  Building a tight envelope with proper mechanical ventilation is important for indoor air quality and the health of the occupants.

“Breathe” may also be attic, or in some installations, wall ventilation.  There are many ventilated attics throughout the country.  A typical ventilated attic will have some sort of a vent at the lower end of the roof, typically under the eave, and a ridge or static vent near the top of the roof.  There are several purposes for this venting, first, it keeps the underside of the roof cool.  This is important in the winter to reduce the chance of an ice dam.  Some claim that keeping shingles cooler in the summer will prolong the life of the roof.  A ventilated attic can also remove any moisture that enters the attic from the conditioned interior of the home.  Any air leaks in the ceiling of a home can move warm moist air into the attic, if the roof is unventilated, this moisture can accumulate on the colder surfaces.  If the surface is below freezing, the moisture will turn to frost.  Frost in an attic is ugly. Not all ventilated attics work.  The photograph shows a ventilated attic. 

This home has air leaks from the conditioned space and the soffit venting was partially blocked leading to the accumulation of frost during the winter.  My advice dealing with ventilated attics in new construction is air sealing is the number one priority in stopping moisture movement through walls and ceilings.  Ventilating the attic is the insurance policy or back-up plan should there be a problem with the air sealing.

I also mentioned wall ventilation, often called a rain screen.  This is simply an air space between the siding and sheeting/house wrap portion of a wall assembly.  The top and bottom of the air spaces are left open so air can move through the space and water can drain out.  This design allows any moisture that should accumulate inside the wall assembly to dry to the exterior.  If you are using polyethylene sheeting as your vapor/air barrier, the only drying potential of the wall cavity is to the exterior.  In my opinion, exterior wall ventilation should be a code requirement when very low perm vapor barriers on the warm in winter side of a wall assembly are allowed or required in cold climates.

As stated in the code reference, the current building code requires no more than 3 air changes per hour with the blower door running at 50 Pascals.  This number is often expressed as 3 ACH50.  By the way, 50 Pascals in roughly the equivalent of a 20 mile per hour wind blowing on all sides of a building at the same time.  When we build a code minimum house with regards to the energy code, which is common, the 3 ACH50 becomes roughly .25 natural air changes per hour during the heating months.  What does all that mean?  It means you are exchanging the entire volume of air inside your home with outside air once every 4 hours, or 6 times per day.  This is considered “tight”!  The natural air exchanges inside a home will fluctuate greatly.  The temperature difference between inside and outside, humidity, wind, building pressures will all change the natural air exchanges.  The .25 is a rough average. The 3 ACH50 code requirement is a minimum.  I have conducted several code compliance blower door tests for contractors and home owners in my area with most of the tests achieving a 2-2.5 ACH50 result.

So, why do we want a “tight house”?  For the homeowner, it’s energy costs, comfort and control. That 10% to 40% of heating and cooling cost number I refer to is lower on a tight house. Being comfortable and having control of the environment within the home are also good for the homeowner.  For the builder, a tight house means less risk and more durability in the assemblies.  If the builder understands how the buildings act as a system, they can choose right materials and construction methods that are based on building science. Good for the builder.

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