Back in my 1990’s post frame building contractor days, we constructed a shop for a client near Moscow, Idaho. We probably didn’t ask enough questions up front and our client didn’t provide enough information to adequately prevent what was initially quite a challenge.
After we had completed construction of this building’s shell, our client poured a concrete slab-on-grade. He placed fiberglass insulation in exterior walls, with a well-sealed vapor barrier. Walls and ceiling were sheetrock and insulation was blown into the attic. Heat was provided by a propane heater.
After the building was occupied, our client called us to advise every one of his windows was leaking!
Turns out these “leaks” were a symptom of a larger problem. Our client had sealed his building so tightly, in order to close an exterior door, a window needed to be open. There was no under slab vapor barrier, nor was a sealant applied. His propane heater was not ventilated to an outside source, adding moisture to interior air (and drawing moisture through his slab). With nowhere to exit, moisture was condensing on the insides of his building’s cooler windows!
Owning and operating an airtight post frame home, shouse (shop/house) or barndominium will increase its energy performance and lower its carbon footprint. However, there are certain things one should keep in mind before building a new airtight post frame building.
A post frame building’s envelope consists of its roof, foundation and exterior walls, doors and windows, and this is what keeps indoor and outdoor air from mixing. When a post frame building envelope is not tight, it can lead to air leakage and drafts, decreasing a building’s overall energy efficiency and increasing utility bills. With a sealed building envelope and upgraded mechanical ventilation systems, energy costs can be controlled and a comfortable indoor environment can be created.
Airtight post frame buildings are passive buildings meeting Passivhaus standards for air leakage. This is a residential construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. This standard recommends (not requires), a maximum design heating load of 10 watts per square meter and windows with a maximum U-factor of 0.14.
Unlike most United States standards for energy-efficient homes, this standard governs not just heating and cooling energy, but overall building energy use, including baseload electricity and domestic hot water.
These buildings have air leakage rates of less than .60 AC/H @ 50 Pascals (2012 IECC Code allows an air infiltration rate up to 3 AC/H @ 50 pascals). Use no more than 1.39kWh per square foot in cooling energy. Use no more than 4,755Btus per square foot in heating energy, and maintain a maximum entire building energy usage ratio of no more than 11.1kWh per square foot.
Airtight post frame buildings are extremely energy-efficient because mixing of indoor and outdoor air is extremely limited, reducing energy bills associated with heating and cooling. Besides a dramatic reduction in energy bills, expect improvements in building comfort, and whole house and heat recovery ventilation system energy efficiency. Moisture infiltration systems will be reduced.
When post frame buildings are constructed with airtightness and energy-efficiency in mind, it can lead to unintentional problems, like excessive moisture and CO2 levels. Thankfully, most of these problems can be corrected with proper installation of a mechanical ventilation system – condensation on exterior walls and windows, excessive indoor humidity, poor indoor air quality, mold and mildew.
Since airtight post frame buildings do not allow for a transfer of indoor and outdoor air, they need one or more mechanical ventilation systems to help ensure a building receives enough fresh air and indoor air, along with excessive moisture and particulate matter, is properly vented outside. This can be accomplished with fans, air ducts and ventilation control systems with sensors monitoring indoor CO2 levels.
Trained professionals can look at a proposed post frame building’s critical systems, including HVAC, lighting and plumbing, and help determine best upgrades to reduce consumption. These may include custom mechanical ventilation systems and sensors to help control indoor air quality and achieve optimum ventilation.
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