by Ben Gries, PE, LEED AP
Energy codes have advanced to the point where small breaches in envelope integrity can lead to claims of HVAC size insufficiency. Each version of energy codes reduces building energy use below the level of its predecessor. This has been accomplished through improved building envelopes, increased HVAC efficiency, reduced lighting power densities and lower internal equipment heat gains. These improvements lead to reduced HVAC equipment capacities and change the impact of each component on the HVAC load.
As building insulation requirements increase, the HVAC equipment decreases in size and capacity. This leads to less energy lost (through the envelope) and less energy used to condition the space (smaller more efficient HVAC uses less energy). This also means there is less HVAC capacity to accommodate poor envelope construction.
When HVAC was first added to buildings, the envelope was the primary factor to consider. At the time, building envelopes were designed to prevent water intrusion, not minimize energy use. Buildings were not insulated, which allowed air to infiltrate. To condition the buildings, HVAC equipment was sized to overcome the air infiltration and lack of insulation.
Today, building envelopes have changed. While still designed to prevent water intrusion, they are also designed to minimize energy loss. This is accomplished through high levels of insulation, air barrier requirements that call for all surfaces and penetration to be sealed, and doors and windows that must limit air infiltration. In addition, the improvement in office equipment and lighting have reduced power consumption and heat rejection to the space.
Code also provides requirements for sizing mechanical equipment to prevent “oversizing” and ensure energy efficient operation. Heating and cooling loads are supposed to be calculated at 70 and 75 degrees respectively, and outside temperatures are to be based on historical climate data. Where equipment sizes were once 300 sqft/ton and 60 btu/sqft, they are now 600 sqft/ton and 25 btu/sqft. The reduction in envelope loads and internal heat from lighting and electronics have reduced HVAC to minimal sizes and drastically improved energy efficiency. Buildings with energy use intensities (EUI) of 85 btu/sqft/yr are now 45 btu/sqft/yr and high-performance buildings use less than 35 btu/sqft/yr with many in the 20-25 btu/sqft/yr range.
This means that any breaches in the envelope can cause significant HVAC equipment performance problems. Missed insulation around window framing or above ceilings, holes in air barriers, gaps around doors — seemingly small oversights can lead to occupant complaints about space temperatures and increased energy use. HVAC systems designed to meet current code requirements do not have adequate capacity to overcome poor envelope construction.
The energy code recognizes this condition and requires envelope testing, either by building pressure testing or visual inspection. This requirement is little recognized and ill enforced. Responsible building owners and design professionals should require envelope commissioning on projects to ensure and verify envelope integrity. By requiring the envelope to be verified and tested, you are ensuring the building will perform as designed, providing occupant comfort with the least amount of energy consumed. Correcting envelope problems during construction is always more cost effective than finding and repairing them after occupancy. A commissioned envelope leads to fewer occupant complaints and less time spent documenting, investigating and correcting these problems during occupancy.