Indoor Environmental Technologies (IET)

Blower Door Pressure Testing, Unvented Attics and Florida Homes


I recently traveled to theCaribbean on a building science investigation. A large multi-national corporation owns several homes there for local executives. These homes had been plagued with dampness and mold growth problems for several years.  After several attempts to correct these problems, IET was called in, as we had work with their local affiliate for some years.

IET performed several types of investigations on this trip, but in this article we’d like to discuss the one that proved most useful, the blower door pressure test.

This method is to determine the air tightness of a structure. This has significant impact on its comfort, environmental quality and energy usage, especially in a hot-humid climate like that of the Caribbean (orFlorida).

Here’s how blower door testing works:

  • All windows, doors, fireplace dampers, etc. are closed.  Exhaust fans for kitchens and bathrooms are turned off.
  • A fabric panel is installed in one of the doorways, with a large, powerful variable-speed axial fan (6800 CFM max) venting to the outside.
  • A manometer (differential pressure meter) has tubes run to inside the home, outside the home and the fan.
  • The fan is turned on and gradually ramped up till it generates a 50 Pascal negative pressure inside the home relative to the outside.
  • The special manometer then calculates the airflow being used to generate this pressure and expresses it in CFM.

Pressure and airflow are entered into a computer program, along with data about the structure such as square and cubic feet, exterior surface area (square feet of walls plus ceiling or roof), AC SEER ratings, climate data, energy cost data, etc.

The program can then be used to calculate various data points. Among them:

  • Air changes per hour at 50 Pascals (ACH50).  This datum is widely used to compare air tightness of structures.  The three homes we tested in the Caribbean showed ACH50 of 16.46, 10.84 (the largest home and a two-story job, both tending to reduce ACH, since volume is inherently larger compared to exterior surface area), and 14.32.  To put this into perspective, the Florida Solar Energy Council recommends 2 to 6 ACH50, while IET prefers to see ACH50 near the lower end of this range.
  • Equivalent leakage area (ELA) or the area in square inches of all leaks combined that are needed to create a 4 Pascal negative pressure.
  • Cost to cool and/or heat the leaked air annually.
  • Estimated annual infiltration in CFM per person. This can be used to determine if the structure requires separate mechanical ventilation to meet ASHRAE ventilation guidelines and, if so, to determine how much ventilation is required.

While the structure is under high negative pressure, IET often works with contractors or homeowners to not only determine how much the structure is leaking, but also to locate and seal these leaks.  Infrared cameras and smoke tubes are very useful in these determinations. Sometimes leaks can be sealed as they are found until the structure reaches the target ACH50. In other cases we locate the highest volume leaks and leave them to be sealed by a contractor. We return after the work has been completed to measure ACH50 again, tracking down more leaks if we still haven’t reached target.  Rinse and repeat.

Where Buildings Leak

Buildings, by definition, leak at their pressure envelopes. For most buildings the pressure envelope consists of the floor (with a slab foundation there is no leakage), the walls and either the ceiling (if the structure has a ventilated attic) or roof (if no attic or unvented attic).  Complex structures with dormers, etc. complicate the situation considerably.

Most Florida homes have a ventilated attic, so the upper pressure envelope is at the ceiling of the top floor.  Unfortunately, ceilings are seldom designed or built to be even vaguely airtight.  Finished drywall is of itself a quite airtight construction method.  However, the tops of the walls are almost never sealed, allowing humid attic/outside air to penetrate down between walls and out into the room at wall penetrations such as pocket doors, medicine cabinets, electrical outlets, etc.  Quite often when inspection shows air infiltration at a wall penetration, particularly on an interior wall, if you actually track down the source of the leak, it’s at ceiling level in the attic.

Air leaks directly through the ceiling drywall are often found at soffits, behind crown moldings, and at all the multiple penetrations for sprinklers, can lights, supply and return vents, exhaust fans, speakers, etc.  One home we inspected had 32 such penetrations in the master bathroom alone. All were leaking air.

Due to the basic design and construction of most homes, locating and sealing each of the possibly hundreds of separate leaks through the pressure envelope in the attic is always time-consuming and is often impractical for accessibility and other reasons.


Our recommendation for the three homes in the Caribbean, and for many homes inFlorida with a ventilated attic, is to move the upper pressure envelope and thermal insulation from the top of the ceiling to the underside of the roof, converting the attic from a ventilated space to a semi-conditioned space.

For additional information on vented attics in an Floridaclimate, see:

By far the most efficient way we have found to accomplish this is by installation of Icynene foam insulation ( on the underside of the roof and sealing all attic vents.  Since the foam is sprayed on and expands after contacting the roof, it effectively seals gaps that are otherwise difficult to locate and seal.

Among others, advantages include:

  • Attic stays cool, usually no more than 5°F to 10°F above the inside conditioned temperature. Compare this to upwards of 140°F in ventedFloridaattics.
  • A lower attic temperature reduces radiant heat from the ceiling down into the home and often allows occupants to set thermostats higher without reducing comfort.
  • The air handler, which is often located in the attic, does not have to struggle against surrounding temperatures of perhaps 140°F to cool supply air down to the target temperature of perhaps 50°F.
  • Small leaks in supply ducting no longer depressurize the home, pulling in outside air. Similar leaks in return ducting no longer directly inject humid air into the home’s HVAC system.

Note:  We do not claim that an unvented attic or Icynene insulation are “magic bullets,” or that they are suitable for every interior dampness situation. Every structure is different and even when these methods are appropriate,howthey are used may make a huge difference in the results. An experienced team is crucial for success.

While most Florida homes are not quite as leaky as ourCaribbeanhomes, reducing airflow through the building envelope and bringing ventilation air under control has multiple benefits for the owners and occupants.  IET highly recommends blower door testing to those who wish to take advantage of these benefits.

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