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Ventilated vs. Unventilated Attic Spaces
April 26, 2011
Which is more energy efficient and why?
Since the latter half of the 1900’s, it was standard practice to build a home with ventilated attic spaces. Unfortunately, in Winter Park’s hot and humid climate, this building technique doesn’t make sense. To understand why, we must understand the origins of the ventilated attic before exploring the technique that is more appropriate for our climate in Winter Park.
During the post World War II housing boom, insulation started to be widely used and homes in colder climates began to experience condensation issues. The homes experiencing problems had insulation above the ceilings and unventilated attic spaces. In colder months, condensation began to appear on the bottom of the roof decking in the attic spaces and gravity conveyed the moisture to the ceilings of the living spaces. This problem did not occur in homes without insulation however; many people blamed the new insulation for the issue. Insulation wasn’t directly at fault for creating condensation, but the use of insulation did affect attic temperatures. In un-insulated homes, attic spaces are warmed by leakage from the living space into the attic. In insulated homes, the attic space temperature is significantly reduced – as heat is retained inside the living space. Moisture from the living space continued to travel into the attic and this created ideal conditions for condensation as the moist air escaped the living space and formed condensation on the cold roof decking.
Building scientists at the time discovered the causes for the issue and recommended the use of ventilated attic space. This technique resolved the condensation issue experienced in insulated homes. Unfortunately for us, this building practice for colder climates was imported into our hot and humid climate. It wasn’t until the late 1990’s that building scientists began to explore different building practices for our climate that improve energy efficiency. They explained that winter temperatures in our region are not low enough to cause roof condensation and that homes in our area face more risk of humidity levels from outside the building than from inside. They also came up with recommendations that would minimize the introduction of hot and humid air into our attic space and also improve the efficiency of cooling homes.
It makes more sense in Winter Park to have a sealed attic space, but this doesn’t mean you can just go out and seal your soffit and roof vents. A sealed attic space means more than losing the vents – it means changing your type of insulation and moving its location to the bottom of the roof decking. The most effective technique is to apply spray foam insulation directly to the bottom of the roof decking. One of the more well-known foam brands is called Icynene, but there are numerous foam products on the market that achieve or exceed its performance.
This insulation change causes the attic space to become part of your conditioned ‘living space’. A conditioned attic space provides one of the most significant benefits for a home’s energy efficiency, as this resolves one of the biggest losses in efficiency of an air conditioning system. It is surprisingly to learn that almost all ductwork leaks air into unintended locations – like the attic (typically between ten and twenty percent in new installations). Another source of energy savings is related to the lower temperature in sealed attic spaces - roughly 5-10 degrees warmer than the living space. On the other hand, ventilated attics can see temperatures soar to 150 degrees and above. It is easy to understand that air conditioning systems perform more efficiently when their ductwork runs through cooler temperatures. Some ancillary benefits of this building technique include: improved building tightness (reduced infiltration of outside humid air), improved indoor air quality (less dust, pollen, dirt), and even stronger construction for hurricane wind mitigation (when structural foam is used).
Homes with sealed attic spaces must also consider interior moisture control for the few weeks in the year that their air conditioning systems are not dehumidifying the air. Sometimes, supplemental fresh air and dehumidification systems are necessary to ensure proper humidity levels inside the home. Well designed mechanical systems overcome potential moisture issues.
The cost to use spray-foam in a home is typically two to three times higher than traditional batt/blown in insulation. This cost premium is partially offset immediately by smaller a air conditioning system (roughly 30% smaller) and over the long run through lower utility bills and improved occupant health.
Architects and builders need to stay up to date as new building techniques prove themselves in the market and improve home construction. If you are considering a renovation or new construction to incorporate a sealed foam attic at your home in Winter Park, be sure to hire qualified and licensed contractors who have proven experience with this building technique.