Unvented Attics
Posted on Feb 19th, 2009 by Doug Marsh VP, CR, CAPS, GBP | 0 Comments
Unvented attics
I heard the other day that, possibly in a few years, that, in Austin at least, loose fill insulation in attics will become a thing of the past. There is now 10 years of data available regarding unvented attics: some of this info is from Florida while a famous test case was run in Las Vegas with similar houses that had tests run on them. An unvented attic is still news to most people because of the prevailing knowledge that attics have moisture in them and that’s why you have to vent them. It turns out that a vented attic is an idea that came out of the 40’s and was universally adopted even though there are a number of climate zones in the United States: Cold/Dry; Hot/Humid; Cold/Humid; Hot/Dry. In these climate zones vented attics behave in different ways. Here in Austin, TX a vented attic is usually full of hot humid air which sits on top of your air conditioned space. HVAC ducting, which is usually insulated only to R 4 or R 6 (code is now R 8), sits in this hot humid space. The furnace/air handler often is located in this same space and is where the cold air originates. The hot moist air in the attic surrounds the unit and the ducting which carries 75 + - degree air and permeates the insulation at the ceiling level compromising the R value of that insulation by as much as 50 percent. Add to that the fact that all the wood framing including the roof deck is a heat sink (the framing heats up and retains heat from the day) which, as the day wears on, continues to radiate heat within the attic.
An unvented attic is achieved by spraying usually, ½ lb +- not toxic foam against the roof deck and at the eaves and gables to completely seal up the attic thereby not allowing hot humid air to get into the attic. This brings the attic within the building envelope. The foam has a low perm rating (moisture penetration rating) and a long heat gain cycle which means that the day to night cooling cycle happened before heat and moisture can penetrate the foam. The result is a cool attic which can be 6-10 degrees warmer than the living space below. This understandably has an enormous effect on cooling costs since any duct leakage occurs within this, now, air conditioned space; the huge heat load on top of the house is eliminated; the air handler is in the cool space. The attic is cool and dry. The HVAC Contractor down sizes the equipment due to the impact that this new attic has on the heating/cooling load on the house. There is more to this as moisture generated within the living space must now be addressed.
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