Thinking about creating an un-vented sealed attic?
Here is an area that deserves a great deal of attention.
Incorrectly assuming that what works in cold climates must work in hot climates, old outdated practices based on rumor, and lack of credible information have caused us to believe that venting an and is the proper method to maintaining a cooler and dryer attic and therefore help lower one’s energy bills. This myth couldn’t be further from the truth.
The fact is that venting an attic in Louisiana’s hot humid marine-like Climate Zone 2 will not only cause more heat and moisture to enter the attic area compared to a sealed non-vented attic but will also cause one to suffer higher energy bills as a result. In our local climate roughly 70% of the total solar heat-gain on our home comes downward through the roof and attic, whereas the walls make up roughly only 30% of the heat-gain.
Commonly-found leaky duct-work, the color of the shingles, lack of insulation, mechanical venting, among many other aspects of the home, will exaggerate the negative effects associated with these heat loads on our homes as well.
To understand how venting an attic actually has a negative impact we must consider the affects that simple indisputable forces of nature have on air movement (convection) and water vapor flow.
Water vapor is water in an invisible gaseous state. Water vapor travels through a process called vapor diffusion. Laws of thermodynamics state that vapor naturally travels from higher vapor pressure, or higher relative humidity, to lower vapor pressure, or lower relative humidity; and from higher temperature to lower temperature (which explains why the outside of your glass off iced tea immediately condensates on a warm humid day). Consider the typical scenario of a house in a hot and humid climate such as Louisiana. As we air condition our homes we are dehumidifying (lowering the vapor pressure) and cooling. Therefore, we are creating an inward driving force of water vapor into the home.
Through vapor pressure and temperature imbalances, water vapor is in a constant state of diffusion (migration) into our homes from the outside.
Air pressure differences will also result in high levels of moisture intrusion. Houses typically exist in a state of depressurization and are pulling moisture-laden air inward through its envelope. Leaky duct-work in the attic and stack affect are two major contributors of these pressure differences which make up a great deal of moisture coming into the home. Finally, as our bathroom power vents, dryer, stove-top exhaust vent, fireplace, water heater flu pipe, etc., push air out of our house moisture-laden air is being pulled into the house at the same rate. Against popular belief, our homes do not “leak air out”, they are constantly sucking air in.
A sealed insulated un-vented attic, however, will minimize or help eliminate some of these temperature, vapor pressure and air pressure differences. Typical phrases used to describe this type of sealed attic are: non-vented attic, enclosed attic assembly, conditioned attic, non-vented roof assemblies and cathedral-ized attic.
So what makes enclosing the attic with spray foam so good? First and far most it is simply insulating your attic, whether its adding to what insulation is already in place or eliminating the need to put insulation on your attic floor during new construction or a major renovation. Secondly, it greatly reduces depressurization of the home which, in return, allows the central air-conditioning system to operate much more efficiently and effectively at removing moisture and reducing temperature. By reducing the attic temperature typically within only 5 to 10 degrees from indoor conditioned room temperature and putting a sealed “cap” on the house stack effect is greatly reduced. Also, the 10% to 30% cool dry air that is mechanically forced out of typical leaky duct-work and into the attic will be contained within this sealed envelope and actually benefit the attic space by passively dehumidifying and cooling it further. In many cases, experienced HVAC contractors intentionally cool the attic with conditioned air from the central system in non-vented roof assemblies. Since the temperature of the air in the duct-work is about 50 to 55 degrees, maintaining a comfortable 80 to 90 degree attic puts much less heat-load on this duct-work rather than what would result from the typical 110 to 140+ degree vented attic temperature. By capturing and holding this cooled dehumidified air leaking from duct work moisture levels in the attic will decrease and so will the opportunity for condensation on duct-work.
Finally, by eliminating ‘attic to house’ pressure imbalances that are typical when venting an attic there will be much less air and moisture forced into the house from this depressurization. Remember, due to leaky duct-work, stack effect, bathroom and cooking exhaust systems, indoor dryers, fire places, and of course vented attics, our homes are always in a state of depressurization which means we are always “sucking” warm and humid outside air into our cool dry homes. Also, the amount of energy that is contained in water vapor is exponentially more energy than is contained in dry air. Therefore, in a vented attic, the size of our air-conditioning unit MUST increase to be able to handle the additional energy that it must remove in order to cool our home. Creating a non vented attic system is a major step in reducing these moisture and additional heat-load concerns.
Building Science – Hot Humid Case Study
Spray in Place Polyurethane Spray Foam Insulation, an Opinion Paper by Dr. Craig DeWitt, Ph.D., PE
Literature Review of the Impact and Need for Attic Ventilation in Florida Homes
Alleviating Moisture Problems in Hot, Humid Climate Housing
Under Deck Sprayed-In-Place Foam Insulation (GAF Warranty)
Energy Efficient Roofing
Pacific Northwest Energy-Efficient Roof Study
Roof and Attic Ventilation Issues in Hot-Humid Climates
Un-Vented Roof Summary
Louisiana’s Builders Guide – A Guide to Energy Efficient Homes 2009 Edition
WARNING TO HOMEOWNERS, BUILDING CODE OFFICIALS, ARCHITECTS AND CONTRACTORS!
EnviroGreen strictly adheres to Best Building Practice and educates its potential clients (and its competition) of these practices. However, the benefits of spray foam explained within the pages of this website hold true only if the proper foam is used with the correct application and installed per manufacturer guidelines according to code and basic common sense. Unfortunately, the Greater New Orleans Market has been flooded with spray foam contractors with little knowledge of building science and correct application. If spray foam is applied correctly then the end product is beautiful and well worth the investment. However, if it is done incorrectly then the end product can become a health & safety issue and an eventual legal liability.