| |
Creating An Unvented Attic
Venting attics
So you're thinking about creating an unvented 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 Incorrectly assuming that what works in cold climates must work in hot climates, old outdated practices based on rumor, and lack of credible information have caused us to believe that venting an attic is the proper method to maintaining a cool attic temperature 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 or marine climate will not only cause more heat and moisture to enter this area compared to a sealed non-vented attic but will also cause one to suffer higher energy bills as a consequence. In typical houses the heat load from hot vented attics contributes well over 10% of the total cooling load required. And, during our extremely hot and muggy summer months we can easily assume up to 20%. To understand this we must consider the affects that simple indisputable forces of nature have on air and moisture flow.
Water vapor is water in an invisible gaseous state. Water vapor travels through a process call vapor diffusion. Vapor naturally travels from higher vapor pressure to lower vapor pressure and from higher temperature to lower temperature. 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 and cooling. Therefore, through vapor pressure and temperature imbalances, water vapor is in a constant state of migration into our homes from the outside and from our attic.
Air pressure differences will also result in moisture travel. Houses typically exist in a state of depressurization and are pulling moisture-laden air inward through its envelope. Leaky ductwork and stack affect are two major reasons for creating these pressure differences which make up for a great deal of moisture into the home.
A sealed insulated unvented attic, however, will minimize or eliminate these temperature, vapor pressure and air pressure differences.
Unvented attics
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 12 degrees from indoor conditioned room temperature and putting a sealed "cap" on the house stack effect (PDF) is greatly reduced. Also, the 10% to 30% cool dry air that is mechanically forced out of typical leaky ductwork and into the attic will be contained within this sealed envelope and actually benefit the attic space by 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 ductwork is about 50 to 55 degrees, maintaining a comfortable 75 to 90 degree attic puts much less heat load on this ductwork rather than would result from the typical 110 to 140+ degree vented attic temperature. By capturing and holding this cooled dehumidified air leaking from ductwork moisture levels in the attic will decrease and so will the opportunity for condensation on ductwork.
Finally, by eliminating 'attic to house' pressure imbalances that are typical when venting an attic there will be much less air and moisture leakage into the house from this depressurization. Remember, due to leaky ductwork, 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. Creating a non vented attic system is a major step in eliminating these concerns.
EnviroGreen has many case studies where homeowners' AC units would continuously run and were unable to cool their homes to desirable temperatures. Many were faced with spending $6,000 to $10,000 on new air-conditioning systems and ductwork because the system was slightly under-sized, the ductwork was leaky, or the cooling capacity and efficiency decreased over the years. After EnviroGreen foamed and sealed these attics these homeowners no longer experienced a need to replace their HVAC systems and ductwork, and paid only a fraction of the cost of doing so. Foaming and sealing their attics actually created an optimally cooler and tighter environment for their AC systems to operate in and allowing them to perform more effectively and efficiently.
A word from our tech guy: While doing inspections on Louisiana homes for almost ten years I have heard so many falsehoods from unknowledgeable HVAC contractors, builders, architects, roofers and contractors concerning unvented attics. The biggest one of these is, "If you seal and un-vent your attic the moisture will build up inside and create attic condensation issues." At which point I always respond, "From what and from where does this moisture build up?" I usually get very colorful answers which include arguments that defy the laws of vapor travel and the mechanics of condensation. Below is a list of just a few resources to view for more information on attic shingle temperatures, roof warranties, and the over-all general importance of creating an unvented attic.
» Hot Humid Climate Case Study for New Orleans, LA
» Moisture Control for New Residential Buildings
» 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
» Unvented Roof Summary
WARNING TO BUILDING CODE OFFICIALS, ARCHITECTS AND CONTRACTORS!!!
The current International Building Code (IBC) requires spray polyurethane foam to be separated from the building occupants with a 15 minute thermal barrier (e.g. ½" sheetrock) or in attics and crawl spaces by an ignition barrier for fire safety reasons.
The exception to this above code is if the spray foam is approved according to ICC Evaluation 4.3.2 to spray to the roof decking without this required ignition barrier. To be approved the product must pass an SWRI Procedure 99-02 Appendix 'C' Crawl Space Comparison Test.
The majority of spray foams sold in the Metro New Orleans Area do not meet such acceptance criteria and therefore are required by international and local building codes to be covered by ½"sheetrock or another necessary ignition barrier.
All EnviroGreen spray foam products meet these stringent testing criteria.
|
| |
|
141 Robert E. Lee Blvd., #227 New Orleans, LA 70124 Phone: 504-344-6762 Fax: 888-893-7259
 |
©2010 EnviroGreen Insulation | Designed by Vektor Web |
|
|
|
