R-value
Vented attic VS. non-vented attic (closed attic assembly)
Fresh air requirements and proper ventilation of appliances
The building science community has made enormous strides over the past 10 years researching and scientifically proving ways to design, construct and operate living and working environments for better performance in sustainability and energy efficiency. Building science studies moisture dynamics, indoor air quality, sustainability, and strongly promotes energy efficient yet affordable building technologies.
Thermacote Systems is committed to expertise in building science and leading foam insulation contractors in this field of study so that our customers can be educated and understand their options. Thermacote Systems encourages owners, builders and subcontractors to view each building as a total system and to synergize the components of that system to work optimally together. By viewing a building as a system instead of separate components, Thermacote Systems strives to achieve greater value and return on investment for our customers. We believe in being good stewards of our company, your investment and our environment.
R-value
R-value is the rating given on the thermal resistance qualities of a material. The r-value per inch of fiberglass, cellulose and open cell SPF is virtually the same. The problem is that r-values are tested in a static vacuum in a laboratory and do not take into account real world conditions such as moisture and air movement. Did you know that a 2X4 has an r-value of 3, a 2X6 has an r-value of 4 and a window with a UV rating of .33 has an r-value of 3. Add those components together and that means that 50% of a building usually has an r-value of about 3.
The advantage SPF has over conventional forms of insulation is that SPF can be an air and vapor retarder, which fiberglass and cellulose cannot. When an architect or builder considers an insulation product, they must consider how the product will perform in all climate conditions for the building’s physical location. SPF has high R-values. For example: 6 inches of open cell spray foam insulation on a roof deck will perform better than R-value of over 38 and 3 inches of closed cell spray foam insulation will perform better than R-value of over 38. Engineers recommend using 3.5 inches of spray foam insulation in a 2 X 4 wall cavity and 6 inches of spray foam insulation on the roof deck. While the laboratory R-value of open cell spray foam insulation at 3.5 inches is R-13 and the laboratory R-value of closed cell spray foam insulation at 1.9 inches is R-13, these products outperform other products because SPF is an air and/or vapor retarder; two defining characteristics which are critical in real world conditions, but irrelevant in a laboratory vacuum testing. Fiberglass and cellulose are not air barriers or vapor retarders.
Vented attic VS. closed attic and the science behind It
(Source of information provided by buildingscience.com)
Unvented roof assemblies are also called closed attics or non-vented attics. A closed attic assembly is created by eliminating all ventilation openings in an attic such as soffits and ridge vents (spindle, gable and mechanical vents) and by creating a thermal boundary and air barrier on the internal shell of a structure. See the visual depictions below.
By moving the air and thermal boundary to the roof deck, air leakage between the occupied space and the attic is mute. The attic becomes a semi-conditioned space by process of air diffusion though ceiling penetrations such as HVAC equipment, exhaust fans, attic access doors or hatches, recessed lighting, and plumbing and electrical holes. The attic does not require ducted supply. A typical closed attic temperature is only a few degrees higher than the conditioned space during the highest summer temperatures. This allows the HVAC system to run more efficiently and to last longer with no condensation problems.
In conventionally built vented attics, pressure created by mechanical equipment, wind and air temperature changes cause uncontrolled air infiltration and movement resulting in great energy waste, increased humidity and strain on HVAC equipment. As conditioned air moves from occupied space to vented attics, more conditioned air is required to replace the significant change in air volume. In hot humid climates like North Texas, outdoor air can condense on HVAC equipment and ductwork located in vented attic spaces causing moisture problems. HVAC systems have to work harder to pull more moisture out of the air.
In unvented roof assemblies, conditioned air movement from attic to outside is eliminated resulting in energy savings, controlled humidity and better indoor air quality. Because the air and thermal envelop is on the external shell of the structure, large thermal gains in the attic are eliminated, overall air changes are significantly reduced and wind driven rain is controlled. Closed attic assemblies also have proven to perform better in case of a fire because the absence of vents reduce wind and air currents that fuel a fire. Property damage can be reduced because fires burn slower because of the air barrier. Closed attic assemblies also allow for increased living space without increasing the footprint of the structure.
Benefits of closed attic assembly
- Complete air sealed structure eliminates air infiltration and air loss resulting in energy savings of 50-70% compared to conventionally built structures
- Greater comfort by controlling humidity levels below 50%
- Healthier indoor air quality by reducing allergens, dust, contaminants
- Quieter living and work environment
- Reduced initial construction costs (size of HVAC, no radiant barrier, no venting requirements)
- Increased living space or conditioned storage space
- Eliminates water intrusion and possible moisture problems from wind driven rain during severe thunderstorms or high winds making the structure more durable
- Increased service life of equipment and appliances in the attic. Closed attic assemblies can double or triple the life of HVAC equipment because every 10 degrees in attic temperature can reduce the life by 50%
Fresh air requirements – build tight with Thermacote spray applied foam, but ventilated right
One of the most substantial benefits of using SPF is its ability to regulate and allow for superior air quality. Because a SPF structure with a Thermacote superior air seal package is so “tight”, it is important that each structure is designed to optimize air quality. A structure utilizing SPF and a Thermacote superior air seal package will realize a very low blower door test score, which is a method used to measure air leakage through a structure’s shell. Some buildings will require mechanical air exchange. This can be done numerous ways with supply, exhaust or balanced mechanical ventilation. Air exchange requirements should always be determined by building scientists or an engineer as part of the thermally engineered building load analysis, or ask your HVAC installer about ventilation in accordance with ASHRAE 62.2.
Proper appliance ventilation
Because SPF is a superior air barrier, properly ventilating gas appliances is extremely important. All gas appliances must be vented to the outside and gas furnaces located in a closed attic should have sealed combustion (also called 90% furnace). Also, exhaust fans and cook top vent hoods should vent to the outside of the structure.
Top of Page
|
Closed attic.
