For example, depending on roof color and orientation to the sun, attic air temperatures can exceed 55 C F when it is less than 38 C F outside. This hot interior can make the ducts and HVAC equipment work much harder to reduce the temperature to comfortable levels.
This is more pronounced if the ducts are leaky and HVAC equipment is drawing air from the attic space itself. In this design, insulation effectively separates the interior and exterior spaces while slowing down moisture flow so the dewpoint is not achieved within the building envelope. The two products most often used in an unvented attic assembly are medium- and low-density sprayed polyurethane foam SPF. In typical construction and climates, building code tables can be followed when using SPF as an insulation and air seal.
However, in cases where a vapor drive is consistently moving in one direction—such as cold storage applications or swimming pools—it is prudent to conduct hygrothermal modeling or calculations to determine if the proposed design is right for the application. Medium-density Moisture calculations of building assemblies i. It has also been tested to perform as an air-impermeable insulation. The physical properties and performance characteristics of SPF allow for the design of unvented attics and crawl spaces with minimal potential for condensation within.
Low-density Hygrothermal modeling of building assemblies and field observations demonstrate low-density SPF can be used in warm and mixed climates without an additional vapor retarder element. However, in colder climates, an additional vapor retarder element is required to prevent the potential for condensation. Low-density SPF has a permeance rating of between 8 to 15 per When tested as part of an assembly, low-density SPF can be an effective air barrier. The result is the physical properties of low-density SPF effectively separate inside and outside temperatures and minimize air infiltration, but allow a higher rate of water vapor transmission than medium-density SPF.
This facilitates the design of unvented attics in warm and mixed climates without an additional vapor retarder, but requires an additional vapor retarder element in colder regions. Either of these systems comes with a premium cost—typically two to three times the price of a blown fiberglass or cellulose vented attic assembly. Combination attic assemblies Modern residential design consists of elevations that can create various attic spaces within the same building.
Dormers are also attached, making it extremely difficult to insulate as an unvented assembly. Furnaces, ducts, and air-conditioning AC equipment run throughout the attic spaces on all levels, except the dormers and the space over the outside deck.
When determining how to insulate this attic space, various assemblies were taken into consideration and a hybrid combination of vented and unvented attic space was planned. The dormers and the attic space over the outside porch were sealed off from the rest of the attic by making a wall of plywood, then insulating the wall with closed-cell SPF. Following this, closed-cell SPF was also installed to the underside of the roof deck.
Building codes and attic assemblies Since , International Code Council ICC supplements to the International Residential Code IRC mean unvented attic assemblies have been accepted by the building codes in residential, but not commercial, applications. IBC requires ventilation in attics and crawl spaces and does not address the unvented attic concept. However, many building code officials have accepted unvented attics on a case-by-case basis when presented with compelling evidence —such as hygrothermal modeling of proposed assemblies—that the assembly will function properly.
The requirements have changed slightly over the years, but many of the elements have remained the same. Medium-density SPF at 51 to 76 mm 2 to 3 in. The vapor retarder classes are important to correctly specify unvented attic assemblies. Warranty Management. For Homeowners. For Pros.
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Roofing Roofing Tools. How Attic Ventilation Works Effective ventilation in an unfinished attic usually includes intake vents down low along the soffits, and exhaust vents up high at the peak or roof ridge. Why Adequate Roof Ventilation is Important Proper ventilation in your attic helps address excess heat and moisture that can otherwise wreak havoc on your home.
Exposure to this excessive heat can warp the roof sheathing and distort and prematurely age the shingles. If you are using loose fill insulation, use sheet metal to create barriers around the openings. If using fiberglass, wire mesh can be used to create a barrier. To completely cover your attic floor with insulation out to the eaves you need to install rafter vents also called insulation baffles.
Complete coverage of the attic floor along with sealing air leaks will ensure you get the best performance from your insulation. Rafter vents ensure the soffit vents are clear and there is a channel for outside air to move into the attic at the soffits and out through the gable or ridge vent. To install the rafter vents, staple them directly to the roof decking. Rafter vents should be placed in your attic ceiling in between the rafters at the point where your attic ceiling meets your attic floor.
Once they are in place, you can then place the batts or blankets, or blow insulation, right out to the very edge of the attic floor. Note: Blown insulation may require an additional block to prevent insulation from being blown into the soffit. While effective at pulling in cooler air, the biggest problem posed by this type of soffit vents is their positioning: Homeowners can too easily inadvertently block them when insulating the attic.
Unfortunately, blocked soffit vents are as just bad as no soffit vents, because they prevent fresh air from freely flowing into the attic. Houses with gable roofs may also have vents located on the side of the house as high as possible within the peak of the gable. Most of the time, their position near the peak of the roof allows heat to dissipate out through its cover.
Releasing all of the heat that rises and gets trapped in the attic can be achieved with one or a combination of the three following vent models in addition to the multipurpose gable vents mentioned above. With square-foot living space at a premium, many homeowners turn to their attics for a little extra room.
When the attic becomes part of the home to be heated and cooled, open-wall gable vents and roof vents are no longer feasible, but the underside of the roof the sheathing and rafters can still get blazing hot without airflow.
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