Why vent attics

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Find a Contractor. Search Search Button. Roofing Roofing Tools. Here we will discuss some of the effects of poor attic ventilation and share 5 practical solutions you can take today. An attic with sufficient ventilation will have equal levels of incoming and outgoing air.

Common conventions call for one square foot of attic ventilation per square feet of ceiling space. However, some authorities recommend one square foot of attic ventilation per square feet. If you live in an area with a humid climate, it may be best to go with the higher standard to ensure you have plenty of ventilation which can save you time and money down the road.

No matter what, be sure to check local building codes to see what the minimum requirement is for your area. Adding roof vents to your attic can ensure warm, moist air is able to escape, preventing heat buildup and condensation. You will need to periodically check your roof vents to make sure they are debris free. 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. A piece of rigid foam board placed on the outer edge of the top plate works very well for this.

Place rafter vents in between the rafters where the ceiling meets the floor. Photo courtesy of Doug Anderson. Add insulation around the rafter vent and out to the edge of the attic floor.

Skip to main content. The absence of cavity insulation in this design yields the highest condensing surface temperature of all of the designs presented. Note that all of the insulation is installed over the top of the roof deck.

In this particular design, the condensing surface is the air barrier membrane installed over the wood decking Photograph 4. This is the type of roof design most appropriate for swimming pools or other humidified building types in cold climates.

It is also one that provides the most durability to the primary components of the structure by keeping these warm and dry. In extreme snow regions it is necessary to add a vented air space between the roof cladding shingles and the rigid insulation to avoid ice damming see the digest on ice dams for more information.

This approach creates a vented-unvented hybrid roof assembly Figure 9. In extreme climates such as high snow load mountain regions a vented space should be provided between the roof cladding and the thermal layer to vent heat.

Instead of installing rigid insulation above the roof deck to control condensing surface temperature, air-impermeable insulation can be installed in direct contact with the underside of the structural roof deck Figure Air-impermeable insulations are typically low density or high-density spray foams Photograph 5. Netted or blown cellulose, fiberglass or rockwool insulation are not considered air impermeable.

In Climate Zones 5 or higher a Class II vapor retarder is required on the interior of the spray foam layer. If a high density foam is used in this assembly, a Class II vapor retarder is not required in Climate Zones 5 or higher as the high density foam itself qualifies as a Class II vapor retarder.

A thermal barrier is required to separate spray foams from occupiable spaces due to the fire performance of spray foam insulations. In Climate Zones 5 or higher see Map 1 the air-impermeable insulation, including any covering adhered continuously to the bottom side should have a vapor permeance of 1 perm or less i. This can be achieved by applying a vapor retarder paint over the interior surface of the low density spray foam or by installing a material layer in contact with the foam that has a vapor permeance of 1 perm or less.

High density spray foam insulation due to its impermeability properties can be installed directly under roof decks in any climate zone without any additional provision for vapor diffusion resistance - including Climate Zones 5 or higher see Map 1.

In this particular instance the high density foam insulation controls the access of interior moisture to the roof deck by air movement and by diffusion. This approach is similar in effect to the approach described in Figure 12 where rigid insulation is placed above the roof deck. Membrane, metal, and shingle roofs can experience night sky cooling that can depress roof deck temperatures significantly below ambient air temperatures. When membrane roofs and asphalt shingles are used it is typically necessary install rigid insulation above the roof deck or install air impermeable insulation below the roof deck.

The thermal resistance thickness of the rigid insulation is climate dependent and moisture load dependent. The higher the interior moisture load the higher the thermal resistance required for the rigid insulation. Membrane roofs and shingle roofs can experience night sky cooling that can depress roof deck temperatures significantly below ambient air temperatures. In general, shingles installed on unvented attic assemblies operate at a slightly higher temperature. This has impacts on the durability of roof assemblies.

A 2 or 3 degree F. All other things being equal, applying the Arrhenius equation Cash et. This is comparable to the effect of the installation of radiant barriers. However, the designs need to be climate sensitive. Control of ice dams, moisture accumulation and heat gain can be successfully addressed by both vented and unvented attic or roof design. The choice of the venting approach is up to the designer.

The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs. However, they require different approaches in different climate locations. Cash, C. Handegord, G. Lstiburek, J. Parker, D.