To Vent or not to Vent the Attic? It’s all about air-sealing and insulation
In the early 2000’s, this was not a relevant question for most homebuilders. Attics were virtually always vented, in line with the thinking that ventilation was the best way to mitigate condensation and moisture issues, ice dams and other damage. Like many elements of home design, however, practices have evolved to seal homes more tightly against energy loss and pollution. Sealing the attic is one such evolution.
In our article “On a home hunt? Make sure Air Quality is on your home inspection list!”, sealing the attic was mentioned. If you are building a new home, you will basically need to decide where the thermal boundary will fall–will it exclude the attic, or include it? Here is a diagram to show what we mean (source: basc.pnnl.gov)
The pink lines indicate where insulation and air barriers will be. In diagram a), the upper thermal boundary is located at the attic floor. Notice that the roofline is broken at the eaves (small hooked part on lower ends) and there is also a “vent” in the ridge of the roof. In diagram b), the upper thermal boundary is located at the attic ceiling (underside of roof), and there are no breaks in the roof line for vents. Diagram a) will require that the attic is vented, but diagram b) will require that it is unvented. Why?
Sealing up uninsulated spaces (like diagram a) will cause the air in the space to become the same temperature as the outside, but without ventilation, moisture cannot escape and mold and rot can form. The alternative is to make sure the uninsulated attic receives plenty of outdoor air ventilation, OR include the attic within the thermal boundary, making it part of the “conditioned space” of the rest of the home (diagram B).
Factors to consider when determining where to locate the thermal barrier in new construction include climate, desire for additional living and storage space, building design and configuration, and location of HVAC.
Acceptable for all climates, providing sealing, insulation and ventilation methods work
Additional living space
Good for truss framing (can’t be used for interior living space anyway)
Good for complex interior ceiling/attic design,
Best for low-slope roofs where it is difficult to seal, ventilate, insulate and provide ventilation space above soffit eaves.
HVAC in attic
Acceptable with good duct sealing and insulation
More resistance against:
Let’s explore the types of ventilation that are common in attics today (source: roofingcalc.com):
Passive roof ventilation using:
Roof vents including box vents and turtle vents
Active roof ventilation using:
Turbine-style (whirlybird) vents
Motorized roof vents
Many people are convinced that “if it’s not broke, don’t fix it”, and this applies to the traditional design of vented attics. In truth, many attics have been able to ward off condensation, rot and mold for decades because they are well-ventilated and properly roofed. If you decide to stay with a vented attic design and still keep the HVAC ducts in the attic, there are two developments that can help save money on heating and cooling.
Laying the ducts on the attic floor and burying them in fibrous insulation can achieve energy savings and can be done at any point, before or after building (although it’s easier whenever new ducts are installed) (basc.pnnl.gov) The key to success with buried ducts is making sure that the ducts are airtight, encapsulating them with foam (ccSPF=closed cell spray foam) if necessary, and deeply burying them in loose insulation. Here is a diagram:
If the ducts are hanging in your attic and you decide to keep the attic vented and ducts where they are, you can have them encapsulated in closed-cell spray foam in order to minimize heat transfer.
Vented attics are not a good place for storage because of extreme heat, cold, and sometimes insect and pest issues. Without the above measures, vented attics are also not the best place for HVAC systems, for the same reasons. HVAC equipment will last longer and perform more efficiently when it’s operating in a temperate, dry climate. Ducts passing through a vented attic must be sealed tightly in order to avoid pulling unconditioned air and dust into your home.
The key to creating an unvented roof assembly is to keep the roof deck – the principle condensing surface in roof assemblies – sufficiently warm throughout the year such that condensation will not occur, or to prevent moisture-laden air from the interior of the home from accessing the underside of the roof deck. (basc.pnnl.gov) This can be accomplished by installing rigid foam insulation over the sheathing, or (more commonly) using spray foam insulation under the sheathing. Both require a high degree of air-tightness to avoid condensation. In an existing home, changing the thermal boundary from the floor to roof or vice-versa can be quite a bit of work, and often it’s best to do/combine this task when reroofing or other modifications, like HVAC system replacement.
For many people, it’s hard to imagine an attic that is not unbearably hot in the summer and frigid in the winter. However, when the roofline is properly insulated and sealed to the walls, you have a space that is less dusty, easier to work in when renovations are made, and a better space for HVAC equipment. It also provides a “buffer space” for all that conditioned air in your home, which tends to rise and escape through the ceiling area. Without excess temperatures bearing down on your ceiling in the summer or trying to vent warm air in the winter, energy costs can be moderated. If you do choose to convert to an unvented attic, be sure that the company you choose to seal and insulate it will do a thorough job of sealing, then apply sufficient spray foam insulation according to local code for your climate.
Whether you decide to go with a vented or unvented attic space, the best choice for air quality and energy efficiency will depend on how well you can seal and insulate. For vented attics, the living space below and any ductwork within need to be sealed and insulated from the attic space, which is an extension of the outside via ventilation. For an unvented attic, the whole attic is an extension of your home and therefore must be sealed and insulated from the outside at the roofline.