Vented or Unvented Attic?
The Approach to the Attic Depends on Whether It's Going to Be a Living Space
Determine whether to insulate the floor or the roof
Using the attic as a living area is a good, green way to take full advantage of the available floorspace for a given house footprint. Why let a whole floor go to waste when the alternative might require building a bigger foundation?
An attic's intended use determines where and how it should be insulated and air sealed. There are basic principles involved, and within each, there are variations. (Isn't that what makes green building so much fun?)
Open-cell vs. closed-cell polyurethane foam
Roofs above conditioned attics are usually insulated with spray polyurethane foam. Builders can choose either closed-cell (2-pound) foam or open-cell (1/2-pound) foam.
Closed-cell foam. When typically installed, closed-cell foam is a vapor retarder as well as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.. When sprayed on the underside of roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , it prevents any interior moisture from reaching the cold roof sheathing.
Open-cell foam, on the other hand, is vapor permeable. When used in a cold climate, the interior face of the foam should be covered with a vapor retarder to prevent interior moisture from diffusing through the foam and condensing against the cold roof sheathing. Some installers spray a vapor-retarder paint on the inside face of cured open-cell foam.
Many building inspectors require that spray polyurethane foam installed in a conditioned attic be protected by a layer of 1/2-inch drywall or an equivalent thermal barrier. The requirement applies both to open- and closed-cell foam.
Batts don't work well between rafters
As long as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. is in place, almost any type of insulation can be installed on an attic floor. Many types of insulation, including fiberglass batts and cellulose, perform poorly between rafters, however.
Problems arise when interior air or water vapor has access to cold roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , leading to condensation. These problems are difficult to prevent with fiberglass batts or cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection..
The best way to insulate rafter bays is with spray polyurethane foam. This creates an air barrier between the attic and the roof sheathing.
In new construction, a conditioned attic can also be created by using structural insulated panels (SIPs) in the roof installation. Similarly, a conventionally framed roof can be insulated from above with rigid foam.
Air-barrier continuity is crucial
One of the biggest challenges builders face when completing an attic is getting the air-barrier details right. When a conditioned attic is insulated with spray polyurethane foam, most air leakage problems are addressed by the insulation. However, builders should ensure air-barrier continuity at the attic perimeter and at chimneys. For more on the topic, see "Air BarrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. Details" below.
Venting, insulation, critters, and R-values
Requirements for unvented conditioned attics are found in Section R806.4 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.. Insulation installed in direct contact with the underside of the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. must be air impermeable.
Venting needs to reach all air spaces above attic insulation [806.1] and have an area greater than 1/150 of the attic square footage [806.2]. This area can drop to 1/300 if 80-50% of the venting is 3 ft. above the eave vents or if a less than 1 perm vapor barrier is on the warm-in-winter side of the ceiling [806.2]. Keep critters out of your vents with 1/8 to 1/4 in. screening [806.1].
Installers need to post a certificate of R-value if it isn't clearly marked on the insulation [1101.4]. In the warmest climates, attic insulation needs to be R-30, but in most cases it's best to have a considerably higher R-value [1102.1]. Whether you enclose recessed lights in an airtight, insulated box, or use properly sealed IC-rated fixtures, inside air shouldn't leak into your attic [1102.43].
Illustration: from Code Check Building 2nd Edition. click to buy .
To vent or not to vent, that is the question.
In the broadest sense, an attic is simply a space between a pitched roof deck and the rest of the house. Depending on how the house has been framed, an attic can be anything from a valuable storage or living area to a space just large enough for heating and ventilating ducts.
When an attic is conditioned, the thermal and air barriers are located in the roof; in an unconditioned attic, insulation and air barriers are in the ceiling that separates the house from the attic. Conditioning an attic raises questions about the type of venting, so these two components need an integrated approach for high performance.
If your pitched roof creates a decent-sized space, why not use it? Detailing the insulation of a roof can be tricky and expensive, but the resulting space can make good sense — economically and environmentally.
In many parts of the country, builders put HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. or plumbing systems in the attic rather than in the living space, where they belong. If these components must go into the attic, then the air barrier and insulation should be moved up to the roof line, making the attic unvented and semi-conditioned. Conditioned or semi-conditioned attics use a little more energy than unconditioned attics, but the amount is insignificant compared to heat loss through ducts and pipes in a vented, unconditioned attic.
By convention, the minimum amount of ventilation is described as a ratio of vent space to ceiling area. Building codes usually require 1 sq. ft. of vent space for every 150 sq. ft. of insulated ceiling; the ratio can be adjusted depending on several factors.
In an unconditioned attic, ventilation serves a number of purposes. The flow of cold air beneath the roof deck in winter helps prevent ice dams and remove moisture that works its way up through the ceiling and insulation. During the summer, vents help carry away the hot air that accumulates under the roof deck, lowering cooling costs in the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. below.
Soffit and ridge vents
The usual way to vent a roof is with continuous soffit vents acting as air inlets and a continuous ridge vent to act as an air outlet. In most cases, ventilation takes place beneath the roof deck. Builders using this method must install vent channels in each rafter bay before insulation is installed.
Insulation should not be allowed to block the flow of air beneath the roof deck. Above the top plates of exterior walls, ventilation chutes should be installed in each rafter bay to keep insulation away from the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . In most cases, a vertical wind dam should be installed at the base of each ventilation chute.
If the insulation is spray foam, it's important to choose rafter baffles that are sturdy enough to resist the forces created by the expanding foam. Many builders prefer to make site-built channels using 1"x1" battens and narrow pieces of rigid foam.
Venting above the sheathing
Vent channels can also be built above the roof sheathing. Builders following this method usually install several layers of rigid foam above the roof sheathing, followed by parallel lengths of 2x4s installed above the foam, each positioned above a rafter. A second layer of roof sheathing is then installed above the 2x4s.
Thick insulation is easier if the attic is unconditioned
Laying down unfaced fiberglass batts or, better yet, blowing cellulose between the joists in an unfinished attic can be the fastest, easiest, and most affordable way to achieve high R-values. As long as a house is designed with an uncomplicated roof, the best way to detail the attic is as vented unconditioned space.
Go to the top of an old farmhouse and what you will almost certainly find is an open attic, its rafters and roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. in plain view. Ventilation could take the form of operable windows or louvered vents in the gable-end walls. Any insulation would be in the floor that divides the attic from the rest of the house.
This is essentially how many houses are built today. The space immediately beneath the roof is “unconditioned.” Whether the attic has enough headroom for storage or is too low to be of any practical use, these houses have one common feature: the attic and the living space below are separated by insulation and an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both..
The two most common problems with unconditioned attics are air leaks at the ceiling plane and not enough insulation, which can result in ice dams in colder climates. It's especially critical to seal air leaks and install adequate insulation at the attic perimeter, where ice damming problems begin. But this is also important in hot climates, where attics can get extremely hot; piles of insulation can make it a lot easier to keep a house cool.
To ensure that there is enough room for insulation at the attic perimeter, use raised-heel trusses or raised plate framing details. If design issues limit the available space for insulation, spray foam may be required at the attic perimeter.
Unconditioned attics are cheap and easy
In an unconditioned attic, it's much less expensive to install a thick layer of insulation on the attic floor than between the rafters, and it's easier to ventilate the roof. Perhaps the biggest bonus of all is that it's a lot easier to pinpoint a roof leak when the roof sheathing isn't hidden by insulation.
Keep roof lines simple
When the attic has headroom for living space, it may be worth it to condition it and insulate the roof. If mechanical equipment and ductwork are located in the attic, it's smart to seal the vents and treat it as semi-conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. — insulated but not heated.
Some people argue that if we don't ventilate wall cavities, why do we need to ventilate roof cavities? Aren't roofs just like walls that are leaning? The answer is, well, sort of.
Conditioned roof assemblies are becoming more common because they are being allowed by local building officials more often. To keep these assemblies dry, they must be airtight because moisture that gets in has a tough time getting out.
A better understanding of how heat and moisture move through building materials, in combination with more sophisticated building products, have cleared the way for successful designs that can be adapted to all climate zones. Just like walls, unvented roof designs should be based on climate. Where temperatures dip low enough to cause condensation, either moisture must be kept away from the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. or the sheathing temperature must be kept high enough to prevent condensation. Exterior rigid foam above the roof sheathing or closed-cell foam insulation under the roof deck can achieve this in all climates.
Unvented roofs can perform well as long as they are properly detailed to limit moisture transfer from the interior. Construction details vary depending on climate, but closed-cell spray polyurethane foam (specifically allowed by Section R806.4 of the International Residential Code) can be used anywhere.
For more information on different ways to create an unvented conditioned attic, see Creating a Conditioned Attic.
Closed-cell spray foam to the rescue
Spraying closed-cell polyurethane foam directly onto the underside of the roof deck to form an air, vapor, and thermal barrier, is a foolproof but expensive way to insulate cathedral ceilings or conditioned attics. Closed-cell foam is denser and heavier than open-cell foams and also has higher R-values. According to a description of this technique by the Building Science Corp., a minimum of 1 inch of closed-cell foam acts as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., while a minimum of 2 inches creates a vapor retarder.
The foam sticks tenaciously to the roof deck and expands to fill cracks and voids that in a cold climate might allow warm, humid air to reach the back of the roof deck and condense. In a hot climate, the foam blocks humid air from entering the house, where it could condense on cooler surfaces.
Closed-cell foam can be combined with other types of insulation, such as fiberglass or cellulose, to get the benefits of an air or vapor barrier and to meet local energy guidelines at a lower cost.
One criticism of unvented roofs insulated with spray foam is that roof leaks are difficult to detect and could cause extensive roof sheathing rot before the homeowner notices any problems.
An air barrier between the living space and the attic prevents the escape of conditioned air and interior moisture into the attic.
In cold climates, some builders use a continuous layer of plastic at the ceiling line as a combination air barrier and vapor retarder. Those using the airtight drywall approach install ceiling drywall with gaskets. In either case, penetrations through the attic floor must be carefully sealed with caulk or spray foam, and attic access hatches must be weatherstripped.
Maintaining this air barrier can be difficult when the ceiling is punctured by recessed lighting fixtures or heating and cooling ducts.
A home's air barrier should always be contiguous with its thermal barrier. In a home with a conditioned attic, the air barrier follows the slope of the rafters. As long as the rafters are filled with spray foam insulation, it's fairly easy to achieve a high-quality air barrier.
Houses with unconditioned attics use a variety of insulation materials. Unless the attic floor is being insulated with spray foam, the integrity of the air barrier must be verified before attic insulation is installed. Most of this air barrier work is performed from the attic. Steps will vary depending on the home's construction details, but may include:
- Installation of site-built boxes around recessed can fixtures. These can be built from scraps of rigid foam, using canned foam as an adhesive.
- Installation of caulk or spray foam to seal the cracks between partition top plates and drywall.
- Sealing oversized holes for plumbing vent pipes and electrical wires.
- Sealing gaps around chimneys.
- Sealing the tops of vertical chases enclosing plumbing pipes, ducts, or flues.
Before the insulation is installed, these steps may also be required:
- Installing insulation dams around the attic access hatch.
- Installing vent baffles and dams above the top plate of exterior walls to protect insulation from wind-washing.
BuildingScience.com: Understanding Attic Ventilation
A Vent on Venting by Joseph Lstiburek, P. Eng.
- Don Mannes/Fine Homebuilding #190
- Daniel Morrison
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