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Vented vs. Unvented Attics
When insulating an unvented roof assembly, prefers closed-cell to open-cell foam because it’s both an air barrier and a vapor retarder.
To finish an unvented cathedral ceiling insulated with closed-cell foam, most codes require a layer of 1⁄2-inch drywall or an equivalent thermal barrier (top).
Depending on local code, the Spray Foam in an unvented, or “cathedralized,” attic (above) may not require drywall covering unless the area is accessible for servicing equipment.
In some cases, the foam may have to be sprayed with an intumescent coating.
How to insulate my attic And Preparing For Winter. Generally, any attic insulation project is going to cost between $2.50 to $3.50 per square foot, insulating attic roof with Spray Foam insulation will save money, types of attic Spray Foam insulation ” Open Cell – Closed cell ” how to insulate an attic roof rafters When properly installed, either type of Spray Foam insulation will act as a perfect air barrier, sealing off all attic bypasses.
Closed-cell polyurethane foam provides the insulation, air barrier, and vapor retarder necessary for an unvented attic assembly
Insulating Unvented Attics With Spray Foam
insulate unvented assemblies. Sometimes the building has a flat roof or a cathedral ceiling that would be difficult or impossible to ventilate. In other cases, the existing framing cavities are too shallow to accommodate a sufficient amount of insulation plus a vent space. And occasionally customers request unvented attics because they make the building more comfortable and energy-efficient.
Why Install Roof Venting?
Traditionally, venting has been used to deal with problems that occur when heat or moisture escapes into the attic
In cold climates, the escaping heat can cause ice dams by melting the snow on the roof. Venting the space above the insulation helps keep the roof cool by carrying this heat away. If moisture enters the attic through the ceiling (usually as an air leak), the vents are supposed to allow it to exit before it condenses on something cold. However, ventilating above fiber insulation comes with an energy penalty.
Fiber insulation is designed to be enclosed in an airtight cavity. When air flows over and through fiber insulation, there is a substantial loss of thermal performance.
Also, most hvac ducts and air handlers leak to some degree, so when these are installed in vented attics, conditioned air is lost to the exterior. And because vented attics are subject to extreme high and low temperatures, additional energy is lost through the thin insulation on the hvac equipment.
In cooling climates, venting the attic can bring humid outdoor air into contact with attic ductwork. If the ducts are not properly insulated, they can be cold enough to cause condensation. Venting and shingle temperature. It’s a common misconception that coderequired venting significantly lowers the summer temperature of the roof surface. In fact, tests have shown that it lowers the surface temperature of asphalt shingles by at most about 5°F.
For many years, roofing manufacturers required that shingles be installed over vented substrates, but today, several companies — including Elk and CertainTeed — will guarantee shingles installed over properly constructed unvented roofs.
In cold climates, the escaping heat can cause ice dams by melting the snow on the roof. Venting the space above the insulation helps keep the roof cool by carrying this heat away. If moisture enters the attic through the ceiling (usually as an air leak), the vents are supposed to allow it to exit before it condenses on something cold. However, ventilating above fiber insulation comes with an energy penalty.
Fiber insulation is designed to be enclosed in an airtight cavity. When air flows over and through fiber insulation, there is a substantial loss of thermal performance.
Also, most hvac ducts and air handlers leak to some degree, so when these are installed in vented attics, conditioned air is lost to the exterior. And because vented attics are subject to extreme high and low temperatures, additional energy is lost through the thin insulation on the hvac equipment.
In cooling climates, venting the attic can bring humid outdoor air into contact with attic ductwork. If the ducts are not properly insulated, they can be cold enough to cause condensation. Venting and shingle temperature. It’s a common misconception that coderequired venting significantly lowers the summer temperature of the roof surface. In fact, tests have shown that it lowers the surface temperature of asphalt shingles by at most about 5°F.
For many years, roofing manufacturers required that shingles be installed over vented substrates, but today, several companies — including Elk and CertainTeed — will guarantee shingles installed over properly constructed unvented roofs.
How Unvented Assemblies Work
A properly constructed unvented attic is immune to the moisture problems that occur in vented assemblies and is much more likely to be energy-efficient.
In an unvented assembly, anything below the insulation — including an attic — is considered conditioned space. Turning the attic into conditioned space saves energy; if heat or air escapes from the hvac equipment, it remains within the conditioned space
If enough energy is saved in this manner, the hvac system can actually be downsized, reducing installation and operating costs. A number of insulation materials can be used in an unvented assembly, but the one with the greatest applicability across the country is SPF. It’s an extremely effective insulation and air barrier all in one, and since it’s sprayapplied, it conforms to irregular shapes that otherwise might be difficult to insulate and seal.
Despite the multiple brands of SPF, there are only two main kinds: open-cell foam and closed-cell foam. Chemically, all brands are nearly identical — contrary to some advertising claims — and contain about the same proportion of
agriculturally derived resin from corn, sugar beets, sugarcane, or soybeans.
None of the spray foams contain formaldehyde or use toxic or ozone-depleting blowing agents. The important differences between products have to do with density, R-value, and permeability.
Open-cell foam. The typical open-cell foam weighs 0.5 pound per cubic foot and has an insulation value of R-3.5 per inch of thickness. This type of foam is relatively permeable; at 5 inches thick it is rated at about 10 perms. Open-cell foam is an air barrier but not a vapor retarder. When sprayed, open-cell foam expands to about 100 times its liquid volume, so it usually has to be trimmed flush to the framing. Fortunately, it’s soft and easy to trim.
Closed-cell foam is denser and less permeable than open-cell material. The typical closed-cell foam weighs 2.0 pounds per cubic foot and provides R-6.6 per inch of thickness. When sprayed, closed-cell foam expands from 30 to 50 times its liquid volume, making it easy to apply without completely filling the framing bay. If the bay must be filled completely, the applicator can overfill it and then trim off the excess.
Trimming closed-cell foam is not as easy as trimming the open-cell material, but it can be done.
In an unvented assembly, anything below the insulation — including an attic — is considered conditioned space. Turning the attic into conditioned space saves energy; if heat or air escapes from the hvac equipment, it remains within the conditioned space
If enough energy is saved in this manner, the hvac system can actually be downsized, reducing installation and operating costs. A number of insulation materials can be used in an unvented assembly, but the one with the greatest applicability across the country is SPF. It’s an extremely effective insulation and air barrier all in one, and since it’s sprayapplied, it conforms to irregular shapes that otherwise might be difficult to insulate and seal.
Despite the multiple brands of SPF, there are only two main kinds: open-cell foam and closed-cell foam. Chemically, all brands are nearly identical — contrary to some advertising claims — and contain about the same proportion of
agriculturally derived resin from corn, sugar beets, sugarcane, or soybeans.
None of the spray foams contain formaldehyde or use toxic or ozone-depleting blowing agents. The important differences between products have to do with density, R-value, and permeability.
Open-cell foam. The typical open-cell foam weighs 0.5 pound per cubic foot and has an insulation value of R-3.5 per inch of thickness. This type of foam is relatively permeable; at 5 inches thick it is rated at about 10 perms. Open-cell foam is an air barrier but not a vapor retarder. When sprayed, open-cell foam expands to about 100 times its liquid volume, so it usually has to be trimmed flush to the framing. Fortunately, it’s soft and easy to trim.
Closed-cell foam is denser and less permeable than open-cell material. The typical closed-cell foam weighs 2.0 pounds per cubic foot and provides R-6.6 per inch of thickness. When sprayed, closed-cell foam expands from 30 to 50 times its liquid volume, making it easy to apply without completely filling the framing bay. If the bay must be filled completely, the applicator can overfill it and then trim off the excess.
Trimming closed-cell foam is not as easy as trimming the open-cell material, but it can be done.
Advantages of Closed-Cell Foam
Both types of SPF are excellent insulation materials, but our company uses closedcell material in unvented assemblies because we think it provides the best overall performance. With it, we can pack more R-value into a small space, which is helpful when the existing rafter bays are shallow; for example, we can get R-30 into a 41⁄ 2-inch space.
In our climate zone, it’s important to avoid excessive vapor diffusion, and we think the best way to do this is to use closed-cell foam. One of the great benefits of closed-cell foam is that if you install it to a thickness of at least 2 to 21⁄ 2 inches, it will have a permeance of 1.0 perm or less.
This means that in addition to being an air barrier, closed-cell foam is a vapor retarder. It’s actually a vapor retarder from both sides, so it ends the debate about which side of the insulation to put the vapor retarder on in climates where interiors are both heated and cooled. Some companies that make both open-cell and closed-cell foam advise insulation contractors not to use the open-cell material in unvented assemblies — or to do it only in certain climates where vapor diffusion will not be a problem.
In conditions of extreme vapor drive — an indoor pool or spa, for instance — it may be necessary to further reduce the permeability of closed-cell foam by coating it with a spray-applied liquid vapor barrier.
In our climate zone, it’s important to avoid excessive vapor diffusion, and we think the best way to do this is to use closed-cell foam. One of the great benefits of closed-cell foam is that if you install it to a thickness of at least 2 to 21⁄ 2 inches, it will have a permeance of 1.0 perm or less.
This means that in addition to being an air barrier, closed-cell foam is a vapor retarder. It’s actually a vapor retarder from both sides, so it ends the debate about which side of the insulation to put the vapor retarder on in climates where interiors are both heated and cooled. Some companies that make both open-cell and closed-cell foam advise insulation contractors not to use the open-cell material in unvented assemblies — or to do it only in certain climates where vapor diffusion will not be a problem.
In conditions of extreme vapor drive — an indoor pool or spa, for instance — it may be necessary to further reduce the permeability of closed-cell foam by coating it with a spray-applied liquid vapor barrier.
Cathedral Ceilings
In a vented cathedral ceiling, the insulation is in contact with the back of the drywall and there’s an air gap (the venting space) above. But in an unvented assembly, the insulation must be against the bottom of the sheathing.
Sometimes, if the rafter bays are unusually shallow, we have to fill them all the way up with closed-cell foam. But because this type of foam has such a high R-value, in most cases we have to fill the cavities only partway.
Contractors often ask about the air space below the foam; most were taught that it’s bad to leave an air space below insulation. This is true of fiber insulations because convection currents can form in gaps and degrade the insulation’s thermal performance. But it is not true of foam, which can’t be infiltrated and is relatively unaffected by surrounding air currents. Any space left below the foam is considered conditioned space.
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Sometimes, if the rafter bays are unusually shallow, we have to fill them all the way up with closed-cell foam. But because this type of foam has such a high R-value, in most cases we have to fill the cavities only partway.
Contractors often ask about the air space below the foam; most were taught that it’s bad to leave an air space below insulation. This is true of fiber insulations because convection currents can form in gaps and degrade the insulation’s thermal performance. But it is not true of foam, which can’t be infiltrated and is relatively unaffected by surrounding air currents. Any space left below the foam is considered conditioned space.
how to insulate my attic ceiling?
how to insulate my attic myself?
how to insulate my attic?
how to insulate my attic door?
how to better insulate my attic?
how to insulate my garage attic?
how do i insulate my attic stairs?
how do i insulate my attic hatch?
how do i insulate my attic ceiling?
how do i insulate my attic door?
how much would it cost to insulate my attic?
