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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.

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The ducts visible in this unvented attic will be concealed after drywall is installed. But because they are in conditioned space, they won't be subject to the extremes of temperature typical of attics.

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 spray-applied, it conforms to irregular shapes that otherwise might be difficult to insulate and seal.

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This barrel ceiling would be difficult to insulate and seal with traditional materials.

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It's an ideal candidate for spray foam, which conforms to its irregular surfaces.

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 closed-cell 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 2 1/2 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.

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Open-cell foam, which expands to about 100 times its liquid volume, typically has to be trimmed flush to framing members — an easy task, since the foam is so soft. Because of its lower expansion rate and higher R-value per inch, closed-cell foam doesn't usually have to be trimmed. When it does, as in this shallow rafter bay.

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The author's crew uses a scraper — in this case a horse curry comb — to clean the framing in preparation for drywall.

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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Fiberglass and cellulose insulation are usually installed in contact with the back of the drywall; the concern is that leaving a space there allows convective air currents to degrade the insulation's thermal performance.

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Because closed-cell foam is unaffected by air movement, the space between it and the drywall is not a problem.

Dealing With Can Lights

It's easier and more energy-efficient to build a cathedral ceiling as an unvented assembly, but dealing with recessed light fixtures can be a real challenge.

There are two issues: how to insulate and seal the area above the fixture, and how to provide enough space around it so it doesn't overheat. Even if the fixture is an IC unit, you can't embed it in foam.

Insulating above. If we're lucky, there will be room to spray a full thickness of foam above the fixture and still maintain the desired 2 to 3 inches of clearance between foam and fixture.

If there isn't enough space or access to spray above a fixture, we sometimes install a piece of nonperforated foil-faced rigid foam above it instead. Before spraying, we mask the fixture to keep it clean, then create an airtight seal by lapping the SPF onto the rigid foam. If the rigid foam butts to framing, we caulk that joint with polyurethane sealant.

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Code requires that a space be left between can lights — even IC-rated cans — and spray foam insulation. In shallow bays, the author's crew installs foil-faced rigid foam above fixtures and creates a seal by lapping the spray foam onto it.

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An alternate method, which may soon be required in California, is to isolate fixtures from the foam by installing them in metal boxes.


Clearances. Few building codes contain specific requirements about clearances between foam and can lights, so it's a good idea to talk to the building inspector about the issue. SPF is such a good insulator it can cause a fixture to overheat, tripping the temperature-limit switch and cutting power to the light. Excess heat could also damage the wire sheathing or even the foam itself.

In California, new code provisions are being developed that will require builders to take one of three measures with recessed lights: leave 3 inches of clearance around a fixture, box around it, or wrap it with 2 inches of mineral fiber. A 3-inch clearance is already required around hot appliance vents.

SPF is compatible with PVC and CPVC, so it's okay to spray it on Romex, PVC pipe, and CPVC sprinkler pipe.

Air Sealing

Any surface we spray will be sealed against the movement of air, but there are always some surfaces we can't spray.

For example, the gaps between doubled-up framing members are too small to spray with foam, yet a significant amount of air can leak through at these spots. It's best to seal these joints during framing by installing compressible foam gaskets between the members. If that isn't done, you can caulk the joints after the foam is installed.

When the gaps are too wide for caulk, we fill them with foam from a can. The canned foam should be the low-expansion type; it contains more closed cells than the high-expansion material. We stay away from the latex foams because they're very permeable.

Fire Resistance

When the unvented assembly is a cathedral ceiling, the foam will be covered with drywall, which is a code-approved thermal barrier. In an attic, though, the rafter bays are not normally covered by drywall, so the issue of fire-resistance comes into play.

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    When insulating an unvented roof assembly, the author 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 (bottom) 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.

This can be a gray area in the code, so be sure to check with your building department before building an unvented attic space. Most codes state that if the attic is accessible for the service of utilities, the foam must be covered with an ignition barrier. Certain water-based intumescent coatings qualify as ignition barriers.

If the attic area is not accessible or is not "accessed for the service of utilities," it may be possible to leave the SPF exposed. Many contractors are confused about how to treat this enclosed attic space. Providing access through a ceiling hatch is okay but not necessary; venting to the room below is prohibited by the fire code.

Other Issues

Unlike fiber insulation, which can be blown through a hose or stuffed into hard-to-reach areas, SPF can't be installed without sufficient access. The applicator must be able to get close enough to the sheathing to spray from 16 to 24 inches away — and do it from pretty much straight on.

Cost. In our area, the installed cost of an average-size closed-cell foam insulation project is between $1.10 and $1.40 per board foot of material.

For R-30, that comes to about $5 per square foot of roof area. That's more than other insulation materials would cost, but not much more if you factor in all of SPF's advantages — future energy savings, increased comfort and moisture control, the greater design flexibility that comes with being able to fit the necessary R-value into small framing cavities, and the possibility that the mechanical system can be downsized.

James Morshead is senior project manager and technical director for American Services Co. in Dublin, Calif.