UPDATED on August 13, 2017 with new concluding paragraphs
Millions of Americans live in states where residential HVAC contractors routinely install ductwork in unconditioned attics. In many cases, these attics also contain a variety of appliances, including air handlers, furnaces, or water heaters.
Because the disadvantages of this arrangement are fairly well known, I’ll mention them only briefly:
- During the summer, attic temperatures often exceed outdoor temperatures.
- Attic ducts almost always have thinner insulation than ceilings, in spite of the fact that the delta-T (that is, the temperature difference) between the air in the ducts and the air in the attic is even greater than the delta-T between the inside of the home and the exterior.
- Most duct seams leak; as a result, supply ducts lose conditioned air to the attic, while return ducts suck in attic air — air which is hot in summer and cold in winter — and bring it to the air handler.
- If access to the attic is through a hatch, servicing any HVAC equipment in the attic is awkward at best.
The bottom line: running ducts through an attic saves money for the builder, but costs the homeowners dearly in increased energy costs.
Ductwork belongs inside a home’s thermal envelope
Ideally, HVAC appliances and ductwork should be located inside a home’s conditioned envelope. In the northern half of the country, appliances and ductwork are routinely located in basements or crawl spaces. If your house has a slab foundation, HVAC appliances can be located in an equipment room and ductwork can be located in interior soffits.
Another solution is to move the insulation from the attic floor to the sloped roof, thereby creating a conditioned attic.
Assuming you want to create a conditioned attic — either during new construction or as a retrofit project — how do you go about doing it? Before getting down to the nitty-gritty details, you’ll need to answer at least five overlapping questions:
Question 1: Where will the insulation go?
There are at least three ways to insulate a sloped roof:
A fourth approach combines between-the-rafters insulation with rigid foam insulation (either above the roof sheathing or below the rafters).
If you install rigid foam insulation above the roof sheathing
Installing rigid foam insulation above the roof sheathing makes a lot of sense. The foam keeps the roof sheathing warm, and therefore dry. Rigid foam also interrupts thermal bridging through the rafters — a big benefit.
Of course, there is a limit to how much foam you can install above roof sheathing; the main problem is the difficulty of hitting the underlying rafters when attaching furring strips or plywood above the foam with long screws. Several builders and homeowners have successfully installed 6 inches of rigid foam on top of a home’s roof sheathing, although some (including Alex Cheimets of Arlington, Mass.) have grumbled about problems driving long screws.
Two layers of rigid foam installed with staggered seams will perform better than one layer of rigid foam with the same total thickness.
Six inches of polyisocyanurate gives an R-value of R-39, which isn’t enough for cold climates. That’s why builders who install rigid foam on top of roof sheathing usually supplement the foam with additional insulation between the rafters.
One way of avoiding the hassle of installing thick insulation above roof sheathing — perhaps in exchange for a different set of hassles — is to install structural insulated panels (SIPs) or nailbase (basically, SIPs with OSB on only one side). It’s possible to order 12-in.-thick SIPs that measure R-48.
For more information on this topic, see How to Install Rigid Foam On Top of Roof Sheathing.
If you insulate between the rafters
Although it’s usually better to install rigid foam above the roof sheathing than it is to install all of the insulation between the rafters, sometimes a homeowner has little choice, and ends up installing all of the sloped-roof insulation between the rafters. A few guidelines concerning insulation installed between rafters:
- Air-permeable insulation (like fiberglass) is risky, since it allows moisture-laden air to reach the cold roof sheathing. If you want to use air-permeable insulation in this location, you have two choices: include a ventilation channel between the top of the insulation and the underside of the roof sheathing, or install rigid foam insulation on top of the roof sheathing to keep the sheathing warm. (Don’t do both, since these two solutions are incompatible.)
- Insulation that is air-impermeable but vapor-permeable (like open-cell spray polyurethane foam) is risky in a cold climate (climate zones 5 or higher) unless the interior side of the insulation is protected with a vapor retarder. According to section R806.4 of the 2009 IRC, such a vapor retarder is required in climate zones 5 through 8 for open-cell spray foam installed to create an unvented attic assembly. Recent research shows that vapor-retarder paint is ineffective when sprayed directly on cured foam, so if you want to use open-cell foam on the underside of roof sheathing in a cold climate, the cured foam should be shaved back even with the bottom of the rafters, and the insulated rafter bays should be protected by a layer of drywall. At that point you can install vapor-retarder paint on the drywall. In a cold climate (climate zones 5 or higher), if you don’t plan to install any drywall, you should use closed-cell foam, not open-cell foam.
- Recent research suggests that open-cell spray foam may be risky in all climate zones, so the safest spray foam insulation to use is closed-cell spray foam. For more information, see Open-Cell Spray Foam and Damp Roof Sheathing.
- Remember that it’s important to meet or exceed minimum code requirements for insulation R-value; beware of spray-foam contractors who try to convince you that below-code-level insulation is adequate. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.
- According to the 2009 IRC (Section R806.4), it’s possible to build an unvented roof assembly with a combination of rigid foam insulation above the roof sheathing and air-permeable insulation in the rafter bays. The code requires that “rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.” The table calls for a minimum of R-5 foam for Climate Zones 1-3, R-10 for Climate Zone 4C, R-15 for Climate Zones 4A and 4B, R-20 for Climate Zone 5, R-25 for Climate Zone 6, R-30 for Climate Zone 7, and R-35 for Climate Zone 8.
- The 2009 IRC also allows another approach: it’s possible to use a combination of air-impermeable and air-permeable insulation in unvented rafter bays, as long as the minimum R-value of the air-impermeable insulation that is “applied in direct contact with the underside of the structural roof sheathing” meets the requirements for condensation control shown in Table R806.4.
- The 2009 IRC defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTM E 2178 or E 283.” Although spray foam insulation and rigid foam insulation can meet this standard, dense-packed cellulose cannot.
- Most rafters aren’t deep enough to accommodate enough insulation to meet minimum R-values required by code, especially if the rafter bays include a ventilation channel. For example, 2×10 rafters, which are 9 1/4 inches deep, provide room for only about 8 1/4 inches of insulation (about R-30 of fibrous insulation) if you ventilate the bay. Some builders solve this problem by sistering, furring down, or scabbing on additional framing below the rafters to deepen the rafter bays.
- Most building codes require spray foam insulation to be protected by an ignition barrier or a thermal barrier. For more information on this issue, see Thermal Barriers and Ignition Barriers for Spray Foam.
If you install rigid foam insulation under the rafters
Installing rigid foam under the rafters has some of the same advantages, as well as the same disadvantages, of installing foam on top of the roof sheathing.
Attaching thick foam overhead is awkward, so few builders install more than 2 inches of foam in this location. That’s why rigid insulation below the rafters is almost never done in isolation; rather, it usually complements insulation installed between the rafters.
One popular insulation combination for sloped ceilings: 2-in. thick foil-faced polyisocyanurate attached to the underside of the rafters and held in place by 1×4 furring strips, with dense-packed cellulose blown into the rafter bays through holes in the rigid foam. (If you choose this method, be sure to include a ventilation channel under the roof sheathing.)
Question 2: Will the insulated roof include ventilation?
Insulated sloped roofs usually include a soffit-to-ridge ventilation channel. Soffit vents allow air to enter the bottom of these channels, and a continuous ridge vent allows air to exit at the ridge. Such ventilation channels work best on simple shed roofs or gable roofs; if your roof has hips, valleys, dormers, and skylights, it will be hard to ventilate well.
The 2006 International Residential Code (IRC) sets out roof ventilation requirements in Section R806. However, in Section R806.4, the IRC permits conditioned attic assemblies without any roof ventilation; according to the code, if ventilation is omitted, only “air-impermeable” insulation can be used in contact with the roof sheathing.
Although the 2006 IRC neglected to include a definition of “air-impermeable insulation,” that omission was rectified in the 2009 IRC, which defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTM E 2178 or E 283.” In other words, an air-impermeable insulation must meet the same airtightness standard as an air barrier material. Although spray foam insulation and rigid foam insulation can meet this standard, dense-packed cellulose cannot.
If you choose to install a fibrous insulation like fiberglass or cellulose between your rafters, you must include a ventilation channel between the top of your insulation and the underside of the roof sheathing.
Even if you insulate between your rafters with an air-impermeable insulation like spray polyurethane foam, you may want to provide a ventilation channel under your roof sheathing. The main function of such a ventilation channel is to separate the roof sheathing from the foam; this facilitates future repairs of sheathing rot.
Polystyrene “ProperVents” are too narrow, too shallow, and too flimsy. The sturdiest vent channels are site-built channels, using 1″x1″ sticks in the upper corners of the rafter bays and thin plywood, Masonite, or rigid foam. If the panels used to build the vent channels are caulked in place, it’s possible to create an effective air barrier to prevent wind-washing from lowering the performance of the insulation.
Remember: ventilated rafter bays are incompatible with rigid foam insulation installed on top of the roof sheathing. If you want to reduce thermal bridging through the rafters of a ventilated roof, you’ll have to install rigid foam insulation under your rafters.
Question 3: Is it important to address thermal bridging through the rafters?
By now, most builders know the difference between the R-value listed on a roll of batt insulation and whole-wall R-value. (The whole-wall R-value is always significantly lower than the label on the batts.) There are several reasons why whole-wall R-values are so low; the most significant reason is thermal bridging through the wall framing.
Thermal bridging can also be a factor that lowers the performance of an insulated roof. It makes little sense to “cathedralize” an attic — that is, to bring the attic into a home’s thermal envelope — in hopes of saving energy, if the roof insulation fails to address thermal bridging through the rafters.
In other words, if your goal is to save energy, do it right. That means installing rigid foam on top of the sheathing or under the rafters.
Question 4: Will any combustion appliances be located in the attic?
Once you’ve tightened up your thermal envelope and brought your combustion appliances indoors, you have to come up with a plan to provide your appliance burners with combustion air. The best solution is to install only sealed-combustion appliances in a conditioned attic.
Each sealed-combustion appliance has two big pipes: one is the flue, and the other conveys fresh outdoor air to the burner.
If you hope to convert an existing unconditioned attic to a conditioned attic, the presence of any atmospherically vented appliances (for example, a gas water heater or a gas furnace) complicates the retrofit work. If you can’t afford to buy new sealed-combustion appliances, you’ll probably be better off leaving your attic unconditioned.
Question 5: Is there any need to condition the attic with a forced-air register?
Okay, you’re coming down the home stretch now. You’ve insulated your sloped roof and your gable walls. You’ve protected any exposed foam with a thermal barrier like drywall. You’ve done your best to limit air leaks in the tricky area at the attic’s perimeter, where the rafters meet the top plates of your exterior walls.
You’ve just created a conditioned attic. Now your HVAC contractor asks you whether or not you want a supply register and a return grille in your attic.
The answer to this question depends on several factors. If you have insulated the roof assembly with closed-cell spray foam, you probably don’t need a supply register or return grille in your attic. If you’ve done a good job insulating and air sealing, the temperature of the air in your attic will approximate indoor conditions, even without a supply register or a return-air grille.
If you have insulated the roof assembly with open-cell foam and you live in a humid climate, however, there is increasing evidence that you probably need to install a supply register and a return grille in your attic in order to keep humidity levels low enough to avoid damp roof sheathing. For more information on this issue, see Open-Cell Spray Foam and Damp Roof Sheathing.
Conditioned attics have a few drawbacks
Creating a conditioned attic is a solution to a fundamental design flaw (locating ductwork or HVAC equipment outside of a home’s thermal envelope). But conditioned attics are not unalloyed blessings; they come with their own set of drawbacks.
These drawbacks include:
- Installing insulation along a sloped roof is always more expensive than installing insulation on an attic floor.
- It’s usually easier to perform air sealing work on the attic floor than to ensure that a conditioned attic is well sealed.
- In a home with a conditioned attic, it’s much harder to locate roof leaks or repair rotten roof sheathing than in a home with an unconditioned attic.
- Damp roof sheathing will always dry faster in a home with an unconditioned attic than in a home with a conditioned attic.
The bottom line: unless you have ductwork in your attic or you plan to convert your attic to living space, an unconditioned attic is usually preferable to a conditioned attic.
A radical approach: Abandoning attic ductwork
If your house has a vented unconditioned attic that includes ductwork, and you are thinking of converting the attic into a conditioned attic, you may be surprised at the high cost of the required work. Creating a conditioned attic is expensive.
In light of the high cost of the work, it’s worth considering a radical option: abandoning the existing forced-air system. In some cases, the cost of installing one or two ductless minisplits is less than the cost of creating a conditioned attic. Moreover, the minisplits will perform at a much higher efficiency than a conventional forced-air system with attic ductwork.
Author’s note: I’d like to provide credit to GBA reader Dana Dorsett for suggesting this last approach.
Last week’s blog: “The Pros and Cons of Advanced Framing.”