©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
UPDATED April 22, 2014
Millions of Americans live in states where residential HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. 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:
The bottom line: running ducts through an attic saves money for the builder, but costs the homeowners dearly in increased energy costs.
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:
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).
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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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.
Although I don't recommend it, it is possible to install all of your sloped-roof insulation between the rafters. A few guidelines concerning insulation installed between 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 1x4 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.)
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 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. 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.
By now, most builders know the difference between the R-value listed on a roll of batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. 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.
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.
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 .
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:
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.
Last week’s blog: “The Pros and Cons of Advanced Framing.”