musingsheader image
Helpful? 9

Creating a Conditioned Attic

Five questions to consider before insulating a sloped roof

Posted on Dec 31 2010 by Martin Holladay, GBA Advisor

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:

  • 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-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures. (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:

  • Will the insulation be installed above the roof sheathing, between the rafters, or below the rafters?
  • Will the insulated roof include ventilation?
  • Is it important to address thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the rafters?
  • Will any combustion appliances be located in the attic — and if so, where will the appliances’ combustion air come from?
  • Is there any need to install a forced-air register to condition the attic air?

Question 1: Where will the insulation go?

There are at least three ways to insulate a sloped roof:

  • Install rigid foam insulation above the roof sheathing.
  • Install insulation between the rafters.
  • Install rigid foam insulation under the rafters.

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-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.

If you insulate between the rafters

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:

  • 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 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., 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 ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. 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, 2x10 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.

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 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.)

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 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.

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 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.

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 is simple: no. If you’ve done a good job insulating and air sealing, the conditions in your attic will approximate indoor conditions, even without a supply register or a return-air grille.

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.

Last week’s blog: “The Pros and Cons of Advanced Framing.”

Tags: , , , , , ,

Image Credits:

  1. Fine Homebuilding
Fri, 02/24/2012 - 11:42

Foil Faced Foam for Venting Channels
by Gary Steinfeld

Helpful? 0

In a 2x12 rafter attic, can I use foil faced foam (Dow Thermax) to create the 2" air channels in the rafter bays and then use fiberglass batt in the bays followed by sheetrock with latex paint. Will the low perm of the thermax cause any problems at this location. Also, shoud I add some rigid foam such as dow blue board below the rafters for a thermal break against bridging: say 1" foam?

Fri, 02/24/2012 - 11:49

Response to Gary Steinfeld
by Martin Holladay, GBA Advisor

Helpful? -1

Yes, you can use rigid foam (for example, Thermax) to create a ventilation channel.

If you have 2x12s with a 2-inch ventilation channel, 1 inch of Thermax will give you an R-value of R-6.5, and the remaining 8 1/4 inches of fiberglass will give you about R-30.5. So you end up with R-37. I don't know if that is enough to meet minimum code requirements; it depends on your climate zone and the local code.

Yes, you can cut down on thermal bridging through the studs by adding a continuous layer of rigid foam insulation under the rafters. I don't think this type of assembly will have moisture problems.

Fri, 02/24/2012 - 12:37

Kraft Free Insulation
by Gary Steinfeld

Helpful? 1

My intention was to use batt insulation with out kraft paper. Is that the correct detail?

Fri, 02/24/2012 - 13:39

Response to Gary Steinfeld
by Martin Holladay, GBA Advisor

Helpful? 0

I don't know your climate, your geographical location, or your local code requirements, so I can't answer your question.

Your detail has to satisfy your local code official, so you can call your local building office for an answer. It rarely matters from a building science perspective whether your fiberglass batts have kraft facing or no kraft facing, as long as you pay attention to air sealing.

Here's more information on vapor retarders: Vapor Retarders and Vapor Barriers.

Thu, 11/08/2012 - 13:22

Edited Thu, 11/08/2012 - 13:26.

Venting Cathedral Ceiling
by David Head

Helpful? -1

Like the previous poster I am thinking of using foil faced 1 inch polyiso for a site built ventilation baffle for 12 inch deep rafters, creating a 2 inch gap. Then spray foam underneath. Should I be worried about vapor condensing on interior foil so that the foam absorbs moisture like a sponge? Is one type of foam better than a another? The house is in climate zone 4c in Northern Ca. Thanks for you time.

Thu, 11/08/2012 - 13:54

Response to David Head
by Martin Holladay, GBA Advisor

Helpful? 0

Q. "Should I be worried about vapor condensing on interior foil so that the foam absorbs moisture like a sponge?"

A. No, for three reasons: (a) The foil layer, even if it is wet, prevents the water from reaching the rigid foam; (b) The foil will be warm (since it is on the interior side of the foam) and will therefore never be cold enough to become a condensing surface; (c) In most cases, the spray foam will stop moisture transfer from the interior to the foil.

Q. "Is one type of foam better than a another?"

A. Closed-cell spray foam has a higher R-value per inch and is less vapor-permeable than open-cell spray foam; for those two reasons, I would say that closed-cell foam is better than open-cell foam in this application. But either one will work.

For more information on this topic, see How to Build an Insulated Cathedral Ceiling.

Thu, 11/08/2012 - 14:11

Cathedral Ceiling Venting
by David Head

Helpful? 0

Thank You very much for you quick reply, you have relieved some of my anxiety of this project. I take that "No, for three reasons: (a) The foil layer, even if it is wet, prevents the water from reaching the rigid foam" also means that water vapor can not entrap itself inside the spray foam and lead to rotting or mold growth on the rafter? Does the vapor just move to dissipate back into the conditioned space?

Sorry for the basic questions, I just do not want to make any mistakes and understand the building science.

Thu, 11/08/2012 - 15:14

Response to David Head
by Martin Holladay, GBA Advisor

Helpful? 0

During the winter, the indoor air is warm and humid. The outdoor air is cold and dry.

The moisture held in the air indoors isn't supposed to go anywhere. It is supposed to stay put. You don't want to have any air or water vapor migrating through your building assemblies. That's why you have an air barrier -- to keep the indoor air in, and the outdoor air out.

Q. "Can water vapor entrap itself inside the spray foam?"

A. No. Closed-cell spray foam is vapor-impermeable. While open-cell foam is vapor-permeable, any water vapor inside the foam isn't trapped. It's free to come and go. Water vapor doesn't hurt spray foam.

Q. "Does the vapor just move to dissipate back into the conditioned space?"

A. Are you talking about outdoor water vapor during hot, humid weather in the summer? If you are, the water vapor should stay outdoors. You can make sure that happens by including an air barrier in your ceiling assembly -- in your case, that would be the foil-faced polyiso. The foil is also a vapor barrier.

If you are talking about water vapor that is inside the house during the winter, the idea is to keep it inside, not allow it to flow through your walls and ceilings.

Tue, 04/30/2013 - 00:54

Creating a Conditioned Attic
by Eric Schroeder

Helpful? 0

Mr. Holladay,
I am writing to you in hopes you that you may possibly add some clarity to a confusing situation for me. While renovating my house a couple of years back I used this sight for a lot of the recommendations in regard to insulating my house. I am in Westchester NY about a mile from the coast zone 5. The house is a wood frame ranch with a pitch of 6/12 with 2x8 rafters and new ¾ in sheathing. My main concern is the attic and roof insulation. Unfortunately, I have Hvac equipment in the attic and I might finish the attic in the future too. There is floor decking in the attic with R-30 bat between the ceiling of the first floor and the attic floor. The sheet rock ceilings were made air tight or as close as possible. I have a sealed door to the attic. My question is this, as per a number of articles and readings (one of which was “Five questions to consider before insulating a sloped roof”) I decided that I would have the contractor create an on-sight baffle to vent the underside of the roof sheathing by using 1x1 and 1” xps foam board to create a continuous channel from soffit vent to ridge pole vent. I then intended to either flash foam with closed-cell and then another type of insulation after that or fill the rest of the 2x8 with closed-cell. Either way I believe I would be short the required code value for insulation (R-38 zone 5?), but thought I would be close and the tighter envelope would still be better over all situation. The problem I have is that all of the South/East facing side of the roof has the 1x1 baffles installed, the other side was never finished, but looking to complete everything now. The info in the last 2 or 3 years has seemed to change (Dr. Joseph Lstiburek article about roof venting). My concern now is that the 1x1 channel with foam board is not large enough even though the IRC says 1” is the minimum. I believed from what I read that 1” would be fine especially since I wanted to give up the least amount of R-value as possible while doing it. The idea was minimum amount of R- value lost, no foam against the deck, and the attic included in the envelope. Now I’m reading all these negative things about potential for mold and other bad things if the channel under the sheathing is less than 2”. So do I spray over the existing baffles and cross my fingers or rip out the baffles and spray against the sheathing? I could try to take the foam board down and increase the channel size up to 1.5 “or 2” , but I would lose some more R-value? What would you do at this point. Any information would be greatly appreciated. Forgot to mention that I am having that side of the roof spec’d out for PV array, if that makes any difference?
Thank you,

Tue, 04/30/2013 - 04:28

Edited Tue, 04/30/2013 - 04:32.

Response to Eric Schroeder
by Martin Holladay, GBA Advisor

Helpful? 0

1. I wouldn't worry about whether or not your 1-inch deep air channel is adequate. It is. A great many attics have smaller air gaps than the one you have provided (for example, those that use Styrofoam Proper-Vents). You have done an above-average job.

2. The most important way to ensure that there are no moisture problems in your ceiling assembly is to pay attention to airtightness. Because you plan to use closed-cell spray foam, you are unlikely to have problems on this score, as long as the spray foam installer pays attention to maintaining air barrier continuity near the soffits, where the air barrier in the sloped roof assembly meets the air barrier in the wall assembly. Since you are worried on this issue, you can always hire a contractor with a blower door to depressurize your attic when the work is done; that way you can check for air leaks and fix them.

3. If you fill the rafter bays with closed-cell foam, you'll end up close to R-38. However, most closed-cell spray foam contractors don't fill framing bays (because the foam is dense and hard to trim when it bubbles past the plane of the framing) -- the framing bays end up "almost full." You might want to consider adding a layer of rigid foam on the interior side of your rafters, to address thermal bridging and to beef up the assembly's R-value.

Thu, 01/16/2014 - 13:54

Roof Assembly without sheathing
by Chris Thomas

Helpful? 0

This seems to be a common theme with my home - I asked a question in the 'Cut and Cobble' article. I need to create a conditioned attic due to the location of the air handler and ductwork. It would also be a great help to use it as storage as it's an old home with limited closets, etc. I'm located in Climate Zone 4. The home has a metal roof on perlins. I cannot insulate from the exterior. The roof has no ventilation (no soffit vents/ridge vent) except from holes from those pesky squirrels.

My hope was to use fiberglass between the rafters and foil-faced foam on the underside. I don't think I would need to worry about wind-washing in the roof assembly.

Any suggestions on how to properly condition this type of attic would be greatly appreciated.


Fri, 01/17/2014 - 08:58

Edited Fri, 01/17/2014 - 08:59.

Response to Chris Thomas
by Martin Holladay, GBA Advisor

Helpful? 0

Everything you need to know is explained in this article: How to Build an Insulated Cathedral Ceiling.

Here are the steps for the type of roof you describe:

1. The air between the purlins (under the metal roofing) provide enough ventilation.

2. The first step is to create an air barrier at the top of each rafter bay. You can either install rectangles of plywood, OSB, or thin XPS or EPS foam. No matter which product you choose, the rectangles have to be air sealed at the perimeter with caulk, canned spray foam, or a high-quality construction tape.

3. You can insulate the rafter bays with fiberglass batts, blown-in cellulose, mineral wool, or spray foam. If the rafter bays aren't deep enough for you to get the minimum R-value of insulation required for your climate zone, you may have to extend the rafters inward (make them deeper) with scabbed framing members.

4. It's important to install an air barrier on the interior of your roof assembly. The simplest air barrier at this location is gypsum drywall.

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!