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13 Helpful?

Creating a Conditioned Attic

Five questions to consider before insulating a sloped roof

Posted on Dec 31 2010 by Martin Holladay

UPDATED on August 13, 2017 with new concluding paragraphs

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.

For more information on this topic, see How to Install Rigid Foam On Top of Roof Sheathing.

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.
  • 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 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 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 reader Dana Dorsett for suggesting this last approach.

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

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Image Credits:

  1. Fine Homebuilding

Dec 31, 2010 9:05 AM ET

Edited Dec 31, 2010 9:07 AM ET.

Great article!
by Allison A. Bailes III, PhD

Great article, Martin! Maybe you could link this article to the one you wrote a while back about contractors putting in lower R-values when they insulate the roofline. That's an important issue that you kind of glossed over in this article but that generates a lot of discussion, especially from the SPF installers who insist on diminishing their installed thickness because of the diminishing returns of extra R-value.

I agree with you that we need to pay more attention to thermal bridging. Here in the Southeast, SPF installers generally go in and spray about 6" of open cell foam between the rafters for an R-value of ~20. Sometimes they spray over the rafters to reduce that thermal bridging, and sometimes they don't. Rigid foam above or below is almost never installed here. The closed cell installers generally put in the same R-value (~20), but I've seen as little as 2" for R-14.

One thing you didn't mention is whether or not you should remove the existing insulation in the flat ceiling if you insulate the roofline in an existing home. I haven't seen data on this, but I've heard that you should remove that insulation to keep the attic close to the temperature in the house, which helps prevent condensation on the underside of the roof insulation. Any thoughts?

Finally, I think you used the wrong word in the last paragraph of your answer to Question 2:

Remember: insulated rafter bays are incompatible with rigid foam insulation installed on top of the roof sheathing.

Didn't you mean to say 'ventilated' instead of 'insulated' there at the beginning of the sentence?

Dec 31, 2010 9:34 AM ET

Edited Dec 31, 2010 9:41 AM ET.

Response to Allison Bailes
by Martin Holladay

Thanks very much for catching my typo, which I have corrected in the text.

Thanks also for all of the points you brought up:
1. As I made clear in my earlier blog, It’s OK to Skimp On Insulation, Icynene Says, I believe that it is essential to comply with minimum code requirements for insulation R-value, even when using spray polyurethane foam. The main reason that spray foam contractors try to circumvent minimum code requirements for insulation R-value is because spray foam is so expensive that it has a hard time competing with other insulation products. I have taken your suggestion to heart and added a reference to this point in my blog.

2. I agree that the Achilles' heel of many spray-foam jobs is that thermal bridging through the rafters is not addressed.

3. In most cases, the existing insulation on an attic floor should be removed when an unconditioned attic is converted to a conditioned attic. However, this recommendation should be addressed on a case-by-case basis. Depending on the climate and the thickness of the existing insulation, it may make sense to leave the old insulation in place.

Dec 31, 2010 9:59 AM ET

Follow Up on Removing Existing Insulation
by John Nicholas

How do you recommend removing the existing insulation? Are there other services out there with truck mounted vacs that could do this? Is is more of a hand and time job?

What about removing extremely deteriorated R-2 Rockwool batts, before blowing cellulose?

Dec 31, 2010 10:19 AM ET

Edited Jan 3, 2011 6:50 AM ET.

Martin: You mentioned that
by Allan Edwards

You mentioned that for "many spray-foam jobs thermal bridging through the rafters is not addressed." Can you point me to any other articles by you or anyone else in the Building Science community on thermal bridging at the rafters? By the way, thanks for another great article.

Dec 31, 2010 10:32 AM ET

Response to John Nicholas
by Martin Holladay

John Nicholas,
1. Contact cellulose insulation contractors for information on removing existing insulation with a vacuum. Many of these contractors offer this service.

2. If you have severely deteriorated old batts, it's probably a good idea to remove them, if only to get rid of the rodent feces. That's probably an old-fashioned job requiring a dust mask, gloves, and many garbage bags.

Dec 31, 2010 10:37 AM ET

Response to Allan Edwards
by Martin Holladay

Thermal bridging links -- (by the way, the "search" function on our GBA site can help you track down the answer to questions like this):

Thermal Bridging

Thermal bridging with spray foam insulation

Insulation Overview

Dec 31, 2010 11:42 AM ET

Pitfalls of different climates
by M. Johnson

I appreciate your coverage of this very interesting topic. In the hot-humid South where I live, it is my understanding there are a number of lawsuits from insulated attics and I believe the #1 problem is rotting roof decking connected with undiscovered water leaks. Research is still ongoing but I do not see a consensus yet as to the wisdom, nor guidelines to avoid trouble.

The methods you describe work well in some climates, but around the country there are several zones with special needs. The simple factor of outdoor dew point being chronically very high or low is important in many situations. Hot-humid climates make it normal to consume a lot of energy, and I would like to hear more from the perspective of a builder in such climates.

Thank you for bringing up the attic subject, it is very interesting.

Jan 1, 2011 12:23 AM ET

Why Does "Green" Require Hazmat Suits
by Riversong

I love the picture at the top of this article. It always amazes me that people would consider "green" any material or method that is IDLH (a NIOSH acronym for Immediately Dangerous to Life and Health) and requires a complete isolation suit for safe installation.

And M. Johnson, you're quite right that studies have demonstrated the danger to the roof sheathing when roofs can't breathe in either direction and take literally months to dry after a leak event. Anything wooden needs to breathe.

Jan 1, 2011 7:36 AM ET

Response to M. Johnson
by Martin Holladay

M. Johnson,
1. You are quite right that installing spray polyurethane foam directly against roof sheathing is controversial, because of the possibility that the foam can disguise roof leaks. That's why I wrote, "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."

2. You wrote, "The simple factor of outdoor dew point being chronically very high or low is important in many situations." If you follow the recommendations provided in this article, however, your roof sheathing will never be cold enough to allow for moisture accumulation in the sheathing due to a dew-point issue -- even (especially) in a southern climate.

3. I disagree with our statement, "Hot-humid climates make it normal to consume a lot of energy." If you design your house well, you will have low energy bills, even in the south. For more information on energy-saving tips for hot climates, see Hot-Climate Design .

Jan 1, 2011 11:02 AM ET

Comment from Joe W
by Martin Holladay

[Joe W. was having trouble with the GBA spam filter, so he sent me the following comment by e-mail:]

I've heard a couple of proposed solutions to increasing the accuracy of screwing SIPs and thick exterior insulation to rafters.

One was to order truss rafters with a screwable flange -- this was advanced as an easily available option.

Another was to double up truss rafters every four feet to provide a wider surface.

Of course, with a timber frame set up, either rafters or purlins would be expected to be wider to begin with.

Are these legitimate suggestions?

Thanks in advance, and happy new year
Joe W

Jan 1, 2011 11:05 AM ET

Response to Joe W
by Martin Holladay

If trusses can be ordered with wide top flanges, that seems like an easy solution -- although some truss-spacing jigs probably won't work with wide flanges.

Doubling up trusses every 4 feet doesn't make sense to me, for two reasons: it's an expensive solution, and you'll want to space your screws closer than every 4 feet.

Jan 1, 2011 10:44 PM ET

A home that uses half the energy of a code built home
by aj builder, Upstate NY Zone 6a

Seal up a home well somehow and stop the flow of heat thru the insulation and a home instantly will use half the energy of yesterdays leaky homes.

Icynene does that and I can take anyone to a home and show you.

The home will have less than R 38 above in complete defiance of Martin's rant that Icynene is selling nonsense. I say Martin who I agree with most of the time is barking up the wrong "R vs. air leaks" tree.

Talk is cheap. I have Icynene installed and it is performing amazingly.

Martin, come visit, I will give you a tour. Then come back to this thread and post your thoughts. I have never seen you post actual hands on experience with Icynene. Stop railing against it till you do have some personal knowledge of the success many of us are having with it.

I am all for going green and entirely cellulose as to insulation. But I am not going to say Icynene does not do a great job. It does. It does for me. It does for my customers. It does for my spray company.

Every good assembly has its' use. What Icynene did for us is cut the size of our HVAC system size in half. It reduced the number of AC run hours to 1/4.

Robert will rant back as to myself and foam and Icynene and using less than "code" R. Rant away. The homes work, I am in them all the time and the HVAC is off most of the time. I have never been in a home that an HVAC system runs so little an amount of time. And they are sized smaller than the installers wanted to size them at my insistence. I will not counter-post Robert. You all do not need an extended warming from that.

peace out. I suggest anyone that if you are trying any assembly that you learn about online to go to sites and learn in person. Talk to satisfied customers. Go to sites that are in the process of being built.

Cellulose is by far the greenest insulation. Icynene is a less green product but it is better at reducing energy demands then Martin is willing to admit.

Jan 2, 2011 3:01 AM ET

unvented attic spaces
by Alex Rockas

I am looking at a home in Santa Cruz, CA that is to be built. It has a rather cut up 4 in 12 truss roof with many areas of California framing. It is fairly complicated with fake rafter tails and a lot of odd framing details. The architect wants closed cell foam (R30) blown in under the roof sheathing with no ventilation. The roof is to be corrugated metal screwed directly down to the plywood sheeting. I am worried about condensation in areas where the foam may not be able to reach (California sections) among other things. Any helpful advice? Thanks, Alex Rockas

Jan 2, 2011 7:48 AM ET

Response to AJ Builder
by Martin Holladay

Did you accidentally post your comment on the wrong page? I don't know what you're talking about. Why are responding to "Martin's rant that Icynene is selling nonsense. ... Icynene is a less green product but it is better at reducing energy demands then Martin is willing to admit."

I didn't attack Icynene insulation. For the record, Icynene spray foam is excellent at filling framing bays completely and does an excellent job of air sealing. For those reasons it always performs much better than fiberglass batts.

Jan 2, 2011 7:50 AM ET

Response to Alex Rockas
by Martin Holladay

Alex Rockas,
1. Please define "California framing."

2. Your architect's plan should perform very well, and shouldn't lead to any condensation problems, as long as the spray foam contractor can reach all areas of the roof sheathing. If some areas of the roof sheathing are not accessible to the spray foam contractor, you need to come up with a plan to insulate those areas.

Jan 2, 2011 8:19 AM ET

Ice damming with OC foam
by Iowa builder

Good topic I will attempt below paste my post from a different thread.

Jan 2, 2011 8:20 AM ET

I am constructing town homes
by Iowa builder

I am constructing town homes in Iowa where the zone is 6. The homes are slab on grade so it requires that all the HVAC to be place in the attic. The entire envelope of the home is done with OC foam. The underside of the roof has anywhere from 6" to 8" of foam and putting my HVAC in conditioned space. I am experiencing major snow melt and ice damming on these roofs when the are completely covered with snow with an outside air temp. at as low as 0 degrees.
I'm feeling I missed a major element in the equation by not sealing the underside of the foam.
I am still able too apply the sealer and am thinking about spraying it with Suppertherm or a good latex paint. But I do not want to trap the moisture at the roof sheathing or worse yet have it raining in the attic.
Sorry to be long winded
Thanks in advance for you comments

Jan 2, 2011 8:29 AM ET

Why Does "Green" Require Hazmat Suits
by Iowa builder

The hazmat suit is for the over spray it sticks to anything and everything and is in the vapor until it is dry.

Jan 2, 2011 8:45 AM ET

Edited Jan 2, 2011 8:47 AM ET.

Response to Iowa Builder
by Martin Holladay

Iowa Builder,
Your roof has at least three problems.

1. The biggest problem is that you installed less than the minimum code requirement for roof insulation in your climate zone. In Zone 6 (according to the 2006 IRC) you need to install a minimum of R-49 of ceiling or roof insulation. If you choose to install open-cell spray polyurethane foam, that means you need a little more than 13 inches of insulation.

You write that you have between 6 inches (R-22) and 8 inches (R-30) of insulation in your roof. That's not enough, as your ice damming problems indicate.

2. Your roof also has thermal bridging problems, since your rafters are uninsulated.

3. Finally, unless you sprayed the cured foam with vapor-retarder paint, your roof sheathing is at risk of accumulating moisture during the winter.

To learn more about solving ice dams, see Prevent Ice Dams With Air Sealing and Insulation.

It's not impossible that your home has an air leak -- for example, at the perimeter of the attic, where the roof foam should join up with your wall's air barrier. Any air leak would certainly contribute to an ice damming problem.

The solution is to beef up your roof insulation by adding rigid foam on top of the existing roof sheathing (and installing a new roof) or by adding insulation on the interior.

Jan 2, 2011 9:41 AM ET

Thanks Martin
by Iowa builder

Thanks for the reply. The 6" would be cutting myself short but I included it in my range of thickness.
I do feel that the thermal bridging is on the trusses you can see that on the frost pattern. I feel comfortable about air leakage for I am getting better blower door test on the total envelope than other homes of the same size. I do have foam over the trusses just not enough I guess primarily where the webs meet the top cord. I will need to correct the problem from the under side they are 4 homes tied together and thats 128 squares of roof. One more thing. It only seems to be bad where there is a large volume of space below the roof and it all freezes at night. I feel I am right at the threshold and sealing will do the trick but don't want two wrongs. I have 86 more homes two do.

Jan 2, 2011 10:04 AM ET

California Framing
by Alex Rockas

California framing (also called blind valley framing) is when you frame a portion of a roof over a roof that has already been sheeted. Here is a link to a page with a rough sketch. It will be hard to spray foam in those areas. Is it feasible to try and seal those spaces with spray? There will be nooks and crannies that easily could be missed. Also, roofers usually seal the top of the sheeting with Grace fire and ice shield before screwing down the metal roof. It is a very effective vapor barrier. Thanks for your help. Alex

Jan 2, 2011 10:33 AM ET

Edited Jan 2, 2011 10:34 AM ET.

Second response to Alex Rockas
by Martin Holladay

Determining whether a spray foam contractor can reach a space in order to install foam isn't rocket science; it's common sense. If a guy in a moon suit has enough room to get in there with his spray wand and move his spray wand in a smooth motion without bumping into anything, the work should be possible. When in doubt, talk to your spray foam contractor.

However, I think you already know the answer to your question. Spray foam is not a good solution when you want to spray the underside of roof sheathing in tight crawl-space areas of attics.

In your case, rigid foam on top of the roof sheathing makes more sense.

Jan 2, 2011 12:49 PM ET

Iowa Builder
by Doug McEvers

Do the roof trusses you are using have an "energy heel", are you getting full insulation out over the plate line? My experience in using energy heel trusses, insulation to code or above and venting the attic space at both the eave and ridge, prevents ice dams. My projects have been in locations with 8,000 to 9,500 heating degree days.

Jan 2, 2011 5:19 PM ET

Iowa builder experiences differ from mine
by aj builder, Upstate NY Zone 6a

Scratching my head as to his melting snow. I have had snow melt problems in the past mostly from their being very deep snow on a roof for more than a short period of time. The insulating effect of the deep snow is the issue along with it being on a building that is heated. We have plenty of "unheated" structures here that suffer not from melting snow but do collapse once in awhile due to weight. Lots of snow, rain and ice can take down any building in this area.

If I ask what is different between our two experiences, the number one difference I see is that I do not have the HVAC or any ducts in my attic spaces. That makes me think they may just be pushing a lot of BTUs into his roof that my homes aren't doing, especially since he is insulated much more than my 5-6 inches.


Jan 2, 2011 5:29 PM ET

Solutions for Iowa Builder type assemblies
by aj builder, Upstate NY Zone 6a

Given that snow is on the roof and is melting and that the HVAC and ducts are in the attic space, vent. Your insulation contractor should have a method that works. If he does not, get another or contact Building Science Corporation and pay for solution that is backed by an insurance policy and contract.

Your project is certainly large enough to afford more expense to solve a possibly horrendous future liability.

Jan 2, 2011 5:35 PM ET

Edited Jan 3, 2011 8:42 AM ET.

Response to AJ Builder
by Martin Holladay

Iowa Builder has a conditioned attic with insulation installed between the roof rafters. Your suggested solution is: "vent."

Obviously, you can't vent a conditioned attic, so I assume you are suggesting that Iowa Builder install 2x4s on top of his existing roof sheathing and vent on top of the roof sheathing. That may help, but it's an expensive solution. If Iowa Builder ends up doing that, I hope he puts down some rigid foam under the 2x4s.

Jan 2, 2011 5:58 PM ET

Different solutions for 4 of the 90 homes he is building
by aj builder, Upstate NY Zone 6a

My suggestions above are mainly to do with his next 86 units. As for the existing units, since I have no issues with snow melt and he does, I think his safest fix for the 4 built units is to add venting above. He could try spraying foam on all the HVAC system too to try to stop the heat flow that I do not have at my homes. I sprayed my ducts in the cellars and love the outcome.

As to expense, yes... but if the melting is a real issue it must be dealt with. No way would I add foam and make a sandwich of any OSB board. Nope. Never ever ever ever. OSB waits for no one to rot. Junk. Has to be dry. Never sandwiched.

For the same price as adding foam sheeting he can spray more foam in the attics which like I said should encapsulate all his ducting and even blank sides of his HVAC. Open cell comes off easy if ever a unit needs to be pulled and replaced. Just leave appropriate areas insulated with wrap style insulation.

Jan 2, 2011 6:03 PM ET

Edited Jan 2, 2011 6:05 PM ET.

How to vent on top
by aj builder, Upstate NY Zone 6a

To vent the first 4 units for the least cost I would go right over the shingles. Add 2x4s then Advantech Zip sheathing taped, then shingle it as per standard methods. The fascia would be lapped and doubled like we do now for thick roof assemblies. Vent added between fascias or choose one of many other set ups.

Done... problem gone, start building the next 86. Worry not.

Jan 2, 2011 10:43 PM ET

Great feedback thanks
by Anonymous

1st. Doug yes I have energy trusses and the exterior sheathing runs right too the deck and notched around the tails and foamed full.
2nd. Aj Yes snow is an insulator but does not melt itself so I feel Martin is on track with the thermal bridging it don't take much water on a roof to make a lot of ice.
A roof over a roof will not happen on a job of mine, the insulating contractor will correct it or remove his product.
This is my first experience in using foam. My last project of 26 duplexes and same style build only vented,R-8 on the HVAC and blown fiberglass. Drove thru there the other day and had a consistent 16 to 20 inches of snow on the roofs and not one icicle.
I wanna be green but I'm not slip sliding on a carbon footprint. ( Just scratching my head here)

Jan 2, 2011 10:46 PM ET

Sorry was my previous post
by Iowa builder

Sorry was my previous post not anonymous

Jan 3, 2011 6:57 AM ET

Edited Jan 3, 2011 1:31 PM ET.

More on minimum R-values
by Martin Holladay

Iowa Builder,
You wrote, "The insulating contractor will correct it or remove his product."

I think it's a good idea for you to hold the insulation contractor responsible for his work. In preparation for meeting with your contractor, I would suggest that you research your local code requirements for minimum ceiling R-values. It's probable that your contractor's spray-foam job doesn't meet minimum code requirements. If the installation doesn't meet code, this is a negotiating point in your favor.

Jan 3, 2011 4:42 PM ET

Duct leakage to outside in homes with ducts in conditioned space
by Brennan Less

I thought it worthwhile to mention that moving duct work into conditioned space does not eliminate all duct leakage to outside. We have measured duct leakage to outside in numerous homes that have brought their ducts inside, and they still register a fair bit of leakage to outside. The reason for this is that the "conditioned" areas that the ducts now leak into are also connected to the outside. It is not difficult to imagine this happening in a drop ceiling chase that is imperfectly sealed. Now, this is certainly not as bad as having that air leak directly into an attic or crawlspace, because heat/cool is imparted to the indoor air before the leakage to outside occurs. But there is still an energy and air delivery problem. I'm sure it goes without saying on this website, that envelope tightness is essential in homes with duct work within the conditioned space. A tight envelope will ensure that duct leakage within conditioned space stays within that space. I add this comment because it is often thought that once ducts are in conditioned space, there is no longer any need to seal the ducts with tape/mastic. So, if you move duct work into conditioned space, be sure that the conditioned space and your ducts are well-sealed, otherwise you've accomplished little. Cheers.

Jan 3, 2011 11:42 PM ET

Creating a conditioned attic without a HVAC unit in the attic?
by Danny Kelly

Seems to me, the only reason to create a conditioned attic is to bring the HVAC system and ductwork into the thermal envelope of the home. If there is no HVAC in the attic - why not just air seal and insulate at the attic level - much easier and less expensive. I am not a HERS rater but I would think adding all of that volume and extra surface area to the thermal envelope would actually decrease the efficiency of the home. Even if you are a foam lover - can't you spray your foam at the ceiling level? Any HERS raters have an opinion on how this would affect the rating?

Jan 4, 2011 12:03 AM ET

Edited Jan 4, 2011 12:33 AM ET.

Bingo Danny
by John Brooks

keeping the thermal envelope at the attic floor level does keep the exterior surface area lower...
and it will use less energy.
It will not change the HERS index because the HERS index will compare the house envelope to it's own geometric twin.

I think the best solution is to keep the envelope at the attic floor with airtight drywall and use more thickness of less expensive insulation.
Increasing the thickness and r-value will improve the HERS index

Jan 4, 2011 6:08 AM ET

Edited Jan 4, 2011 9:13 AM ET.

Response to Danny Kelly
by Martin Holladay

If you don't have any HVAC equipment or ductwork up there, and you don't intend to use your attic for living space, then it is clearly best to leave the insulation level on the attic floor. I agree with you and John Brooks: that makes air sealing easier and insulating cheaper.

It also makes it easier to spot and repair roof leaks, which is a big plus.

I've added a few sentences at the end of the blog to address these issues.

Thanks again for your comments.

Jan 4, 2011 9:47 PM ET

Thanks Martin
by Danny Kelly

Thanks for the clairifcation. I thought your article was clear by the way - was mainly responding/questioning AJ's post/practice and wondering if would afect HERS. John - thanks for the reply.

Jan 5, 2011 3:40 PM ET

Construction details you alluded to
by M. Johnson

Martin you said "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."

Do you have any articles with more discussion of that? Something with pictures is most welcome. I have been looking for a comprehensive discussion of how this might be done with low probability of trouble in a hot-humid climate.

Thank you -- Mark Johnson

Jan 5, 2011 4:04 PM ET

Response to Mark Johnson
by Martin Holladay

Mark Johnson,
No, I can't think of a specific article that discusses the concept.

However, people have been building site-built ventilation chutes in rafter bays for many years, so we aren't discussing a new technology or technique.

Although these ventilation chutes are usually used with fiberglass or cellulose insulation, there is no technical reason they can't also be used with spray polyurethane foam, as long as the panels are rigid enough not to collapse under the pressure of the expanding foam.

Jan 5, 2011 5:18 PM ET

My spray company will do vented roof decks
by aj builder, Upstate NY Zone 6a

My spray company, as I remember said they would spray right over the store bought foam vent chutes. For odd sized spacing one would have to make these fit well or make your own like has been posted many times on this site. He had no issues with venting by the way, though I really want to try an unvented non heated attic assembly and it is working beyond my expectations. Much better than detractors state it should. Open cell if a tiny bit of water gets to sheathing will let the sheathing dry. That is a fact. Much less chance of rot verses closed cell. If I went closed cell I would purchase pressure treated sheathing. It would be worth every penny.

Jan 5, 2011 7:56 PM ET

Edited Jan 5, 2011 7:57 PM ET.

Spray foam on the roof deck
by Mark Parlee

I wrote an article that appeared in the JLC on Repairing a Rotting Roof June 2010 that dealt with a few shortcomings of the Icynene, when not installed considering all of the parameters. While this was not just an insulation problem but an indoor humidity problem as well, some things could have been done to alleviate this problem at the time of original construction.
Iowa Builder feel free to get in touch with me for additional perspective.

Jan 8, 2011 9:08 PM ET

Edited Jan 9, 2011 5:52 AM ET.

Making the Attic a Conditioned Space
by Green Mountain Realty

Taking the building envelope to the roof is a good approach and will save energy - another approach is to use radiant coated deck sheathing, seal all ceiling attic penetrations and blow in R-60 insulation as it will cost less than spray foam and should give at least the same energy efficiency to the home.

Mar 26, 2011 6:32 PM ET

What about thermal bridging in the ceiling joist
by A.K. Harrison

If I used 2x10s would I spray cellulose to the top of the joist and add an extra 2 inches? Would the 2 inches be enough to eliminate thermal bridging? What is the best barrier to use between the ceiling joists and the plywood for cellulose?

Mar 26, 2011 9:09 PM ET

Response to A.K. Harrison
by Martin Holladay

Q. "If I used 2x10s would I spray cellulose to the top of the joist and add an extra 2 inches? Would the 2 inches be enough to eliminate thermal bridging?"

A. If you are using spray cellulose, you would need to install framing members perpendicular to the rafters to address thermal bridging. Two inches would go a long ways toward addressing thermal bridging.

Q. "What is the best barrier to use between the ceiling joists and the plywood for cellulose?"

A. I'm not sure what you mean. If by "joist" you mean "rafter," and if by "barrier" you mean "ventilation chute material," the answer is "plywood, fiberboard, or rigid foam." If you mean something else entirely, please explain.

Apr 20, 2011 10:41 PM ET

You need to cite your sources
by Gerard Celentano

Sorry for long post. I find this article a bit more informative than previous ones but the opening premise is flawed (IMHO) and hence doesn't seem worthy of much discussion. I find it curious that I asked a question touching on some of the topics in this article a few months back. The advice you guys gave seems to contradict what this article is saying today.

I have a problem with opening premise is that it costs "homeowners dearly". I'd like that quantified since I'm pretty sure that implementing the recommended upgrades would cost homeowners dearly.
I tend to rely on my experience so forgive me if I can't quote DOE or IRC. I have a house in the mid Atlantic with the very scourge of energy efficiency you refer to (leaky air handler in the dreaded uninsulated attic/crawlspace). Your definition of costing "dearly" is about $1000 extra to run the AC in the summer. I can't see how I can effectively insulate the attic (as you suggest) with an ROI of less than 7 years. So even though it's costing me dearly, I have better places to put my resources.

I have another house in New England the same energy scourge and I paid big bucks to spray foam the rafters. I calculate that I save about $0 (that's Zero) in AC costs. Maybe it's a hundred bucks a year, but with rising energy costs my ROI should be sometime before I expire (I'm in my late 40's).

Your delta T argument goes down the right path but is meaningless without duty cycle; ie. it may only be for short duration. This is why a frugal New Englander (or one good at math) would not insulate his attic to save cooling costs. Even my house near the Nation's Capital peaks at about 50% during the hottest 6 hours of the hottest days. Thank goodness it's oversized by today's standards because it's a much cheaper solution to solve the issues raised in your opening argument than the ones you proposed.

I hate to pick; but your thermal bridging argument only makes sense in the context of an opportunity cost or comparative analysis. Diminishing returns are valid issues that real people have to struggle with. Instead of glittering generalities you need to quantify how much energy you'll save jumping from R1.5 to R20 in contrast to R20 to R50? Then quantify the perceived comfort, up front and recurring costs, and ROI to substantiate your claim.

Thank you

Apr 22, 2011 12:25 PM ET

Good points Gerard
by Scott Sanders


I have to agree with the idea behind you points on the cost effectiveness of retroactively adding insulation to existing houses and also looking at the incremental savings of additional energy saving strategies in new construction (such as 2" of under slab insulation vs 16"). Regarding existing houses, a cost/benefit analysis definitely should to be performed, and unfortunately, the benefit is frequently not realized within the desired time frame (before an owner wants to sell the house for example).

The two "devil's advocate" points that I would like to mention are
1) The externalized costs of heating/cooling your house (the ambiguous environmental impacts of remote power generation and transmission) are not reflected in the price you pay for gas and electric. Therefore the cost benefit analysis may work out in more situations than we think.
2) In new construction the incremental costs of increased energy efficiency seem to intuitively be cost effective because the labor costs will have little change. I can't name a study off the top of my head, but I bet there is one out there from LBNL or Building America that backs this up. There is of course a tipping point where they are cost in-effective, but I don't know of any energy modeling software that can accurately help determine this point.

Apr 24, 2011 6:02 AM ET

Response to Gerard Celentano
by Martin Holladay

Thanks for your post; you bring up many important issues.

First of all, the point of my article is to provide advice to anyone who wants to create a conditioned attic, with the aim of making sure they do a good job. I never intended to encourage people to install insulation between the rafters of an existing home. That's why I wrote, "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."

According to your own estimates, this fundamental design flaw is costing you $1,000 a year. For you, the investment in insulation to create a conditioned attic isn't worth it, because the payback period is 7 years. That's your decision. Some other homeowners are very happy to make investments in their homes with a 7-year payback.

I am happy to stipulate that many energy-efficiency measures don't make much sense when analyzed in terms of payback. The main reason for these long payback periods is the fact that fossil fuels are quite cheap, and are sold at prices that don't include external costs like damage to the planet's climate.

Apr 25, 2011 12:51 PM ET

rigid foam under rafters - fire protection required?
by T Conover

Thanks for the article. If you install the rigid foam under the rafters in an unoccupied/non living space attic, does the foam need to be covered with drywall?

Apr 25, 2011 12:59 PM ET

Response to Troy Conover
by Martin Holladay

The answer to your question can be found in section R316.5.3 of the 2006 IRC. If the attic space is "entered only for purposes of repairs or maintenance," you can dispense with the requirement for a thermal barrier and instead install an ignition barrier (a less stringent option than a thermal barrier). But ignition barriers aren't nothing -- many people choose drywall, which must be at least 3/8 inch thick to qualify as an ignition barrier.

If you install Thermax foil-faced polyiso, some building inspectors will allow you to dispense with the requirement for an ignition barrier.

Dec 3, 2011 11:20 AM ET

rigid between rafters
by joell solan

Thanks for thinking so much about insulation, i'm still new to thinking about this so please bear with me.

I live in the Northwest, am planning to gradually finish a 600 sq foot attic space with cathedral ceilings, new roof put on about 5 years ago, before we moved in. my rafters are only 2x4 and i don't want to lose much living space. My tentative plan has been to put 2-2.5 in. polyiso between and 2 in. under rafters for now. This won't get us to code but would be better than what we have now. now i've been thinking that someday the roof will need to be replaced and i could add rigid on top.

Which brings me to my questions:

if "ventilated rafter bays are incompatible with rigid foam insulation installed on top of the roof sheathing" does this approach make sense?

and since poly-iso is faced, can i use 2 layers on top of each other and if so do i create a sandwich with the faced surfaces or layer them with facing to the interior? thanks

Dec 3, 2011 11:38 AM ET

Response to Joell Solan
by Martin Holladay

Don't bother to include ventilation. Instead, fill the 3.5 inch rafter bays with 3.5 inches of polysio (one 2-inch layer, and one 1.5-inch layer). Seal all gaps with canned spray foam. Then install a continuous layer of 2-inch polyiso under the rafters to stop thermal bridging.

Using several layers of foil-faced foam is fine -- there is no problem with making a sandwich like this, except that it is slow, fussy work.

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