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Prevent Ice Dams With Air Sealing and Insulation

Roof ventilation and rubber membranes are admissions of defeat

Posted on Oct 1 2010 by Martin Holladay

During snowy winters, many northern homes are plagued by ice dams. If your house suffers from wet ceilings during the winter, you may be ready to call up a contractor. Be careful, though: since most contractors don’t understand the causes of ice dams, they often suggest the wrong solution.

Ice dams form when a home’s escaping heat warms the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and melts the underside of the snow layer on the roof. Water trickles down the roof until it reaches the cold roofing over the eaves, where it freezes. After a while, the ice at the eaves gets thicker and thicker, forming an ice dam. Eventually, water backs up behind the ice dam. If the water reservoir is large enough, it can back up under the roof shingles and damage ceilings. (Image #4, below, depicts the complicated shape of a typical ice dam more accurately than the simplified drawings in Images #2 and #3.)

The four possible solutions to ice damming are:

  • Sealing air leaks between the warm interior and the attic or cathedral ceiling.
  • Adding more ceiling insulation.
  • Improving ventilation between the top of the insulation and the roof sheathing.
  • Installing a rubberized membrane under the roofing.

While the first two of these solutions can reduce or eliminate the problem, the last two solutions are the equivalent of waving a white flag and admitting defeat.

Step one: seal the air leaks

Most ice dams are caused by flaws in a home’s 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.. If escaping indoor air finds its way to the underside of the roof sheathing during the winter, the heated air raises the temperature of the sheathing. That’s bad.

One energy expert who made a career of correcting ice-dam problems was the late Tony Woods, a Canadian physicist and founder of CanAm Building EnvelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. Specialists in Mississauga, Ontario. After diagnosing problems in hundreds of homes, Woods knew from experience that most ice dams were caused by air leaks.

I interviewed Woods a few years ago for an article in Energy Design Update. “You can’t say to the consumers, ‘Will you please look in your attic?’ — because they don’t want to look into the attic,” Woods told me. “Who does the consumer go to when he has a problem? It’s always the roofer. The roofer will replace any rotten wood and try to talk the consumer into buying a new roof.”

Woods recalled one home that was plagued by leaks every winter. “The lady had called three roofers in just two or three years,” Woods told me. “The first roofer persuaded her to replace the roof.” The following winter, however, her ceiling was once again wet.

“The second year, she brought in a second roofer. He said the first roofer hadn’t ventilated properly, and he built a second roof over the first roof. But she still had the same problem. Then, when the third roofer came on, I was called to look at the house. There were pot lights galore. … When we said that the problem is not the roof, the lady burst into tears.”

Unless you have a blower door handy, the only way to locate air leaks into an attic is to crawl up and look around. “Most of the sealing can be handled effectively with two-component polyurethane spray foam,” Woods told me. “Usually you have to move some of the insulation aside to expose the top plates. You can seal ducts by spraying, in most instances. Occasionally we will cut pieces of drywall to box the can lights. Also, more than 50% of exhaust fans are not connected to the outside, so we extend the exhaust ducts.” Although Woods often advised homeowners that they needed more insulation, he told me that “most of our work is stuffing and foaming holes.”

Step two: check your insulation level

Once you’ve plugged your air leaks, check your insulation levels. The latest version of the International Residential Code requires R-49 ceiling insulation in climate Zones 6, 7, and 8. (That area includes northern Idaho, northern Utah, Wyoming, South Dakota, Minnesota, Wisconsin, northern Michigan, upstate New York, Vermont, New Hampshire, and Maine.)

In these areas, ceilings need a minimum of 14 inches of fiberglass batts, cellulose, or open-cell spray foam. If you’re using blown-in fiberglass, you’ll need about 20 inches to achieve R-49.

In climate zones 4 and 5 — an area that includes northern Arizona, northern New Mexico, and Tennessee — you’ll need a minimum of R-38 insulation in your ceiling. That means at least 11 inches of fiberglass batts, cellulose, or open-cell spray foam, or about 15 inches of blown-in fiberglass.

In homes where there isn’t enough room to get R-38 or R-49 at the perimeter of the attic, the best thing to do is to install as much closed-cell spray polyurethane foam as the space permits. In some cases, it may be necessary to install additional rigid foam insulation on top of the existing roof sheathing. (That can only be done if you are replacing the roof.)

Two more points about attic insulation:

  • Attic insulation must completely cover the top plates of a home’s exterior walls.
  • Code insulation requirements represent the legal minimum. Most green builders choose to exceed these minimum requirements.

Step three: improve ventilation between the insulation and the roof sheathing

Once you’ve done your best with steps 1 and 2, it’s worth considering step 3: ventilation.

In the world of ice-dam prevention, ventilation is capitulation. By recommending ventilation, a builder is saying, “I wasn’t able to include enough insulation to prevent the roof sheathing from being warmed by escaping building heat. So I guess I’ll use another method to cool the roof — I’ll ventilate the underside of the roof with exterior air.”

Although it is an admission of failure, this type of roof ventilation often makes sense. It’s a kind of insurance.

When using roof ventilation to address ice dams, remember:

  • Ventilation should always be the third, not the first, weapon in your arsenal. Distrust any expert who advises using ventilation as the first step toward solving an ice dam problem.
  • If ventilation channels are improved without any attempt to perform air sealing work, ventilation improvements can make an ice dam problem worse or increase a home’s fuel bills. (By depressurizing an attic, an effective ridge vent often increases air leakage through the ceiling, bringing more heat than ever against the roof sheathing.)
  • The best ventilation channels include a balance of soffit vents and ridge vents. Attics do not need gable vents.
  • In a cathedral ceiling, provide an air barrier between the top of the insulation and the ventilation channel. Site-built ventilation chutes (foamed or caulked in place) are far preferable to polystyrene Proper-Vents.
  • All attic ventilation systems require an insulation dam (wind-wash protection) at the perimeter of the attic, facing the soffits.

In the past, code-minimum insulation requirements were woefully inadequate, and almost every cold-climate home leaked enough heat to generate ice dams. Many building inspectors, noting that attic insulation was code-compliant, falsely concluded that the only way to stop ice dams was by improving ventilation.

In recent years, building codes have raised the bar on attic insulation. These days, if your ceiling is airtight and is insulated to the latest building code requirements, ice dams are far less likely.

Wayne Tobiasson, a researcher at the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire, has for many years advised builders to use attic ventilation to address ice dams. Recently, however, Tobiasson has modified his advice. “I have backed off of some of my statements about ventilating in cold regions,” Tobiasson explained at the 2010 Westford Symposium on Building Science. “As we increase the amount of insulation in roofs, there is less and less need to ventilate.”

Although so-called hot roofs (roofs without any ventilation) can work well, ice dams can still form on a hot roof if the snow is deep enough. (Very deep snow acts like insulation. If your roof is covered with two feet of fluffy snow, the bottom of the snowpack is insulated from cold outdoor temperatures. That raises the chance that melting will occur.)

So, if your climate is very snowy, you probably want to stick with a cold (ventilated) roof.

For more information on this issue, see All About Attic Venting.

Step four: cover your roof sheathing with rubberized membrane underlayment

There are lots of roofers out there with a sure-fire cure for ice dam problems: just cover your entire roof with Ice & Water Shield.

Run, don’t walk, away from such a roofer.

These are the guys who have completely thrown in the towel. They have no plan to seal air leaks into your attic. They have no plan to improve your insulation level. They’ve given up on ventilation. In fact, they have no plan whatsoever to prevent ice dams. This is what they’re saying: “Yup, your roof is going to get an ice dam. Maybe a big one. We can’t plug your energy leak, so we’ll just let the ice dam develop. The Ice & Water Shield will probably keep your ceiling dry.”

That said, Ice & Water Shield is relatively cheap insurance. Used properly — extending from your eaves to a point that is 3 feet higher than the plane of your exterior wall — Ice & Water Shield will limit damage from ice dams that form due to unusual weather conditions or some idiot who disturbed your attic insulation.

But remember: ventilation and rubberized membranes should be the last weapons in your arsenal. First, seal your air leaks and beef up your ceiling insulation.

[Author's postscript: The topic of ice dams, like many other building science topics, is multifaceted, and a blog-length article can't pretend to be comprehensive. There's always more to say on the topic, and the discussion that follows in the comments below is informative. Particularly pertinent are Bill Rose's observations on roof geometry (Oct. 1 comment) and the Oct. 12 comments by John B. and Brian T. concerning the heat flow from a masonry chimney to the roof sheathing in an unconditioned attic. Keep those comments coming!]

Last week’s blog: “Radiant Barriers: A Solution in Search of a Problem”

Tags: , , , , , , ,

Image Credits:

  1. Image #1: Daniel Morrison
  2. Image #2: Building Science Corporation
  3. Image #3: University of Minnesota Extension (
  4. Image #4: Canada Mortgage and Housing Corporation

Oct 12, 2010 10:45 PM ET

ventilation redux
by tom Barthelemy

Martin - as you say, venting will prevent ice dams but sometimes at the cost of energy efficiency. thing is, the roof where I drilled the pitifully inadequate holes thru the lookouts already had lots of insulation - done in the early 90's so it consisted of 14" of fiberglass, but that's not so slacky. Ice dams were not an issue there, but a couple other bays that were vent-blocked by skylites had premature shingle degradation.

My conclusion is that venting works for you in several ways. You were hasty to call it a white flag.

Oct 13, 2010 10:10 AM ET

response to martin
by TGray

Thanks. Yes I could have tacked up a wash dam carefully scribed to the profile of the 14" wide chute and then close all gaps w/ spray foam. With 120 lf of soffit I guess I just said it has to be good enough. My concern is if what I did was correct and recommended how on earth could I convince a normal homeowner or typical insulation contractor to do all this?

On the 2nd question about the hip roof you refrerred be to Kohta's post- but that is dramatically different- 6" polyiso above deck and spray in each rafter bay. I am talking about a basic builder home, hip roof, no soffit, batts @ ceiling plane, unconditioned attic with no intentional venting. You could never make that right without removing gutter, fascia, fix up eve condition (pinch point) reinstall gutter and fascia w/ venting. By your comments above you might recommend removal of all batts, air seal at ceiling plane then spray the deck and eves to R38 with no venting?

Oct 13, 2010 10:48 AM ET

Second response to TGray
by Martin Holladay

Plenty of unconditioned attics work fine without perfect soffit-to-ridge venting. Some have gable vents, and some have no vents or incidental air leaks that function a little bit like venting.

If the ceiling is sealed against air leaks, so that humid indoor air can't enter the attic, such attics can perform well. If you want to prevent ice dams, though, you certainly need a well-sealed ceiling -- no air leaks! -- and very deep insulation. I'm partial to R-50 or R-60 cellulose.

Oct 13, 2010 11:08 AM ET

Missed issue...
by Anonymous

I have read (most) of the above comments, and still don't have an answer for my specific roof. I live in sunny Southern Colorado, in the mountains. We get a lot of sun, but a lot of snow. My house has a 5/12 pitch, but one side is southern oriented to get the maximum passive solar benefit. The drawback, is that half of my roof is northern oriented. With charcoal colored shingles, the south side melts off, heating the roof, and assumable the attic space, while the north face gets no sun, and is buried in up to 3' of snow. Combine that with a chimney running into the attic space, and you have 2 separate heat sources, that aren't mentioned above on how to best manage them. There was also no mention of ice melt cables anywhere. I understand the 4 basic steps to solve the ice dams, but if the logistics of the roof (orientation, color) and the physical aspects of the attic (chimney) all provide heat into the attic space, then what is my best option to resolve my north face ice dams? By the way, thank you for your assistance. As I've read above, there's a million "experts" out there, all claiming that they know the only way. I appreciate your objectivity and assistance.

Oct 13, 2010 11:20 AM ET

Response to Anonymous
by Martin Holladay

If your analysis is correct, and the only two significant heat sources for your attic are solar gain and a chimney, here are your options:
1. You can certainly insulate the section of your chimney that passes through your unconditioned attic. I described details in my October 12 comment.

2. If the solar gain through your south roof is still enough to melt snow on your north roof, then you can either experiment with improved ventilation, or you can install spray polyurethane foam insulation on the underside of your north-slope roof sheathing.

Oct 14, 2010 10:23 AM ET

anonymous - ventventvent
by tom Barthelemy

If you have enough insulation, then the solution to your Colorado roof is to ventilate the cavity - preferably with a ridge vent, to get rid of the sun's heat. Ventilation, Insulation, tight construction - all three are parts of the package and it is real unwise to cherrypick one or two of them.

Oct 15, 2010 5:23 AM ET

by John B.

Thanks, Martin for the concise response. This site is great help to alot of folks!

Oct 30, 2010 7:10 AM ET

What do we do about flat roofs in the city that have ice dams?
by Anonymous

Do flat roofs have the same causes for ice dams (insulations, lack of ventilation etc.)? What do we need to do to correct the leaking usually seen in one corner of the home? Do we also need to add insulation?

Oct 30, 2010 12:25 PM ET

Response to Anonymous
by Martin Holladay

Seal air leaks in your ceiling plane and add insulation.
Most flat roofs aren't ventilated, and shouldn't be.

Dec 16, 2010 4:12 PM ET

Who do you call
by Anne

What type of person do I call to look at fixing the air leaks? Is this anyone who does insulation? I have a one and a half-storey - no attic, so should I wait until I'm going to replace the roof (due anyway) and then get a separate insulation person to come first and do step 1?
Thanks for any advice you can offer.

Dec 16, 2010 4:24 PM ET

Response to Anne
by Martin Holladay

The type of contractor you are looking for is called a home-performance contractor, a home energy rater, an energy auditor, or a blower-door contractor. Whoever you hire should have (and use) a blower door.

You can ask your state energy office of your local utility's energy conservation office (if your utility has one) to identify a local contractor with a blower door. Or you can visit the Web sites of RESNETor BPI -- click the links -- and search their databases for a contractor near you.

Jan 29, 2011 4:00 PM ET

Half-hot roof
by Eldan

I just had my cape-style home reinsulated. In the side attics we went with 7.5" open cell spray foam in the rafters from the soffits top the tops of the knee walls. Above the sloped and horizontal ceiling is 6"-8" of cellulose. Airsealing was performed. The top attic has a vent in each gable. So the top half of my roof is cold and vented (perhaps inadequately) and the bottom half is unvented. Roof pitch is 7/12.

This winter has brought near record snowfall to northern New Jersey and I've got a lot more snow accumulation my roof than most of my neighbors. But I've also got ice dams and most of my neighbors don't. Those houses aren't capes though, and my overhangs are wider. There's a lot of different factors involved I suppose.

I've been observing things daily and it appears the snow is melting from the top half slightly faster than the bottom half. I suppose this means I should try to increase ventilation in the top attic. If I'm right, how would you go about it? And if I'm wrong?

Jan 29, 2011 5:03 PM ET

Response to Eldan
by Martin Holladay

You don't need more ventilation; you need more insulation (and possibly air sealing work).

6 to 8 inches of cellulose isn't much -- that's about R-21. Much less than minimum code requirements.

Call back the cellulose truck! Your open-cell spray foam could have been thicker, too -- its only about R-26.

Jan 29, 2011 8:15 PM ET

Response to Martin, #63
by Eldan


I think the fact that there's still many inches of snow on my roof today while my neighbors' roofs are nearly bare or have the shingle pattern visible indicates that we've done a relatively good job air sealing and insulating. The ice dams may just be this year's anomaly but of course I'm seeing them as bitter irony.

I forgot to mention that the 8" of cellulose in the top attic is on top of the old fiberglass. I'm not sure what the total R-value is. In the sloped ceiling with 2x8 rafters the old fiberglass was removed and replaced with cellulose. Obviously there's no room for R-49 there no matter what I do. There will always be a band of minimum insulation in the middle of my roof. How does that impact the four solutions?

Jan 30, 2011 6:36 AM ET

Another response to Eldan
by Martin Holladay

It's hard to diagnose your problem over the Internet. Here are some possibilities:

1. It's possible that your ceiling still includes air leaks that are heating your roof sheathing.

2. It's possible that the band of R-26 insulation is allowing enough heat to reach your roof sheathing to cause problems.

If you can't afford to add more insulation to your roof (for example, by installing rigid foam insulation on top of your existing roof sheathing, followed by new roofing), you may have to live with the occasional ice dam. Installing wide Ice & Water Shield will minimize damage to you ceiling from roof leaks.

Jan 31, 2011 2:09 PM ET

Flat roofs
by WesM

It was not uncommon for flat roofs to be constructed with voids, or attic spaces, certainly those of a certain vintage. Drainage slopes were often constructed from separate framing on top of the primary structure, and these spaces were not always vented, as they probably should be. My own house, built around 1950, has this type construction. When the existing built up roof membrane is due for replacement my intent is to remove the slope framing and install sloped rigid insulation on top of the roof joists and install a two ply SBS membrane. This will eliminate the unvented void under the membrane, increase the insulation and create the slope to drain to the scupper location.

It is unclear to me if the person in post #58 is having leaking inside the home in that corner, or if there is water spilling over the edge of the roof, or at a scupper drain location. If it is leaking inside he house I would inspect the roof for obvious damage and soft spots. The junction with the parapet is always due for scrutiny. I currently have a location on my roof where it appears there is a hole in the membrane at the toe of the metal flash over the cant strip. I believe this is the source of the water infiltration I was seeing inside this fall.

Jan 31, 2011 4:16 PM ET

Icynene roof deck, peel and stick membrane, climate zone 4
by Jon Rasich

I am building an unvented attic/cathedralized 2nd floor with icynene on the roof deck. Inspector insists upon the "peel and stick" membrane from the eave up several feet up the roof--that is, enough to cover a portion of my insulated roof deck. This seems to be both unnecessary--the entire roof will be cold and won't create a melt/refreeze condition--and dangerous--won't the roof "want" to dry to cold/low RH exterior in the winter but be stopped by the membrane?

Roof has a 14/12 slope, which would seem to further mitigate ice damming. Should I continue to push back with the inspector.

Thanks for any advice.

Jan 31, 2011 4:30 PM ET

Response to Jon
by Martin Holladay

Just because you are insulating with Icynene doesn't mean you can't have ice dams. How much Icynene will you install? What's the R-value?

Also, what's your climate?

You wrote that you plan to install "Icynene on the roof deck." That's unusual; are you sure you didn't mean to write UNDER the roof deck?

Icynene-insulated roof decks usually dry to the interior, because Icynene is vapor-permeable, and because most types of roofing are not very vapor-permeable.

Feb 15, 2011 9:46 PM ET

Edited Feb 15, 2011 9:48 PM ET.

Roof Insulation Retrofit
by Charles Scarborough

I am insulating a roof under a laundry room which has had extreme ice dam formation. I live in far NE PA. My roof is metal with snow brakes. I am installing 4 inches of XPS spay can foamed in place with a 1.5 inch air gap below the deck. There are roof vents cut in below the ridge. Also will have 2 inches of polyiso strapped across the bottom of the rafters. The XPS extends over the top plate and sealed with foam. This will make the attic space a unheated storage area. Will this be sufficient to stop the ice dams from forming? I had to replace the gutters twice before and then I added the snow brakes. I still have my gutters but I have 4 foot icicles too. Any comments and insights would be appreciated.

Feb 16, 2011 6:29 AM ET

Response to Charles Scarborough
by Martin Holladay

It's impossible to be sure from your descriptions whether the work you propose will prevent all future ice dams. A few comments:

1. XPS is different from "spray can foam." XPS is extruded polystyrene, a type of rigid foam that comes in sheets.

2. It's hard to determine what your final roof assembly R-value will be, because you haven't identified the foam you are using. Two inches of polyiso has an R-value of about R-13. You are in NE Pennsylvania, which is either climate zone 5 or climate zone 6. If you are in climate zone 6, then the 2006 IRC requires a minimum of R-49 in your ceiling. That means that after installing the 2 inches of polyiso, you still need an additional R-36.

3. Your plan to install 4 inches of "spray can foam" won't meet R-36, since that would require your "spray can foam" to have an R-value of R-9 per inch -- an impossible value. It sounds like you need to increase the R-value of the insulation you plan to install.

4. Your plan will only work if you succeed in installing an effective air barrier at your ceiling plane. I suggest you do what it takes to add R-49 to your ceiling.

Feb 16, 2011 5:02 PM ET

Roof Insulation
by Charles Scarborough

Martin. Thank You for responding. I apologize for not being clearer. What I meant to say was that there will be 4 inches of XPS cut to fit between the rafters and that they will be spray foamed in place from a can at the edges. I looked at the map and I believe I'm Zone 6. Will the Polyiso be enough of an air barrier across the rafters? I could possibly add additional layers to up the R-value as there is room to fit it. With 4 inches of XPS and 2 inches of Polyiso I believe that is R33. Another 2 inches will bring it to R46. Close but not to code. Will I gain any extra R value with the reflective side down towards the attic? The information mentions an additional R value gain. Thanks for your time and I am sorry for my poor description previously.

Feb 16, 2011 5:09 PM ET

Response to Charles Scarborough
by Martin Holladay

Upon re-reading your original question, I now understand what you have in mind.

Yes, your plan will work -- and the polyiso will make a good air barrier, as long as the seams are taped. If you install 4 inches of XPS and 4 inches of polyiso, you will be close to code requirements, and you should greatly reduce your chance of ice dams.

Is there any reason you don't want to use polyiso between the rafters instead of XPS?

Feb 16, 2011 6:34 PM ET

by Charles Scarborough

Martin, I am concerned that if a roof leak ever occurred that the polyiso would absorb water and possibly be a problem especially if it happened during the cold months where it could refreeze in the insulation. Maybe this scenario isn't possible. Since this assembly will make it difficult to determine when a leak occurs I thought XPS would send the water down the rafter bay without absorbing any. I realize that I would possibly see any leak at the soffits but how long the leak existed before I noticed it concerned me. I appreciate all your knowledge and advice that you freely share with the rest of us. It is much appreciated.

Feb 17, 2011 6:00 AM ET

Knowledge About Roofs & Ice Dams
by Tina Gleisner

I'm amazed by the depth of the discussion here ... and now I know I need to learn more & find some experts from Canada/Midwest to help my readers understand these problems where they're VERY important as NH isn't anything like what most of you are talking about.

I totally get that you start with sealing, then insulation plus ventilation or you'll end up with moisture problems in the attic. What I don't understand is why there's no discussion about roof ice melt solutions like the one offered by Bylin Engineered Systems (described at

In my mind, you first want to prevent the heat loss but knowing you can't stop (except cold roof) all the heat loss, you next want to prevent ice dams so beyond removing snow, there are ice melt solutions or metal roofs.

PS My readership is homeowners, so in lay terms I've written a series of articles and welcome comments (or send me an article to add to the series) ...

Feb 17, 2011 6:46 AM ET

Edited Feb 17, 2011 9:13 AM ET.

No, Tina -- no!
by Martin Holladay

In my article, I described why ventilation and rubber membranes are a form of capitulation, chosen by contractors who have given up any hope of stopping heat from escaping from your home. But I failed to explain the obvious: even worse are contractors who solve the problem by INCREASING the heat escaping from your home.

You have provided a link to a company promoting "heating panels delivering 36 watts per square foot" for installation on top of your roof. As I often write -- run, don't walk, away from such contractors.

If a homeowner installs just 40 square feet of these advertised heating panels, they'll be wasting the same amount of electricity as would be required to provide all of the space heat for a Passivhaus building.

It's hard enough these days to convince builders that we need to make our buildings well insulated and tight. I don't think I have the strength to battle the idiots who want to attach electric heating panels on top of our roof shingles.

Mar 11, 2011 11:47 AM ET

Great article. I agree that
by Michael Lichy

Great article. I agree that too many people address the symptoms of ice dams and not the cause; however I disagree with the "ventilation and rubberized membranes should be the last weapons in your arsenal." statement. Proper ventilation and an ice dam protection membrane are required by the model building codes and most local building codes. They should not be considered "the last weapons in your arsenal". They should be included into the design from the very beginning. What I am trying to say is; they are just as important as air sealing and insulation to the prevention of ice dams (ventilation) and roof leaks (ice dam protection membrane).

Again, great article. It is one of the best articles I have read about preventing the formations of ice dams.

Michael H. Lichy
RC Lichy & Associates, Inc.

Mar 11, 2011 11:53 AM ET

Response to Michael Lichy
by Martin Holladay

Residential building codes do not require ventilation for all roofs. In the 2006 International Residential Code, for example, the steps necessary for unvented roofs are set out in Section R806.4.

Apr 21, 2013 12:41 AM ET

Retro 1964 hip with dropped soffit
by Paul Kuenn

In 2006 I did all the painful sealing of top plates on a 4:1 single level ranch (mine). This took considerable effort to reach with a hoe and vacuum and my longest 36" foam gun just to catch edge of holes in plate. Then I stripped off the vinyl siding and added 2" EPS not knowing at the time (life w/o Utube) that I should have gone with 3-4 more inches of insulation. Walls are 2x4 and hip rafters are 2x6 with old standard notch to bring the soffit down below the plate.

That means I had very little space to add insulation when I blew in 18" of cellulose over the old crappy fiberglass between the ceiling joists. I have a new recycled rubber slate roof with lots of solar thermal collectors and PV so I can't do much with that. I'd love to raise the roof to add higher heel but that would be an engineering feet tipping the roof one side at a time. Yikes!

The big question>>> can I add (if I remove the aluminum vented soffit again, cut off the upper section of my EPS to expose top plate & push back the old fiberglass or new cellulose) enough spray foam (what would be the best?) to stop heat loss at the upper room edges. That's really the biggest heat loss shown on the gun and I see a little mold growing in two North wall corners inside.

I will be taking off the siding again to add at least 3 more inches of EPS and furring strips. Rebuilding the window boxes (again) to warm up the walls. Blower door showed I got us to 1.2 and would love to add some of the new Lunos HRVs in a few walls (no room for regular duct work). My solar thermal runs floor heat.

Oh yah, one more biggy. I'd cut the tail ends of the rafters and support the roof overhang like on the hot roof (Alaska style) and the new EPS would cover the rafter ends. Basically everything covered but the plastic vent tubes I installed in every rafter bay (except hip corners where NOTHING can fit).

Let me have it Martin, another PK adventure with spouse looking for a way out...


Apr 21, 2013 6:17 AM ET

Response to Paul Kuenn
by Martin Holladay

If you have a ventilated unconditioned attic with limited space between the top plates of your perimeter walls and the roof sheathing, the obvious choice for insulation in this area is closed-cell spray foam. Remove all the existing insulation back from the soffit to a location where there's room for 16 inches of insulation, and install spray foam.

The solution isn't perfect, but it's the best you can do under the circumstances.

If you want to gain room for another inch or two of insulation, you can remove the ventilation chutes, and convert your attic into an unvented attic. If you did a good job air-sealing your ceiling, this solution may work fine with no further modifications. If there are signs of moisture problems after you do this, you might need to install insulation under all of the attic roof sheathing, converting your attic into a conditioned unvented attic. That costs money, of course.

Apr 21, 2013 7:41 PM ET

Minimal top plate insulation
by Paul Kuenn


You not only fixed my problems getting to this Web site but also gave me a good solid approach - all on a Sunday! Great thought going to full depth at change from closed cell to cellulose at 16" depth. I had thought of an unvented attic but hate the thought of spending a summer's worth of afterwork evenings again up there. I think once was enough. However, it's still better than raising the whole roof.

On that note: In our cold and snowy area Appleton (near Green Bay), WI, do you think that more than 2" sealed EPS/XPS would be needed under the rafters for an unvented attic? I have more than 18" of cellulose ontop of the old fiberglass for ceiling insulation. If treated like a wall with 2/3 insulation on the outside... is the attic like the wall interior in that situation?

FYI - we've never had any ice dams even with two feet of snow on the roof for months. I was more worried about the load on those 50 year old 2x6s.

Your advise is always appreciated. PK

Apr 22, 2013 4:53 AM ET

Response to Paul Kuenn
by Martin Holladay

If you want to be sure that you won't have any condensation problems in an unvented conditioned attic in your climate, I have to give you the conservative, cautious response: follow the building code.

You are in climate zone 6. In your climate zone, you need a minimum of R-25 of closed-cell spray foam under your roof sheathing to create an unvented conditioned attic, according to the 2009 IRC (Section R806.4). More information here: Creating a Conditioned Attic.

I'm not saying that less insulation wouldn't work. I'm just saying that the code requires a conservative approach.

Apr 22, 2013 11:04 PM ET

Minimal top plate insulation
by Paul Kuenn

Thanks for the link!

Very good article that I've missed in searches. I like the idea and knowing codes are worthless for real world scenarios, would think overspraying to cover rafter bottoms would be a good control on frosting in below zero temps if there was (isn't there always?) some moisture. I'll have to get a quote as that sounds like a big job (but worth investigating). I'll see how doing the outer edges works first. PK

Feb 21, 2014 10:49 AM ET

EPA Graphic
by Joe Skowan

This is the most comprehensive article and ensuing conversation I've ever seen about ice dams! Good points all around. I thought that this graphic I found might add to the conversation...the US EPA actually posted an infographic about common air leaks in a home to educate homeowners about it. I thought it was helpful.


Common air leaks - EPA.jpg

Feb 21, 2014 11:21 AM ET

Reply to Joe Skowan
by Martin Holladay

Thanks for the feedback. For more information on sealing these air leaks, see Air Sealing an Attic.

Feb 24, 2014 12:22 PM ET

Lots of good advice, but...
by Horst Fiedler

I see there are some proponents of cellulose insulation. Good stuff, but you have to understand mice and other rodents love the stuff too. Non-irritating and easy to tunnel through.

I didn't read all the experts' opinions, but was the effect of sunlight addressed? I noticed in our area the severity of ice dams seem to correspond to the amount of sunshine we get, especially in late winter.

Feb 24, 2014 12:29 PM ET

Response to Horst Fiedler
by Martin Holladay

I've done a lot of remodeling, and I've found far more evidence of mice in fiberglass batts than in cellulose. I think that the borates added to the cellulose make cellulose insulation less attractive to mice.

You're right that sunlight can contribute to snow melt and ice dams. This is the only cause of ice dams or icicles on unheated buildings like garages and sheds. Of course, ice dams on unheated buildings are minor and rare compared to ice dams on heated buildings, which is one sign that sunlight is only a minor contributor to ice dams.

Mar 5, 2014 6:44 PM ET

One more important solution: heat cable to clear gutters
by Mark Hays

Thanks Martin for this very helpful post on ice dams. Based on a number of articles on GBA and Fine Homebuilding, we sprayed the underside of our roof deck with 10" of foam. This sealed the air leaks, significantly reduced our heating costs and eliminated most of the ice dams and huge icicles that grew on our roof in the winter.

But not all. On a sunny but cold day, our well-insulated and dark roof warms up. Snow melts and runs down to the white gutter that is hanging in the cold air. Voila -- an ice dam that blocks the gutter and builds up until it pushes water up under the shingles.

This problem is 80% better than before we foamed the roof deck, but until we replace the roof and add more membrane protection, the only solution is a heat cable clipped to the gutter. I connected it to a switch, and the cable clears the iced up gutters in a couple of hours.

As you noted, one blog post cannot cover all of the details. GBA readers need to know, however, that the best insulation, venting and membranes often will not stop all of the ice dams on a roof with gutters -- and heat cable is the only option.


Sep 25, 2014 8:54 PM ET

Fiberglass batts as filler under rafter vents
by Venkat Y

My attic has rafter vents but the space between the vent and the top plate of the exterior walls is stuffed with what seems like folded fiberglass batts.Is this typical and acceptable or should I have them replaced with polyisocyanurate boards (cut to form and properly air sealed with foam all around) as shown in the videos on this page?

I have fiberglass blown in insulation in the attic about 14 inches (Climate Zone 5a) and I have had condensation around the ceiling edges this past winter.

Thanks in advance.

Sep 26, 2014 5:32 AM ET

Edited Sep 26, 2014 5:33 AM ET.

Response to Venkat Y
by Martin Holladay

If you have seen evidence of condensation at the perimeter of your ceiling, there is clearly a problem. Windwashing of the fibrous insulation at this location is the most likely explanation. So, yes: the work you describe will help.

Step one is to create an air barrier and insulation dam as you describe. This should be located as far as possible toward the exterior of your wall, at the exterior edge of your top plate.

The next question is whether there is enough vertical room between the top plates of your walls and the underside of your roof sheathing to get an adequate thickness of insulation. You're aiming for at least R-38 (that means at least 11 inches of blown-in fiberglass) to meet code. If you don't have 11 inches of space available, you might want to use spray foam (or stacks of carefully installed rigid polyiso, with air-sealed edges) at the perimeter of your attic.

Jan 26, 2016 11:58 AM ET

Solving Cape ice dam; unconditioned to conditioned living area.
by Tony Matuszak

Thank you, I have learned much from this article, the discussions, and am hoping I can get some advice.

The house is a cape in western NY that has an ice dam problem on the southwest windward side. The cape has no overhang. The attic is sloped front and back, unconditioned, and has a single layer of original 4” fiberglass batt between the 8” attic floor rafters. There is a plywood floor which does not extend to the last three feet towards where the roof rafters meet the outer walls. There are two gable vents and the attic is presently used for storage.

We will be doing the work by ourselves. We want to have the present attic become living space with storage behind the newly constructed kneewalls. The center ceiling would not be cathedral. There are no dormers and I will not be constructing any.

Unless convinced otherwise, I was not going to foam due to possibly a chemical mistake vs. temperature and such. I have researched diy with Froth-Pac spray foam. I have taken in your information on having no venting and spraying foam direct to the sheathing. I am not counting out spray foam as the final decision.

For venting, I have researched the Smart Vent and Air Vent products where a cut away is made in the roof sheathing 6” from the roof ends. This have favorable reviews but I do wonder if the first six inches without having direct venting underneath it makes for a not complete solution. I would build the venting channel with the 1 x 1’s as you suggest. This seems like the only venting fix or construct a short three or four inch overhang. I could use some input on this.

Above the wall plate, the roof/wall junction starts with 2 inches of space with 7 ½” space below the plate in the ceiling rafter channel. So I cannot get R50 at the roof/wall with rigid board. Your recommendation here is to foam spray as first choice with rigid board wind dam second. So I am not sure which way to go here.

For the lower slope behind the kneewall and the upper slope above the flat center ceiling, I can go R50. It’s the loss of usable room footage in the living area of trying to get R50 with rigid board which leads me to two thoughts. How much depth of foam spray would it take to get to R50? Or get this section of slope roof to R27.5 in the channel and one layer of 1 1/2” board over the rafters, giving a total of R35. Is this section of slope with the lower R going to cause snow to melt into water at this section, possibly moving an ice dam further up the slope instead of near the gutter?

I have not priced a company coming it to foam spray yet versus the cost of myself installing with foam board or myself doing the spray foam.

Thank you for your time I appreciate some insight.

Jan 26, 2016 12:21 PM ET

Response to Tony Matuszak
by Martin Holladay

Q. "How much depth of foam spray would it take to get to R-50?"

A. If you install closed-cell spray foam, you would need between 7 3/4 inches and 8 1/2 inches of spray foam, depending on the brand. If you install open-cell spray foam, you would need about 13 1/2 inches of spray foam.

Q. "Is this section of slope with the lower R going to cause snow to melt into water at this section, possibly moving an ice dam further up the slope instead of near the gutter?"

A. That's a hard question to answer, but the answer is "maybe." It depends on the circumstances. The higher the R-value you can achieve, the better.

It never hurts to repeat the obvious: the best solution to your problem is to install one or more layers of thick rigid foam above your roof sheathing, followed by new roofing. Here are links to two relevant articles that may help you:

Insulating a Cape Cod House

How to Install Rigid Foam On Top of Roof Sheathing

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